tag:blogger.com,1999:blog-74858630640993070592024-03-13T15:41:37.606-07:00Back Alley AstronomyIntermittent news from the Milky Way & beyondBucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.comBlogger70125tag:blogger.com,1999:blog-7485863064099307059.post-20120934825781510022017-05-31T13:48:00.000-07:002017-06-10T19:32:30.656-07:00The Small Mars Problem<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-IJhAZ9Y0VkM/WS8rhcykUPI/AAAAAAAAA9Q/t2UTaj8ZMhQ-6UulBIVNaAn2GVoZSHbegCLcB/s1600/Small%2BMars%2BProblem.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" data-original-height="912" data-original-width="1600" height="364" src="https://3.bp.blogspot.com/-IJhAZ9Y0VkM/WS8rhcykUPI/AAAAAAAAA9Q/t2UTaj8ZMhQ-6UulBIVNaAn2GVoZSHbegCLcB/s640/Small%2BMars%2BProblem.jpg" width="640" /></a></div>
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Figure 1.</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;"> All planets
and dwarf planets orbiting within 6 astronomical units (AU) of our Sun, shown
at their relative diameters. <o:p></o:p></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">From afar, our Solar System looks regular and
well-organized. At its center is a large, massive sphere of incandescent gases
(the Sun) surrounded by eight smaller and less massive spheres of heavier
elements (the planets) distributed in concentric orbits out to a distance of
about 4.5 billion km/2.8 billion miles. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The orbital distribution of the eight planets also seems
regular, at least at first glance. Smaller, rocky worlds are confined to the
inner system, while larger, gaseous worlds dominate the outer system. Planet
sizes follow a curve, rising from the inner to the middle planets and then
declining again from the middle to the outer planets. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">In more specific terms, mass and radius increase along
with distance among the three planets closest to the Sun (Mercury through
Earth). Both parameters peak at the orbit of the fifth planet, Jupiter, which is
almost a dozen times the radius and more than 300 times the mass of Earth. Then,
from Jupiter through Uranus, the seventh planet, both mass and radius decline
substantially along with distance from the Sun.<o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">But this orderly progression of planet sizes has two
notable interruptions: Mars and Neptune. If the distribution of planets were truly
regular, Mars would be larger and more massive than Earth, and Neptune would be
smaller and less massive than Uranus. Instead, the Red Planet has only 53% of
Earth’s radius (0.53 Rea) and 11% of its mass (0.11 Mea), while the Azure
Planet, at 17.2 Mea and 3.9 Rea, has about 98% of the radius of Uranus but 119%
of its mass.<o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">How did that happen? <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">In the present post I’m going to ignore the oddity of
Neptune and concentrate on the Martian half of the question. My rationale is
that Mars occupies our system’s classical habitable zone, and therefore – along
with Earth and Venus – plays a critical role in theories of the habitability of
extrasolar planets. If mass had been more uniformly distributed in the inner
Solar System, Mars would be more massive than it is. If its mass were in the
range of 1 to 2 Mea, Mars would likely be able to sustain a magnetic field,
plate tectonics, surface water, and long-term habitability. Therefore, if we
want to understand the potential system architectures that might support
life-bearing planets, we need to understand why Mars is so small.<o:p></o:p></span></span></div>
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<span style="font-size: large;"><b><span style="font-family: "georgia" , serif;">Figures
1</span></b><span style="font-family: "georgia" , serif;"> and <b>2</b> highlight the Small
Mars Problem and the Great Martian Gap, which is the name I just invented for the
general depletion of mass between Earth and Jupiter. The planet Mars and the
dwarf planet Ceres orbit within this gap at 1.52 AU and 2.77 AU, respectively. With
a little more than 1% of the mass of our Moon, Ceres accounts for fully one-third
of all mass in the Asteroid Belt, which is concentrated between 2.2 and 3.3 AU
(the latter boundary provided by the 2:1 resonance with Jupiter's orbit; Jewitt
et al. 2009). The entire region between the orbits of Earth and Jupiter
contains less than 0.12 Mea, with little Mars accounting for 99% of the total. By
contrast, the region extending inward from Earth’s orbit to the Sun contains
1.87 Mea, yet Earth, the most massive object, accounts for only 53% of the
total.<o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">Figure 2</span></span></b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">. The
Great Martian Gap</span><o:p></o:p></span></div>
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<a href="https://2.bp.blogspot.com/-OJfcsG8mzXc/WS8rpNV8eNI/AAAAAAAAA9U/T0U2hCwqf5oaDYQRPUVStpU0-9w-WqnIQCLcB/s1600/The%2BGreat%2BGap.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" data-original-height="600" data-original-width="1554" height="246" src="https://2.bp.blogspot.com/-OJfcsG8mzXc/WS8rpNV8eNI/AAAAAAAAA9U/T0U2hCwqf5oaDYQRPUVStpU0-9w-WqnIQCLcB/s640/The%2BGreat%2BGap.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , sans-serif; font-size: 10pt;">Blue numbers along the bottom refer to
astronomical units (AU), where the Earth/Sun separation = 1. Planets are shown
at their relative sizes and relative distances from the Sun, with separate
scales for radius and distance. As astronomers have long noted, mass is
severely depleted between the orbit of Jupiter at 5.2 AU and the orbit of Earth
at 1 AU (see Weidenschilling 1977).<o:p></o:p></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">zigzag
migration<o:p></o:p></span></span></b></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Recent studies by Konstantin Batygin & Greg Laughlin
(2015) and by Sean Raymond & colleagues (2016) have presented conflicting scenarios
to explain the Small Mars Problem and the Great Martian Gap. Both involve
zigzag migratory paths for Jupiter during the primordial phase of system
evolution. <o:p></o:p></span></span></div>
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<span style="font-size: large;"><a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html"><span style="font-family: "georgia" , "serif"; mso-bidi-font-family: Arial;">Batygin &
Laughlin</span></a><span style="font-family: "georgia" , serif;"> based their approach on earlier models
by Kevin Walsh & colleagues (2011) and Pierens & Raymond (2011), in
which Jupiter formed in the outer Solar System (somewhere beyond 3 AU) and then
migrated first inward and then outward again. The popular name for this scenario
(blogged </span><a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html"><span style="font-family: "georgia" , "serif"; mso-bidi-font-family: Arial;">here</span></a><span style="font-family: "georgia" , serif;"> and </span><a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html"><span style="font-family: "georgia" , "serif"; mso-bidi-font-family: Arial;">here</span></a><span style="font-family: "georgia" , serif;">) is the Grand Tack. According to Batygin & Laughlin,
these maneuvers not only swept most solid mass out of the region exterior to
Earth’s present orbit, but also created an instability that emptied the region interior
to 0.7 AU. <o:p></o:p></span></span></div>
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<span style="font-size: large;"><a href="http://backalleyastronomy.blogspot.com/2016/07/jupiter-re-ascending.html"><span style="font-family: "georgia" , "serif"; mso-bidi-font-family: Arial;">Raymond &
colleagues</span></a><span style="font-family: "georgia" , serif;"> took a very different approach based
on the “inside out” model of planet formation presented by Chatterjee & Tan
(2014). Contrary to his own earlier work, Raymond’s group proposed that Jupiter
formed in the inner Solar System near the Sun, and then migrated first outward,
then inward, and finally outward again, depleting the region inward of Venus
and wreaking havoc beyond Earth. <o:p></o:p></span></span></div>
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<span style="font-size: large;"><b><span style="font-family: "georgia" , serif;">sweeping secular resonances </span></b><span style="font-family: "georgia" , serif;"><o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">While the Grand Tack has been more widely discussed and
endorsed than the inside-out scenario, both explanations have been faulted. Now
Benjamin Bromley & Scott Kenyon (2017) present an alternative approach in
which “sweeping secular resonances” with Jupiter’s orbital motion, rather than
any migratory scenario, become the mechanism for clearing the Great Martian
Gap. Their model implies a less dramatic but equally consequential role for
Jupiter, and I suspect that it can be extended to explain similar gaps observed
in the architecture of multiplanet systems around other stars.<o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">To develop their model, Bromley & Kenyon (hereafter
BK17) conducted extensive numerical simulations based on earlier work by and
with their collaborators Makiko Nagasawa and Edward Thommes (Nagasawa et al.
2007, Thommes et al. 2008). They also note recent work on the same problem by Xiaochen
Zheng & colleagues (2017). <o:p></o:p></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , serif;">BK17 begin with the familiar theoretical construct of the
Minimum Mass Solar Nebula (blogged </span><a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html"><span style="font-family: "georgia" , "serif"; mso-bidi-font-family: Arial;">here</span></a><span style="font-family: "georgia" , serif;">). They assume that a dusty gas nebula (generally known
as a </span><a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html"><span style="font-family: "georgia" , "serif"; mso-bidi-font-family: Arial;">protoplanetary
disk</span></a><span style="font-family: "georgia" , serif;">) is present at the outset of their
simulations. Jupiter is fully formed at its current semimajor axis of 5.2 AU (<b>Figure 2</b>), having cleared a gap in the
disk for 1 AU on either side of its orbital path. A swarm of planetesimals orbits
inward of this gap, while Saturn orbits well beyond it. Both gas giants exert
gravitational effects on their surroundings, and the disk itself has gravity. In
addition, the orbit of Jupiter is slightly eccentric, but probably less so than
its present value of 0.05. BK17 assume an eccentricity of 0.03 in their
simulations.<o:p></o:p></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , serif;">The key factor in their approach is the </span><span style="font-family: "symbol";">n</span><i><span style="font-family: "georgia" , serif;">5</span></i><span style="font-family: "georgia" , serif;"> resonance (“nu-5,” Greek letter <i>nu</i> with superscript <i>5</i>), a
“secular” or “very long-term” resonance between the motion of the
protoplanetary disk and Jupiter’s orbital period. BK17 define the nu-5
resonance as the location “where the local apsidal precession rate matches
Jupiter’s rate of precession,” and note that a planetesimal or protoplanet at
this location will be perturbed by Jupiter’s gravitational influence onto a
highly eccentric orbit. The likely result will then be either collision with
another planet or protoplanet, engulfment by the Sun, or ejection from the
Solar System. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">In the early Solar System, when the gas disk was still present,
the nu-5 resonance was located in the vicinity of the present Asteroid Belt
(Zheng et al. 2012). As the gas dissipated, the resonance moved inward,
destabilizing (“shaking up”) the orbits of protoplanets and planetesimals and
effectively clearing out a substantial mass in solids. After the gas was
completely depleted, the nu-5 resonance reached its present position inside the
orbit of Venus. This sweeping shake-up created the Great Martian Gap while
leaving behind enough mass to build Earth and Venus, as well as their two
by-blows, Mercury and Mars.<o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">BK17 discovered that several different factors were
critical to reproducing the mass of Mars and the present-day Asteroid Belt
within the time constraints provided by the known age of Mars. These include the
mass of the perturbing planet, its distance from the system habitable zone, and
the timing and speed of the sweeping secular resonance generated by its orbital
motion. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Regarding <b>mass</b>,
BK17 find that only a “Jupiter-mass planet” can produce the magnitude of
perturbation required to induce a shake-up in the protoplanetary disk of a
Sun-like star. Unfortunately, they don’t provide a precise value for the necessary
mass – for example, would an object of Saturn’s mass (95 Mea) be sufficient?
They also note that a “super-Earth” would be massive enough to produce sweeping
secular resonances in an M dwarf system, likely referring to an object in the
range of 1-10 Mea (see, e.g., Kenyon & Bromley 2009). <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Regarding <b>orbital location</b>,
they find that the masses of Earth and Mars depend sensitively on the semimajor
axis of Jupiter at the time of the sweeping resonance. If Jupiter had been
substantially farther from the Sun, the resonance would never have reached the
orbit of Mars, and Mars would have grown much bigger than it actually did –
presumably massive enough to support a habitable environment. But if Jupiter
had been substantially closer to the Sun, and thus closer to the system
habitable zone, the resonance would have inhibited the formation of Earth in
the same way that it stunted the growth of Mars when Jupiter was at 5.2 AU. Instead
of one living planet, our system would have none at all. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Regarding the <b>timing</b>
of the sweeping secular resonance, BK17 note that its schedule is determined by
the lifetime of the protoplanetary disk. As we saw in an <a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html">earlier
post</a>, the system age when gas dissipation commences can fall anywhere
between 1 and 10 million years. At the early end of that range, according to
BK17, dissipation accompanied by shake-up would have extremely negative
consequences for rocky planet formation, as it would destroy planetesimals
before they had time to accrete into protoplanets. At the latter end, however,
the effects would be modest, since accretion would already be well advanced,
potentially permitting the growth of Earth-size planets out to a distance of 3
AU. In the case of our Solar System, we can assume that the shake-up happened
before a system age of about 4 million years, given radiometric evidence that
Mars was fully formed by then. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The rate of disk dispersal also matters. Although many studies
have found that gas dissipation happens rapidly, requiring less than half a
million years from start to finish (Williams & Cieza 2011), variation is
inevitable: some disks take longer than others to disperse. BK17 find that the
relative speed of dissipation strongly affects the outcomes of secular
resonance sweeping. If the gas dissipates quickly, the resonance sweeps inward at
the same rate, resulting in minimal disruption of the planetesimal population. If
the gas dissipates more slowly, the resonance becomes increasingly more destructive,
clearing larger and larger quantities of solid mass from the system. <o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">Figure 3.</span></span></b><span style="font-family: "georgia" , serif;"><span style="font-size: large;"> Calaveras street sweepers on the Day of the Dead</span><o:p></o:p></span></div>
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<a href="https://4.bp.blogspot.com/-TLxaN4kXELg/WS8rx5Ke_HI/AAAAAAAAA9Y/BG111UKe28giKlN0bJDa-frN1N7wHiIAQCLcB/s1600/Street%2BSweepers%2Bby%2BJose%2BGuadalupe%2BPosada%2B-%2B850px.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" data-original-height="516" data-original-width="850" height="388" src="https://4.bp.blogspot.com/-TLxaN4kXELg/WS8rx5Ke_HI/AAAAAAAAA9Y/BG111UKe28giKlN0bJDa-frN1N7wHiIAQCLcB/s640/Street%2BSweepers%2Bby%2BJose%2BGuadalupe%2BPosada%2B-%2B850px.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , sans-serif; font-size: 10pt;">From a print by José Guadalupe Posada
(1852-1913)<o:p></o:p></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">extrasolar
asteroids and orbital gaps<o:p></o:p></span></span></b></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Although BK17 are interested primarily in the evolution
of our Solar System, they attempt to generalize some of their results to
extrasolar locales. Their chief concern is the occurrence of extrasolar analogs
of the Asteroid Belt. They argue that most systems with a gas giant in
Jupiter’s approximate location (i.e., just outside the system <a href="http://backalleyastronomy.blogspot.com/2016/11/accretion-with-migration-in-radially.html">ice
line</a>, where accretion is maximized) will experience a sweeping secular
resonance whose outcome will be a ring of rocky debris in the inner system. While
they concede that few such structures have been discovered to date (<a href="https://en.wikipedia.org/wiki/HD_69830">HD 69830</a> is most familiar),
they attribute these limited findings to the difficulty of discerning modest
aggregations of warm debris even around nearby stars. In the future, they predict,
more sensitive searches will be more successful. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">I suggest that the implications of their model are much
broader than their relevance to extrasolar asteroid belts, and far more dispiriting.
If sweeping secular resonances are common in systems with cool gas giants, then
the outlook for habitable planets is even less promising than I thought. Here’s
why.<o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">An important focus of this blog is the possibility of
Solar System analogs – that is, exoplanetary systems containing cool giants
whose orbital parameters would permit the survival of Earth-mass planets (0.5-2
Mea) in the local habitable zone. (For recent posts on this topic, see <a href="http://backalleyastronomy.blogspot.com/2017/01/2016-backyard-bonanza.html">here</a>
and <a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html">here</a>.)
My January search of the <i><a href="http://exoplanet.eu/">Extrasolar Planets Encyclopaedia</a></i> identified
<a href="http://backalleyastronomy.blogspot.com/2017/01/2016-backyard-bonanza.html">17
such systems</a> located within 60 parsecs/196 light years. All center on
Sun-like stars in the range of 0.85-1.15 Solar masses, so their habitable zones
have boundaries similar to those proposed for our own system (0.99-1.70 AU;
Kopparapu et al. 2013). <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Among the Jupiter analogs in these systems, semimajor axes
range from 3 AU to 5.2 AU, and more than half orbit inside 4 AU. According to the
findings of BK17, virtually all these systems will have experienced a sweeping
secular resonance very similar to the one they propose for the Solar System.
Because all but one of the 17 confirmed Jupiter analogs orbits closer to the local
habitable zone than does our own Jupiter, the depletion of mass in this favored
region is likely to be even more extreme than it was at home. Therefore,
habitable planets appear to be less likely in the existing sample of Solar System
analogs than they are in the Solar System. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">To put it another way: BK17 have just shown that gas giant
planets are even more unfriendly to the formation and survival of habitable
planets than we <a href="http://backalleyastronomy.blogspot.com/2012/11/dwarfs-vs-giants.html">already
suspected</a>. It’s not enough for the giant to reside outside the system ice
line in a configuration that permits an Earth-like planet to maintain an
Earth-like orbit. The giant must also be distant enough from the central star that the sweeping
secular resonance generated by its orbit was insufficient to clear solid mass
from the local habitable zone. Even Jupiter managed to evacuate mass from more
than half of the radial extent of our own habitable zone, drastically reducing
our system’s potential to produce life-bearing planets. Now it looks like
extrasolar Jupiters might be still more likely to foreclose the possibility of
life around other stars. <o:p></o:p></span></span></div>
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com3tag:blogger.com,1999:blog-7485863064099307059.post-57327363917974536802017-05-03T23:10:00.001-07:002017-05-31T15:16:01.000-07:00TRAPPIST-1 and Kepler-11: Revised Masses<div class="separator" style="clear: both; text-align: center;">
<a href="https://4.bp.blogspot.com/-wEFM2E2o38I/WQrEz_MxCgI/AAAAAAAAA8k/PMpbjmz4AbgdE_8TpIO2I0ujXOh61s3kwCLcB/s1600/TRAPPIST-1%2BMasses%2Bfrom%2BWang%2B2017.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="550" src="https://4.bp.blogspot.com/-wEFM2E2o38I/WQrEz_MxCgI/AAAAAAAAA8k/PMpbjmz4AbgdE_8TpIO2I0ujXOh61s3kwCLcB/s640/TRAPPIST-1%2BMasses%2Bfrom%2BWang%2B2017.jpg" width="640" /></a></div>
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<b><span style="font-family: "arial" , sans-serif;">Figure 1.</span></b><span style="font-family: "arial" , sans-serif;"> Revised masses
and radii for the seven planets of TRAPPIST-1, as estimated by Wang &
colleagues (2017). Results from Gillon & colleagues (2017) are shown for
comparison. Image source: Figure 5 of Wang et al. 2017, with new labels. <span style="font-size: 10pt;"><o:p></o:p></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , serif;">A few months ago, Michaël Gillon & colleagues reported
a remarkable seven-planet architecture for a nearby ultra-cool red dwarf, </span><a href="http://backalleyastronomy.blogspot.com/2017/03/trappist-1-and-seven-dwarfs.html"><span style="font-family: "georgia" , "serif";">TRAPPIST-1</span></a><span style="font-family: "georgia" , serif;">. In
their analysis, six out of the seven transiting planets in this tightly packed
system have densities in the range of Earth, Venus, and Mars – and no fewer
than three occupy the system habitable zone. Those findings were based in part
on a 20-day campaign of nearly continuous observation by the Spitzer Space
Telescope.<o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Now Songhu Wang & colleagues have presented a rather
different picture of the TRAPPIST-1 family, based on more than 70 days of
monitoring by the Kepler Space Telescope during the K2 Mission. Even though Kepler’s
current precision is inferior to that of Spitzer, the availability of data
covering a much longer period of time still permits a more robust
characterization of these planets than was possible for Gillon’s group. Like
their predecessors, Wang & colleagues analyzed variations in transit times
to estimate the masses and densities of the planets. Thanks to their augmented
dataset, they were able to include all seven in their calculations, and not
just the inner six. <o:p></o:p></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , serif;">The periods, semimajor axes, and equilibrium temperatures
of the TRAPPIST planets are unchanged, and only two of them have smaller radii. Nevertheless, many planetary masses,
and all densities, are dramatically different. <b>Figure 1</b> and <b>Table 1</b> contrast
the results of Wang & colleagues with those of Gillon & colleagues. <b>Figure 2</b> depicts the planets at their
relative sizes and densities according to Wang’s group (revised from the first
figure in my </span><a href="http://backalleyastronomy.blogspot.com/2017/03/trappist-1-and-seven-dwarfs.html"><span style="font-family: "georgia" , "serif";">previous post</span></a><span style="font-family: "georgia" , serif;"> on TRAPPIST-1).<o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">Table 1.</span></span></b><span style="font-family: "georgia" , serif;"><span style="font-size: large;"> Comparison of TRAPPIST-1 parameters from
Wang et al. and Gillon et al.</span><o:p></o:p></span></div>
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<a href="https://1.bp.blogspot.com/-2EDaw2X1Rqc/WQrFAiBn3QI/AAAAAAAAA8o/WovA3-JHemYr-cGjXpRGp-6myQvqJgp-QCLcB/s1600/TRAPPIST-1%2BPlanetary%2BParameters%2B-%2BWang%2Bversus%2BGillon.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="140" src="https://1.bp.blogspot.com/-2EDaw2X1Rqc/WQrFAiBn3QI/AAAAAAAAA8o/WovA3-JHemYr-cGjXpRGp-6myQvqJgp-QCLcB/s640/TRAPPIST-1%2BPlanetary%2BParameters%2B-%2BWang%2Bversus%2BGillon.gif" width="640" /></a></div>
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<span style="font-family: "arial" , sans-serif;">Period is expressed in Earth days; radius, mass, and
density are expressed in Earth units.<o:p></o:p></span></div>
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<span style="font-family: "arial" , sans-serif;">(</span><span style="color: blue; font-family: "arial" , sans-serif;">W</span><span style="font-family: "arial" , sans-serif;">) = Wang et al. 2017; (G) = Gillon et al.
2017.<span style="font-size: 10pt;"><o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">a super mercury among super ganymedes?<o:p></o:p></span></span></b></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">As we compare the findings of these two teams, we need to
remember that both of them reported their results with large uncertainties (as
shown in Table 1). For five out of seven planets, the results on mass from both
groups are formally equivalent. The exceptions are planet <b>e</b>, for which the difference between Wang’s highest estimate and
Gillon’s lowest is only 2% of an Earth mass (0.02 Mea), and planet <b>h</b>, for which Gillon’s group reported no
mass at all. Moreover, for the three innermost planets (<b>b, c, d</b>), the densities estimated by Wang’s group are formally
consistent with a rocky composition like Earth’s, again within uncertainties. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">But if we focus on the interpretations that each group
actually prefers, the differences become too wide to bridge. According to Wang’s
group, only the three innermost planets might be rocky in composition, with
planet <b>c</b> requiring major enrichment
in iron to explain its large mass. Planets <b>b</b>
and <b>d</b>, on the other hand, must be either
depleted in metals or enriched in volatiles, or both, to achieve their proposed
densities. At 62% and 72% of Earth, respectively, their closest analogs in our
Solar System are the Moon and Mars. <o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">Figure 2.</span></span></b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">
Revised densities for the planets of TRAPPIST-1 </span><o:p></o:p></span></div>
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<a href="https://4.bp.blogspot.com/-5R4BEc2lit8/WQrFUYJLPCI/AAAAAAAAA8s/S1pOp_6DU3w7j-0-8YpJRjGVOCupFyDqwCLcB/s1600/TRAPPIST-1%2BSystem%2BArchitecture-Wang2017.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="574" src="https://4.bp.blogspot.com/-5R4BEc2lit8/WQrFUYJLPCI/AAAAAAAAA8s/S1pOp_6DU3w7j-0-8YpJRjGVOCupFyDqwCLcB/s640/TRAPPIST-1%2BSystem%2BArchitecture-Wang2017.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , sans-serif;">The seven planets of TRAPPIST-1 are shown at
their relative sizes, with colors corresponding to the densities estimated by Wang
et al. 2017. Yellow shading marks the system habitable zone. Except for planet <b>c</b>, all densities are lower than those
preferred by Gillon et al. 2017. This result suggests an internal composition
with a rocky core enveloped in a layer of ice. (Update of Figure 1 in a </span><a href="http://backalleyastronomy.blogspot.com/2017/03/trappist-1-and-seven-dwarfs.html"><span style="font-family: "arial" , sans-serif;">previous post</span></a><span style="font-family: "arial" , sans-serif;"> on the same system.) <span style="font-size: 10pt;"><o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The next three planets (<b>e, f, g</b>), which occupy the habitable zone, have densities similar
to those of Ganymede, Titan, and Callisto, the largest moons of Jupiter and
Saturn. The bulk composition of those moons is approximately one-third water
ice and two-thirds rock (Hussmann et al. 2015). A similar abundance of ice,
possibly accompanied by depletion in metals, is needed to explain the relatively
large radii and low masses of this temperate TRAPPIST trio. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">For planet <b>h</b>,
the smallest and coolest member of the family, the estimated density has such
large uncertainties that we can say only that the numbers are equally consistent
with a substantial hydrogen envelope (like Uranus), a composition completely
dominated by hydrogen (like Saturn), or a rock/metal object with a modest
percentage of water ice but no gaseous hydrogen at all (like Europa).
Nevertheless, the transit timing data also provide an upper limit on this
object’s mass, so we know that planet <b>h</b>
is too lightweight to retain a hydrogen atmosphere unless it is constantly
replenished by volcanic outgassing. Therefore, this little world’s bulk composition
is probably similar to that of the three planets in the habitable zone. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">From the new perspective offered by Wang &
colleagues, the seven planets of TRAPPIST-1 present far more variety in bulk
composition and surface environments than earlier data suggested. Indeed, if we
accept the accuracy of these new findings – and I don’t see why we shouldn’t –
we can no longer describe TRAPPIST-1 as a system with several Earth-like
planets. Instead, we see a single terrestrial planet (<b>c</b>) enriched in iron, with an equilibrium temperature too high to
permit surface bodies of water, accompanied by six lightweight planets that variously
resemble scaled-up versions of Mars (density 0.71 Earth) and the three largest
moons in our Solar System (Callisto, Titan, and Ganymede; respective densities
0.33, 0.34, and 0.35 Earth). Because three of the least dense planets (<b>e, f, g</b>) occupy the system habitable
zone, our most optimistic conjecture is that they are ocean worlds with liquid
seas sloshing atop layers of high-pressure ice (Kuchner 2003, Leger et al. 2004).
<o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;"><br /></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">This isn’t an especially promising outlook for anyone who
seeks exotic alien organisms, but if your models allow low-density ocean
planets to support life (Noack et al. 2016), you can still imagine undulant sea
creatures populating the hydrospheres of one or more of these little worlds. <o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">Figure 3.</span></span></b><span style="font-family: "georgia" , serif;"><span style="font-size: large;"> Exotic
aquatic life on Earth</span><o:p></o:p></span></div>
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<a href="https://3.bp.blogspot.com/-ybNJsXx4gbY/WQrFlw6kC9I/AAAAAAAAA8w/WzbyvrzUJKo2Zqel8sJNGhaVAU670JmvACLcB/s1600/Nembrotha%2Bcristata%2B%252B%2BHapalochlaena%2Blunulata.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="248" src="https://3.bp.blogspot.com/-ybNJsXx4gbY/WQrFlw6kC9I/AAAAAAAAA8w/WzbyvrzUJKo2Zqel8sJNGhaVAU670JmvACLcB/s640/Nembrotha%2Bcristata%2B%252B%2BHapalochlaena%2Blunulata.jpg" width="640" /></a></div>
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<i><span style="font-family: "arial" , sans-serif;">Nembrotha cristata</span></i><span style="font-family: "arial" , sans-serif;"> (left), a
tropical sea slug, by Chriswan Sungkono; <i>Hapalochlaena
lunulata</i> (right), a highly venomous octopus native to the Philippines, photographer
unknown. <span style="font-size: 10pt;"><o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">younger
star, fatter planets<o:p></o:p></span></span></b></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , serif;">In
related news, a team led by Megan Bedell has offered revised masses and
densities for the six transiting planets of Kepler-11. This is the </span><a href="http://backalleyastronomy.blogspot.com/2012/05/kepler-11-as-testbed.html"><span style="font-family: "georgia" , "serif";">benchmark system</span></a><span style="font-family: "georgia" , serif;"> for all
studies of extrasolar planetology, as it was the first place where astronomers could
obtain sufficiently precise data on the transit times of multiple interacting planets
to permit estimation of their masses. Back in 2011, when the system was
announced, everyone was shocked to learn that planets not much heavier than
Earth could support greenhouse atmospheres inflated with hydrogen and helium. <o:p></o:p></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , serif;">By now,
of course, that weirdness is pretty well digested, but Kepler-11 still retains
the power to amaze. With transit data extending over the full Kepler mission,
this system has already benefited from repeated analyses that led to revisions
(mostly downward) in the masses of its six planets. Improved results became
available for the first time in 2013, when Jack Lissauer and colleagues
published new physical and orbital parameters for the whole system (</span><a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html"><span style="font-family: "georgia" , "serif";">blogged here</span></a><span style="font-family: "georgia" , serif;">).<o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Now,
four years later, we have another update. It’s important to note that the new findings
are not based on any new transit data. (As far as I know, no transits of
Kepler-11 have been observed since the termination of data collection by the
Kepler Mission in 2013.) Instead, the revised parameters are based on precise observations
of the host star. Although previous studies have always noted a close
resemblance between Kepler-11 and our Sun, Bedell & colleagues go further:
they characterize the star as a “Solar twin.” <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Contrary
to Lissauer’s group, who estimated a stellar age in the range of 7 to 10
billion years, a stellar mass 96% Solar (0.96 Msol), and a stellar radius 105%
Solar (1.05 Rsol), Bedell’s group finds that the star is a bit younger than our
Sun (3.2 ±0.9 billion years versus 4.55 billion years), with a larger mass (1.04
Msol) and a slightly revised radius (1.02 Rsol). Because most planetary data
depend sensitively on the properties of the host star, these new values lead to
further revisions in our understanding of the planets. <o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">Table 2.</span></span></b><span style="font-family: "georgia" , serif;"><span style="font-size: large;"> Comparison of Kepler-11
parameters from Bedell et al. and Lissauer et al.</span><o:p></o:p></span></div>
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<a href="https://3.bp.blogspot.com/-2HnQx2nvyxg/WQrFy9jyDZI/AAAAAAAAA80/WZFyHz6dSQE449I0EhGNHrF_JtdB2ACZQCLcB/s1600/Kepler-11%2Bparameters%2Bafter%2BBedell%2B2017.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="158" src="https://3.bp.blogspot.com/-2HnQx2nvyxg/WQrFy9jyDZI/AAAAAAAAA80/WZFyHz6dSQE449I0EhGNHrF_JtdB2ACZQCLcB/s640/Kepler-11%2Bparameters%2Bafter%2BBedell%2B2017.gif" width="640" /></a></div>
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<span style="font-family: "arial" , sans-serif;">Notes: <i>a</i>
= semimajor axis in astronomical units; period = orbital period in days.<o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">B 17</span></b><span style="font-family: "arial" , sans-serif;"> = Bedell et al. 2017; </span><b><span style="color: blue; font-family: "arial" , sans-serif;">L 13</span></b><b><span style="font-family: "arial" , sans-serif;">, </span></b><b><span style="color: red; font-family: "arial" , sans-serif;">L 11</span></b><span style="font-family: "arial" , sans-serif;"> = Lissauer et al. 2013, 2011.<span style="font-size: 10pt;"><o:p></o:p></span></span></div>
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<span style="font-size: large;"><b><span style="font-family: "georgia" , serif;">Table 2</span></b><span style="font-family: "georgia" , serif;"> compares three generations
of data on Kepler-11. </span><span style="font-family: "georgia" , serif;">It’s readily
apparent that most of the new parameters offered by Bedell’s group represent a
reversion in the direction of the initial findings from 2011. Specifically, all
the masses proposed by Bedell’s group are larger than the ones published by
Lissauer & colleagues in 2013, as are all radii except for planet <b>g</b>, where we see no change. Yet in
comparison with the 2013 update, the latest estimates have less extreme consequences
for planetary composition. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">As the authors note, the upward revisions in masses and
radii result in an average increase of almost 50% in the planets’ bulk density.
But the big picture stays mostly the same. As before, all Kepler-11 planets
with well-constrained masses are more lightweight than Uranus (14.5 Mea). As
before, planets <b>c</b> through <b>f</b> are unambiguously puffy, requiring
hydrogen envelopes to bring their ample radii in line with their relatively
puny masses (all <10 Mea). <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The most consequential change involves planet <b>b</b>, to which the parameters announced in
2013 allowed Lopez & Fortney (2014) to attribute a bulk mass fraction in
hydrogen of about 0.5%. Their interpretation seems counterintuitive – I mean, how
could an object under 2 Mea retain any hydrogen at all after aeons of extreme
irradiation? Nevertheless, to my knowledge, it has been broadly accepted. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The newly estimated density of 0.445 Earth strengthens
the argument that planet <b>b</b> is an amalgam
of rock and ice, rather like Europa, which has a bulk density of 0.55 Earth. No
lightweight envelope is needed to explain its radius. Although I doubt that the
findings of Bedell’s group are the last word on Kepler-11b, they offer a
physically plausible model of the bulk composition of this benchmark extrasolar
planet. <o:p></o:p></span></span></div>
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<a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><span style="font-size: large;"><i><span style="font-family: "georgia" , "serif";">Click
here for a permanent index by topic of blog posts</span></i><i><span style="color: blue; font-family: "georgia" , "serif";"><br />
</span></i><i><span style="font-family: "georgia" , "serif";">on
Back Alley Astronomy</span></i></span></a><span style="font-family: "georgia" , "serif";">
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<b><span style="font-family: "arial" , sans-serif;">REFERENCES<o:p></o:p></span></b></div>
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<b><span style="font-family: "arial" , sans-serif;">Bedell M</span></b><span style="font-family: "arial" , sans-serif;">, Bean JL, Meléndez
J, Mills SM, Fabrycky DC, Freitas FC, Ramírez I, Asplund M, Liu F, Yong D.
(2017) Kepler-11 is a Solar Twin: Revising the masses and radii of benchmark
planets via precise stellar characterization. <i>Astrophysical Journal</i> 839, 94.<o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">Gillon M</span></b><span style="font-family: "arial" , sans-serif;">, Jehin E,
Lederer SM, Delrez L, de Wit J, Burdanov A, Van Grootel V, Burgasser A, Triaud
A, Opitom C, Demory B-O, Sahu DK, Bardalez-Gagliuffi D, Magain P, Queloz D.
(2016) Temperate Earth-sized planets transiting a nearby ultracool dwarf star. <i>Nature</i> 533, 221-224. Abstract: </span><a href="http://adsabs.harvard.edu/abs/2016Natur.533..221G"><span style="font-family: "arial" , sans-serif;">2016Natur.533..221G</span></a><span style="font-family: "arial" , sans-serif;"><o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">Gillon M</span></b><span style="font-family: "arial" , sans-serif;">, Triaud A,
Demory B-O, Jehin E, Agol E, Deck KM, Lederer SM, de Wit J, Burdanov A, Ingalls
JG, Bolmont E, Leconte J, Raymond SN, Selsis F, Turbet M, Barkaoui K, Burgasser
A, Burleigh MR, Carey SJ, Chaushev A, Copperwheat CM, Delrez L, Fernandes CS,
Holdsworth DL, Kotze EJ, Van Grootel V, Almleaky Y, Benkhaldoun Z, Magain P, Queloz
D. (2017) Seven temperate terrestrial planets around the nearby ultracool dwarf
star TRAPPIST-1. <i>Nature</i> 542, 456-460.
Abstract: </span><a href="http://adsabs.harvard.edu/abs/2017Natur.542..456G"><span style="font-family: "arial" , sans-serif;">2017Natur.542..456G</span></a><span style="font-family: "arial" , sans-serif;"><o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">Hussmann H</span></b><span style="font-family: "arial" , sans-serif;">, Sotin C,
Lunine J. (2015) Interiors and evolution of icy satellites. In <i>Treatise on Geophysics, Volume 10: Physics
of Terrestrial Planets and Moons</i>, ed. G. Schubert. Elsevier B.V.<o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">Kuchner MJ</span></b><span style="font-family: "arial" , sans-serif;">. (2003)
Volatile-rich Earth-mass planets in the habitable zone. <i>Astrophysical Journal</i> 596, L105-L108.<o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">Leger A</span></b><span style="font-family: "arial" , sans-serif;">, Selsis F, Sotin C,
et al. (2004) A new family of planets? “Ocean Planets.” <i>Icarus</i> 169, 499-504.<o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">Lissauer JJ</span></b><span style="font-family: "arial" , sans-serif;">,
Jontof-Hutter D, Rowe JF, Fabrycky DC, Lopez ED, Agol E, et al. (2013) All six
planets known to orbit Kepler-11 have low densities. <i>Astrophysical Journal</i> 770, 131. Abstract: </span><a href="http://adsabs.harvard.edu/abs/2013ApJ...770..131L"><span style="color: #3778cd; font-family: "arial" , sans-serif; text-decoration-line: none;">2013ApJ...770..131L</span></a><span style="font-family: "arial" , sans-serif;"><o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">Luger R</span></b><span style="font-family: "arial" , sans-serif;">, Sestovic
M, Kruse E, Grimm SL, Demory B-O, Agol E, Bolmont E, Fabrycky D, Fernandes CS,
Van Grootel V, Burgasser A, Gillon M, et al. (2017) A terrestrial-sized
exoplanet at the snow line of TRAPPIST-1. In press.<o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">Noack L</span></b><span style="font-family: "arial" , sans-serif;">, Höning D,
Rivoldini A, Heistracher C, Zimov N, Journaux B, Lammer H, Van Hoolst T,
Bredehöft JH. (2016) Water-rich planets: How habitable is a water layer deeper
than on Earth? <i>Icarus</i> 277, 215-236.<o:p></o:p></span></div>
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<b><span style="font-family: "arial" , sans-serif;">Wang S</span></b><span style="font-family: "arial" , sans-serif;">, Wu DH,
Barclay T, Laughlin GP. (2017) </span><span style="font-family: "arial" , sans-serif;">Updated masses for the TRAPPIST-1 planets. In press. Abstract:
</span><a href="http://adsabs.harvard.edu/abs/2017arXiv170404290W"><span style="color: blue; font-family: "arial" , sans-serif;">2017arXiv170404290W</span></a><span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"> <o:p></o:p></span></div>
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-34967004236253624482017-04-18T23:57:00.000-07:002017-04-19T13:32:11.298-07:00HD 219134 Scorecard: 5 planets, 2 transiting<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-VSXkhny8nYg/WPcJjsMgTAI/AAAAAAAAA8E/TL1Ni6rpCksNNNM_fSiiGepTXui2RFfOACEw/s1600/HD%2B219134%2Bsystem%2Barchitecture%2B-%2BGillon%2B2017.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="584" src="https://3.bp.blogspot.com/-VSXkhny8nYg/WPcJjsMgTAI/AAAAAAAAA8E/TL1Ni6rpCksNNNM_fSiiGepTXui2RFfOACEw/s640/HD%2B219134%2Bsystem%2Barchitecture%2B-%2BGillon%2B2017.gif" width="640" /></a></div>
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Figure 1.</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;"> Architecture of the mixed-mass planetary system around
HD 219134, a nearby K dwarf, as characterized by Gillon et al. 2017 and Johnson
et al. 2016. All five planets are shown at their approximate relative sizes. Planets
<b>b</b> and <b>c</b> are observed in transit, so their radii are known. The radii of
the other planets are based on those of planets with similar masses and measured
radii (e.g., planets <b>d</b> and <b>e</b> are assigned the same radii as Neptune
and Saturn, respectively). See <b>Table 1</b>.</span><span style="font-family: "verdana" , sans-serif; font-size: 10pt;"><o:p></o:p></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , serif;">Fresh from their discovery of </span><a href="http://backalleyastronomy.blogspot.com/2017/03/"><span style="font-family: "georgia" , "serif";">four new Earth-size planets</span></a><span style="font-family: "georgia" , serif;"> transiting
TRAPPIST-1 (a minuscule M dwarf in the Sun’s back yard), Michaël Gillon &
colleagues have contributed exciting new data to our developing picture of
another nearby exoplanetary system: HD 219134. The host star is a metal-rich K3
dwarf located only 6.55 parsecs away (21 light years) in the direction of
Cassiopeia. Gillon & colleagues confirm a previous report by Motalebi &
colleagues (blogged</span><span style="font-family: "georgia" , "serif";"> </span><a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html"><span style="font-family: "georgia" , "serif";">here</span></a><span style="font-family: "georgia" , "serif";">) that the
innermost planet (<b>b</b>) is visible in transit,
while announcing that the second planet (<b>c</b>)
also transits. In addition, they present radial velocity masses for both transiting
planets (hereafter <b>tranets</b>), as well
as a refined radius for planet <b>b</b>.
For both objects, these values are consistent with a purely rocky composition. <o:p></o:p></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , serif;">After the 2015 announcement by Motalebi & colleagues
(hereafter M15), two subsequent studies offered additional data and analyses of
the HD 219134 system: Vogt & colleagues (hereafter V15, blogged </span><a href="http://backalleyastronomy.blogspot.com/2015/10/a-different-picture-of-hd-219134.html"><span style="font-family: "georgia" , "serif";">here</span></a><span style="font-family: "georgia" , serif;">) and Johnson &
colleagues (hereafter J16, blogged </span><a href="http://backalleyastronomy.blogspot.com/2016/04/hd-219134-take-three.html"><span style="font-family: "georgia" , "serif";">here</span></a><span style="font-family: "georgia" , serif;">). Many findings from these
studies overlap, but others are mutually inconsistent, including results on the
precise number of planets and their approximate masses and periods.
Nonetheless, all three studies agreed that HD 219134 hosts two or more low-mass
planets on hot and warm orbits, plus one gas giant less massive than Saturn outside
the system ice line. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">In their new study, Gillon & colleagues confine their
attention to the inner system, where they identify a total of four low-mass
planets. This result contrasts with the three small planets proposed by M15 and
the five preferred by V15. Along with their transit findings, Gillon’s group also
report the latest radial velocity data on all four planets from continuing observations
with the HARPS-N spectrograph. In passing, they affirm the presence of the
giant planet but do not comment on the conflicting results of the earlier
studies. <b>Table 1</b> summarizes their
findings on planets <b>b</b> through <b>f</b>, supplemented by the findings of J16
on planet <b>e</b> (which J16 called planet
<b>h</b>, even though no one has proposed
as many as seven planets – i.e., <b>b</b>
through <b>h</b> – for this system).</span></span></div>
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<span style="font-size: large;"><b><span style="font-family: "georgia" , serif;">Table 1.</span></b><span style="font-family: "georgia" , serif;"> Revised
system parameters of HD 219134</span></span></div>
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<a href="https://4.bp.blogspot.com/-4Exh7nPiSh8/WPcJ5-5E0ZI/AAAAAAAAA8I/Z2hetvEnPsgc3e0lcGpvJ36cLERjG000ACLcB/s1600/HD%2B219134%2Bsystem%2Bparameters%2B-%2BMarch%2B2017.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="150" src="https://4.bp.blogspot.com/-4Exh7nPiSh8/WPcJ5-5E0ZI/AAAAAAAAA8I/Z2hetvEnPsgc3e0lcGpvJ36cLERjG000ACLcB/s640/HD%2B219134%2Bsystem%2Bparameters%2B-%2BMarch%2B2017.gif" width="640" /></a></div>
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<i><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Tags:</span></i><span style="font-family: "arial" , sans-serif; font-size: 10pt;"> Period = orbital period
in days; a = semimajor axis in astronomical units (AU); Radius = radius in
Earth units; Mass = mass in Earth units; Teq = equilibrium temperature in
Kelvin. Data on planet <b>e</b> are based
on those for planet <b>h</b> in Johnson et
al. 2016. All other data are based on Gillon et al. 2017.<o:p></o:p></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">conflicts
resolved?<o:p></o:p></span></span></b></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">We’ve now seen four successive studies of HD 219134
conducted by three different scientific teams. (I regard Gillon et al. 2017 as
the same team as M15, since the two author groups overlap substantially and
both report HARPS-N data.) Our picture of planet <b>b</b> is largely unchanged from the results of M15 and V15, except that
we have a firmer understanding of its radius. With the new transit data and
improved radial velocity data on planet <b>c</b>,
we can also be confident of that planet’s mass, radius, and density, with no
change in its estimated period. Even planet <b>d</b> retains virtually the same period reported by M15 and V15, although
its mass falls between the values they provided. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Although planet <b>f</b>
was not included in the analysis of M15, and a potential planet with a similar
periodicity was rejected by J16, the object reported by Gillon & colleagues
looks a lot like the planet <b>f</b>
proposed by V15: its period is virtually the same, while its mass is a bit
lower – a minimum of 7.3 Earth masses (7.3 Mea) instead of 8.9 Mea. Notably,
the newly published period of planet <b>f</b>
appears to place it just outside a 2:1 mean motion resonance with planet <b>d</b>. Similar period ratios have appeared
in several compact low-mass systems discovered by the Kepler Telescope. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Consistent with M15 and J16, Gillon & colleagues
implicitly reject V15’s proposed planet <b>g</b>
at 94 days. They remain silent on the parameters of the system’s gas giant, to
which J16 assign a period of almost 6 years and a semimajor axis of about 3 AU.
<o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">mass
distribution<o:p></o:p></span></span></b></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The new data suggest that HD 219134 harbors a substantial
mass in refractory elements within a space much smaller than the region bounded
by Mercury’s orbit in our system (see <b>Table
1</b>). Gillon & colleagues argue that the two innermost planets, <b>b</b> and <b>c</b>, have minimal volatile constituents, with radial velocity data
indicating a combined refractory mass of about 9 Mea for this pair. Although we
remain ignorant of the radii of the next two planets, <b>f</b> and <b>d</b>, most exoplanets
with comparable masses (in the approximate range of 5 to 20 Mea) and comparable
thermal environments (cooler than Venus) support substantial envelopes of
hydrogen and helium. Existing studies suggest that the bulk mass composition of
such planets is generally between 1% and 30% hydrogen/helium (see Lopez &
Fortney 2014). Therefore, given their combined radial velocity masses of 23.5
Mea, we can estimate an aggregate mass of 16-22 Mea in refractory elements for
this cooler, fatter pair, with hydrogen and other volatiles accounting for the
remaining 1-7 Mea. That brings the total refractory mass of the four inner planets
into the range of 25-31 Mea. <o:p></o:p></span></span></div>
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<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Altogether, the radial velocity data indicate a total
mass of about 33 Mea for the inner system of HD 219134, placing it in roughly
the same ballpark as the aggregate mass of the six-planet systems around
Kepler-11 (about 30 Mea) and Kepler-20 (about 55 Mea). All these endowments are
much richer than the aggregate mass of the four terrestrial planets in our
Solar System, which collectively sum to 1.98 Mea. <o:p></o:p></span></span></div>
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<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">stingy
spitzer scheduling<o:p></o:p></span></span></b></div>
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<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The late, lamented Kepler Mission demonstrated the need
to collect continuous photometric data over a period of years in order to
characterize the inner reaches of any planetary system. The longer the sequence
of light curves, the more robust the resulting analysis of potential transits. In
light of that history, I was surprised to learn that Gillon’s group was able to
observe only two transits each for planets <b>b</b>
and <b>c</b>. Their observing sessions with
the Spitzer Space Telescope were confined to periods of 6.5 to 7.5 hours
centered on the transit window predicted for each planet. As a result, we have
no idea whether any of the other planets orbiting HD 219134 can be observed in
transit. Definitive findings one way or the other would dramatically improve
our understanding of this system’s distinctive architecture and constrain the
degree of coplanarity among the orbits of the inner planets <o:p></o:p></span></span></div>
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<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Notably, Spitzer had to monitor TRAPPIST-1 for 20 consecutive
days in order to untangle the orbits of its seven known planets, and even that
allotment was too brief to obtain a clear picture of the outermost planet,
which has a period just shy of 19 days. To illuminate the inner system of HD
219134 would require an observing run in excess of 200 days – evidently an
impossibility at present. What we humans need is a whole array of space-based
observatories staring year after year at all the most interesting stars in our
neighborhood. <o:p></o:p></span></span></div>
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<br /></div>
<div class="MsoNormal">
<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">architecture
and habitability<o:p></o:p></span></span></b></div>
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<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , serif;">As I discussed in an </span><a href="http://backalleyastronomy.blogspot.com/2016/04/hd-219134-take-three.html"><span style="font-family: "georgia" , "serif";">earlier post</span></a><span style="font-family: "georgia" , serif;"> on HD
219134, this system can be characterized as a “rich mixed-mass system,” defined
as a planetary system with at least two low-mass planets plus at least one gas
giant on a wider orbit. Including our Solar System, about a dozen such configurations
are known. They share important similarities. In most of them – including HD
219134 – two or more low-mass planets are observed in transit, while the gas
giant tends to be the outermost of the known planets. (The exceptions to the
latter generalization are Kepler-87, Kepler-89, and the Solar System, each of
which includes a gas giant with a low-mass planet on an exterior orbit.) <o:p></o:p></span></span></div>
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<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">In more than half of the known systems –Kepler-48,
Kepler-68, Kepler-87, Kepler-167, HD 219134, HD 10180, and our Solar System –
we note a gap between the inner system of low-mass planets and the outer gas
giant. Except for our own system, we can’t be sure whether any of these apparent
gaps is truly empty. Some might be occupied by one or more planets that are
misaligned with the others, and thus not visible in transit, or hiding a planet
or planets that are too lightweight for radial velocity observations to detect.
<o:p></o:p></span></span></div>
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<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , serif;">In any case, such gaps are significant, because in HD
219134 and several other systems, they correspond to the classical habitable
zone. That’s why I’d like to see lots of follow-up observations of HD 219134,
accompanied by analyses of the orbital stability of potential Earth-mass
planets occupying the space between 0.5 and 1 AU. Three such analyses have
already been conducted for the habitable zone of HD 10180, with conflicting
results (as blogged </span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="font-family: "georgia" , "serif";">here</span></a><span style="font-family: "georgia" , serif;">).<o:p></o:p></span></span></div>
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<br /></div>
<div class="MsoNormal">
<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">beware
of outsiders<o:p></o:p></span></span></b></div>
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<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Equally significant is the mechanism responsible for
creating and maintaining these orbital gaps. The difficulty of identifying such
a mechanism is underscored by the case of our Solar System, where we still have
no widely endorsed scenario to explain why mass is severely depleted between
the orbits of Earth and Jupiter, and altogether absent between the orbits of
Mars and the Main Belt asteroids. Jupiter is probably involved, but the details
are elusive (see Batygin & Laughlin 2015, Raymond et al. 2016). <o:p></o:p></span></span></div>
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<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The question of orbital gaps and missing mass in this
subset of system architectures is subsumed by a more general concern regarding the
role of longer-period gas giants in systems with multiple low-mass planets on
hot or warm orbits. Several recent studies have addressed the stability of
compact multiplanet systems in the presence of an outer giant, whether seen or
unseen (Hands & Alexander 2016, Hansen 2017, Becker & Adams 2017, Huang
et al. 2017, Jontof-Hutter et al. 2017, Read et al. 2017). The results suggest
that “friendly” giants, meaning those permitting the survival of several small,
closely spaced planets, need to be cool in more ways than one. Not only must
they follow orbits well-separated from the inner planets, in regions where
insolation and equilibrium temperatures are lower; they must also be
dynamically cool, with minimal eccentricities (deviations from circularity) and
inclinations (deviations from coplanarity). <o:p></o:p></span></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The most delicately balanced configuration – and thus the
one most easily upset – contains several planets continually observable in
transit. This balance can be maintained only a) if no gas giant is present or b)
if the giant is either precisely aligned with the inner ensemble or very widely
separated from it. Higher values of eccentricity, inclination, and mass, as
well as lower values of semimajor axis, will lead to perturbations of the inner
planets. In a perturbed regime, their inclinations might oscillate, such that transits
periodically cease for certain planets and then resume after an interval, or
become permanently misaligned, such that only one planet, or none at all, is observed
in transit. In cases of extreme excitation of inclinations and eccentricities,
some or all of the inner planets would be lost altogether. <o:p></o:p></span></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , serif;">For example, Becker & Adams found that, among 18
Kepler systems with at least 4 transiting planets each, potential gas giant
companions would have to maintain semimajor axes of 10 AU or more to avoid
perturbing the inner planets. They also made more specific predictions for several
interesting systems featured in previous blog posts: </span><a href="http://backalleyastronomy.blogspot.com/2015/08/the-blazing-wasp-47s.html"><span style="font-family: "georgia" , "serif";">WASP-47</span></a><span style="font-family: "georgia" , serif;">, </span><a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html"><span style="font-family: "georgia" , "serif";">Kepler-11</span></a><span style="font-family: "georgia" , serif;">, </span><a href="http://backalleyastronomy.blogspot.com/2013/04/holy-grail-earthman.html"><span style="font-family: "georgia" , "serif";">Kepler-62</span></a><span style="font-family: "georgia" , serif;">, </span><a href="http://backalleyastronomy.blogspot.com/2014/02/a-new-design-for-planetary-systems.html"><span style="font-family: "georgia" , "serif";">Kepler-90</span></a><span style="font-family: "georgia" , serif;">, and </span><a href="http://backalleyastronomy.blogspot.com/2016_09_01_archive.html"><span style="font-family: "georgia" , "serif";">Kepler-20</span></a><span style="font-family: "georgia" , serif;">. <o:p></o:p></span></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The inner system of WASP-47 consists of a gas giant
flanked by two low-mass planets inside a semimajor axis of 0.10 AU; the outer
system contains another gas giant at 1.36 AU. Becker & Adams concluded that
the orbit of the outer giant must be approximately coplanar with those of the
inner tranets, or else they would become mutually misaligned and no longer be observable
in transit. For Kepler-11, Kepler-62, and Kepler-20, their conclusions were
even more restrictive: none of these systems could harbor an additional planet
of 30 Mea or more between 1 and 30 AU without upsetting the clockwork orbits of
the inner tranets. (Notably, Jontof-Hutter & colleagues reached a less
restrictive conclusion for Kepler-11, ruling out any slightly inclined
Jupiter-mass planets within 3 AU.) For Kepler-20, a total of six low-mass
planets are known, but only five are observed in transit, given the misalignment
of planet <b>g</b>. Becker & Adams
propose that an undetected gas giant on a cool orbit might be responsible for
this configuration. <o:p></o:p></span></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">In an analogous study, Read & colleagues investigated
the stability of systems containing short-period tranets and non-transiting
giants at larger semimajor axes, including two rich mixed-mass systems,
Kepler-48 and Kepler-68. For Kepler-48, they concluded that the outer giant
must be closely aligned with the inner system, whereas for Kepler-68, available
data provided no strong constraints on inclination. <o:p></o:p></span></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">The upshot of this group of studies is that compact inner
systems of low-mass planets can typically tolerate cool giants only when the
latter are well separated and well aligned. Accordingly, Brad Hansen included an
evocative short title for his recent article on the stability problem: <i>Beware of Outsiders</i>. Once we have more
data on HD 219134, comparable analyses should be able to constrain the
alignment of planet <b>e</b>. <o:p></o:p></span></span></div>
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<br /></div>
<div class="MsoNormal" style="page-break-after: avoid;">
<b><span style="font-family: "georgia" , serif;"><span style="font-size: large;">tranets in near space<o:p></o:p></span></span></b></div>
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<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">HD 219134 now warrants a throng of superlatives: it’s the
nearest star with a tranet of any description, the nearest Sun-like star with a
tranet, the nearest with more than one tranet, the nearest with a terrestrial
tranet, and the nearest with more than one terrestrial tranet. In order of
increasing distance from the Sun, its closest rivals are <b>Gliese 436</b> at 10.2 parsecs (one hot tranet more massive than
Neptune), <b>Gliese 1132</b> at 12 parsecs
(one hot terrestrial tranet), <b>TRAPPIST 1</b>
at 12.1 parsecs (seven terrestrial tranets), <b>55 Cancri</b> at 12.3 parsecs (one very hot, massive terrestrial
tranet), <strong>LHS 1140</strong> at 12.47 parsecs (one temperate terrestrial tranet), <b>Gliese 1214</b> at 13 parsecs
(one hot, puffy tranet about half the mass of Uranus), <b>HD 189733</b> at 19.2 parsecs (one transiting Hot Jupiter), <b>HD 97658</b> at 21 parsecs (one hot tranet
more massive than Neptune), <b>Gliese 3470</b>
at 29 parsecs (one hot Uranus-mass tranet), <b>HAT-P-11</b> at 38 parsecs (one hot tranet more massive than Neptune),
and <b>Kepler-42</b> at 39 parsecs (three warm
terrestrial tranets in the nearest Kepler system). All the more distant tranets
are either Hot Jupiters or low-mass planets discovered by space-based
telescopes (Kepler and CoRoT). <o:p></o:p></span></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Among the 12 transiting systems located within 40 parsecs
(130 light years), only one includes a transiting gas giant on a hot orbit (HD
189733), a reminder that the well-known species of Hot Jupiters is actually quite
rare. More than half of these nearby transiting systems center on M dwarfs, while
only two systems with rocky tranets orbit Sun-like stars (HD 219134 and 55
Cancri). Coincidentally or not, these two are also the only mixed-mass systems
in the group. <o:p></o:p></span></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-family: "georgia" , serif;"><span style="font-size: large;">Given so many superlatives and distinctions, it’s safe to
predict that we’ll be hearing more news from HD 219134 for years to come. <o:p></o:p></span></span></div>
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<div align="center" class="MsoNormal" style="text-align: center;">
<a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><span style="font-size: large;"><i><span style="font-family: "georgia" , "serif";">Click
here for a permanent index by topic of blog posts</span></i><i><span style="color: blue; font-family: "georgia" , "serif";"><br />
</span></i><i><span style="font-family: "georgia" , "serif";">on
Back Alley Astronomy</span></i></span></a><span style="font-family: "georgia" , "serif";">
<o:p></o:p></span></div>
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<div class="MsoNormal" style="page-break-after: avoid;">
<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">REFERENCES<o:p></o:p></span></b></div>
<div class="MsoNormal">
<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Batygin K, Laughlin G.</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Becker JC, Adams FC.</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;"> (2017) Effects of unseen additional planetary perturbers on compact
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Hands TO,
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Hansen B.</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;"> (2017) </span><span style="background: white; font-family: "arial" , sans-serif; font-size: 10pt;">Perturbation of compact planetary
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Huang CX</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">, Petrovich C,
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Jontof-Hutter D</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">, Weaver BP, Ford EB, Lissauer JJ, Fabrycky DC. (2017) Outer
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Motalebi F,</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;"> Udry S, Gillon M, Lovis C, Ségransan D, Buchhave LA,
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Raymond SN</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">, Izidoro A, Bitsch B,
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<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Read MJ</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">, Wyatt MC,
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<div class="MsoNormal">
<b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">Vogt SS</span></b><span style="font-family: "arial" , sans-serif; font-size: 10pt;">, Burt J, Meschiari S, Butler RP, Henry GW, Wang S, Holden B, Gapp C,
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-61254976024846540202017-03-18T00:51:00.000-07:002017-04-18T23:48:21.265-07:00TRAPPIST-1 and the Seven Dwarfs<br />
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<a href="https://3.bp.blogspot.com/-Y4H6jBpNe9A/WMzlkCB0iuI/AAAAAAAAA7Y/giLgqr8-J-gWFtwatEnIk1JH18GUlj5FQCLcB/s1600/TRAPPIST-1%2BSystem%2BArchitecture.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="574" src="https://3.bp.blogspot.com/-Y4H6jBpNe9A/WMzlkCB0iuI/AAAAAAAAA7Y/giLgqr8-J-gWFtwatEnIk1JH18GUlj5FQCLcB/s640/TRAPPIST-1%2BSystem%2BArchitecture.jpg" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> The
seven planets of TRAPPIST-1 are shown at their relative sizes, with colors
corresponding to the densities estimated by Gillon et al. 2017. The area shaded
in yellow represents the system’s habitable zone. Planet <b style="mso-bidi-font-weight: normal;">f</b> is the most similar to Earth in radius, but not in mass or
density (see <b style="mso-bidi-font-weight: normal;">Table 1</b>). The density
of planet <b style="mso-bidi-font-weight: normal;">h</b> has not been estimated
yet. </span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The TRAPPIST-1 system has been collecting superlatives
since it was first announced just 10 months ago. With a mass only 8% of our
Sun, the system’s ultra-cool M dwarf host is the smallest and least massive star
ever detected at the head of a family of <i style="mso-bidi-font-style: normal;">bona
fide</i> planets. Michaël Gillon & colleagues initially reported three
Earth-sized objects (<b style="mso-bidi-font-weight: normal;">b, c, d</b>) transiting
this rosy little orb in May of 2016. All have periods shorter than 10 days. With
their discovery, TRAPPIST-1 became the nearest star known to host multiple
transiting planets (hereafter <b style="mso-bidi-font-weight: normal;">tranets</b>),
as the system is located only 39 light years (12 parsecs) away in the </span></span><a href="http://backalleyastronomy.blogspot.com/2016/03/the-nearest-20-parsecs.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-size: large;">Sun’s back yard</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">. In
addition, these objects constitute the first multi-tranet system ever detected
by a ground-based telescope – the TRAPPIST instrument in La Silla, Chile. All
the other multi-tranet systems known at the time of the discovery were
originally detected by the Kepler Telescope, a space-based observatory on an
independent orbit around our Sun.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Gillon & colleagues determined that these three tranets
did not have hydrogen atmospheres, given their small radii. On theoretical
grounds, the astronomers also argued that they were more likely to include a
significant icy component than to be purely rocky objects, given our current
understanding of the </span></span><a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-size: large;">protoplanetary disks</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">
surrounding very low-mass stars. The discovery team concluded by expressing
their hopes for more precise characterization of the system parameters with
more powerful telescopes.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Those hopes have now been partially fulfilled, and the
view is astounding. Three weeks ago, Gillon’s team made global headlines that
momentarily eclipsed the nonstop buffoonery of the American president. In
brief: four additional tranets have been confirmed around TRAPPIST-1, bringing
the total to seven. The new discoveries were made on the basis of an extensive
observational campaign using the Spitzer Space Telescope (Gillon et al. 2017). Then,
just this week, members of the same scientific team reported follow-up data
obtained by the K2 program, which uses the Kepler Telescope (Luger et al.
2017). The new findings further constrain the system parameters.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Analyses of the data provided by all telescopes established
that at least six of the seven planets experience transit timing variations.
These data enabled an estimate of the masses and densities of all but planet <b style="mso-bidi-font-weight: normal;">h</b>, the outermost. Not only do the six inner
tranets resemble Earth and Venus in size and mass – three of them (<b style="mso-bidi-font-weight: normal;">e, f, g</b>) occupy the system habitable
zone (see <b style="mso-bidi-font-weight: normal;">Figure 1</b>).</span></span></div>
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</div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">As a colorful header in </span></span><a href="https://www.thesun.co.uk/news/2944894/nasa-says-trappist-1-solar-system-could-be-teeming-with-exotic-alien-lifeforms/"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-size: large;">The
Sun</span></span></i></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"> proclaimed, “Nasa says TRAPPIST-1 solar system could be
teeming with ‘exotic’ alien life forms!” </span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><br /></span></span></div>
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<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Comparing TRAPPIST-1 to
our Solar System</span></span></div>
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<a href="https://4.bp.blogspot.com/-3S7lGoUzjJI/WMzlxK-wuOI/AAAAAAAAA7c/6O1lQ5mAk2A4TbHEtf4alpE1VH00T0UOQCLcB/s1600/TRAPPIST-1%2BSize%2BComparison%2Bwith%2BSolar%2BSystem%2B-%2Beso1706d.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://4.bp.blogspot.com/-3S7lGoUzjJI/WMzlxK-wuOI/AAAAAAAAA7c/6O1lQ5mAk2A4TbHEtf4alpE1VH00T0UOQCLcB/s640/TRAPPIST-1%2BSize%2BComparison%2Bwith%2BSolar%2BSystem%2B-%2Beso1706d.jpg" width="616" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Image Credit: European Southern Observatory<o:p></o:p></span></div>
<br />
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">three terrestrial
planets in the liquid water zone</span></span></span></b><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">If TRAPPIST-1 were a G-type star like our Sun, or a
K-type star like Epsilon Eridani or </span></span><a href="http://backalleyastronomy.blogspot.com/2016/04/hd-219134-take-three.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">HD 219134</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, this
news would rank among the greatest discoveries in the history of science. As it
is, however, the primary of this fascinating system is barely a star at all.
It’s a red dwarf of spectral type M8, even smaller and dimmer than </span></span><a href="http://backalleyastronomy.blogspot.com/2016/08/the-perils-of-proxima.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Proxima Centauri</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">. It’s
so lightweight that it barely crosses the threshold between stars, which are
the sites of </span></span><a href="https://www.youtube.com/watch?v=4MFVhqcRpN4"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">nuclear fusion</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, from
brown dwarfs, which are not.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">But the problem isn’t the diminutive size or puny
luminosity of TRAPPIST-1 (see <b style="mso-bidi-font-weight: normal;">Figure 2</b>
for perspective). It’s the evolutionary history of M dwarfs, as detailed in an </span></span><a href="http://backalleyastronomy.blogspot.com/2016/08/the-perils-of-proxima.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">earlier blog post</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">. During
the first billion years of their development, these stars are hotter and more
prone to destructive outbursts, and they emit a much higher flux of X-rays,
than during the far longer period of their maturity. Any planet that currently
orbits in the habitable zone of an M dwarf – including Proxima Centauri b and
TRAPPIST-1e, 1f, and 1g – was subject to high temperatures, high levels of
extreme ultraviolet radiation, and frequent stellar flares for hundreds of
millions of years, potentially stripping away its reservoir of water and much
of its atmosphere.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Even the writers for <i>The Sun</i> know a thing or two about M dwarfs and the problem of extrasolar
habitability. The article I cited above also engaged in a little analysis: </span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Now [Nasa’s]
top scientists are trying to work out whether these worlds are abundant with
extraterrestrial beings – or as dead as a terrestrial doorknob. Nasa boffins suggested
the planets “could harbour exotic lifeforms, thriving under skies of ruddy
twilight.”</span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">However,
they could also be barren wastelands because their parent star is a red dwarf,
a relatively cool type of sun that could wipe out early lifeforms before they
have a chance to evolve into sentient beings.</span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">“A
bumper crop of Earth-size planets huddled around an ultra-cool, red dwarf star
could be little more than chunks of rock blasted by radiation, or cloud-covered
worlds as broiling hot as Venus,” Nasa warned. (Hamill 2017)</span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">--------------------------<o:p></o:p></span></span></span></div>
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b></span></span><br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> TRAPPIST-1 system parameters</span></span></span><br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://1.bp.blogspot.com/-ANrRzSvQeKw/WMzl-xE2r0I/AAAAAAAAA7g/MsP-rgz62WMeuCES9pdL7QOt6Ul-jUfvACLcB/s1600/TRAPPIST-1%2BSystem%2BParameters.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="216" src="https://1.bp.blogspot.com/-ANrRzSvQeKw/WMzl-xE2r0I/AAAAAAAAA7g/MsP-rgz62WMeuCES9pdL7QOt6Ul-jUfvACLcB/s640/TRAPPIST-1%2BSystem%2BParameters.gif" width="640" /></a></div>
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p></o:p></span></span></span><br />
<i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Tags:</span></i><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Period = orbital period
in days; a = semimajor axis in astronomical units (AU); Mass = mass in Earth
units; Radius = radius in Earth units; Density = density in Earth units; Teq =
equilibrium temperature in Kelvin. All data derive from Gillon et al. (2017)
and Luger et al. (2017). </span><br />
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span><br />
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------------------<o:p></o:p></span></div>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">system architecture</span></span></span></b><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Now let’s get behind the headlines and put this discovery
in context (see <b style="mso-bidi-font-weight: normal;">Table 1</b>). A key
detail that was omitted by the mainstream media is the system’s unique orbital
architecture. Among the 3,593 exoplanetary systems in the current census, just
three are reported to host as many as seven planets. Those are </span></span><a href="http://backalleyastronomy.blogspot.com/2014/02/a-new-design-for-planetary-systems.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-90</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, </span></span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">HD 10180</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, and
now TRAPPIST-1, which immediately becomes the odd man out. Unlike TRAPPIST-1, the
first two systems center on Sun-like stars: Kepler-90 has a mass of 1.13 Solar
masses (Msol), HD 10180 of 1.06 Msol. These two stars also host a greater
diversity of planets than TRAPPIST-1. Each supports at least one gas giant and six
smaller planets with likely masses in the range of Super Earths, gas dwarfs,
and </span></span><a href="http://backalleyastronomy.blogspot.com/2016/07/k2-and-tweens.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">tweens</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">. By contrast, all seven
planets orbiting TRAPPIST-1 have estimated masses within 50% of Earth and
Venus, suggesting that all are probably rocky, with at most a small contribution
from volatile constituents. This is the most uniform family of planets among known
systems with at least six of them.</span></span><br />
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</div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">TRAPPIST-1 is also the most tightly packed collection of
planets discovered to date. The outermost planet of HD 10180 has a cool semimajor
axis of 3.4 AU, while the outermost planet of Kepler-90 orbits at about 1 AU (the
same as the distance of Earth from the Sun). Among the known six-planet
systems, the outermost planets of </span></span><a href="http://backalleyastronomy.blogspot.com/2016_09_01_archive.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-11</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> and </span></span><a href="http://backalleyastronomy.blogspot.com/2016_09_01_archive.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-20</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> have
semimajor axes of 0.46 AU and 0.35 AU, respectively. By contrast, the semimajor
axis of the seventh planet (<b style="mso-bidi-font-weight: normal;">h</b>) of TRAPPIST-1
is only 0.06 AU, similar to the orbit of a Hot Jupiter. In this regard, the system’s
nearest rivals are </span></span><a href="http://backalleyastronomy.blogspot.com/2016/01/a-data-driven-year.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-444</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, with
five subterrestrial planets orbiting within 0.08 AU, and Kepler-80, with five terrestrial
and gas dwarf planets within a similar radius. Both of the latter systems
center on K-type stars, demonstrating that ultra-compact architectures are not unique
to ultra-cool M dwarfs.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Because we can observe all seven planets in transit, we
know that the TRAPPIST-1 system is completely </span></span><a href="http://backalleyastronomy.blogspot.com/2012/10/mass-matters.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">“flat” or co-planar</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">: all seven
orbit in the same spatial plane. All also appear to be engaged in orbital
resonances, since the periods of each pair of adjacent planets can be expressed
as the ratio of two small integers. From planet <b style="mso-bidi-font-weight: normal;">b</b> to planet <b style="mso-bidi-font-weight: normal;">h</b>, the
ratios are 8:5 (<b style="mso-bidi-font-weight: normal;">b/c</b>), 5:3 (<b style="mso-bidi-font-weight: normal;">c/d</b>), 3:2 (<b style="mso-bidi-font-weight: normal;">d/e</b>), 3:2 (<b style="mso-bidi-font-weight: normal;">e/f</b>), 4:3 (<b style="mso-bidi-font-weight: normal;">f/g</b>), and 3:2 (<b style="mso-bidi-font-weight: normal;">g/h</b>) (Gillon et al. 2017, Luger et al. 2017). These
commensurabilities translate into relationships extending to sequences of three
and four TRAPPIST-1 planets, both adjacent and non-adjacent.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">The best-known example of an orbital architecture with at
least three objects in a resonant chain appears in Jupiter’s satellite system,
where the periods of Io, Europa, and Ganymede conform to a ratio of 4:2:1. This
relationship is known as a <b style="mso-bidi-font-weight: normal;">Laplace resonance</b>,
in honor of the first astronomer to describe it (</span></span><a href="https://fr.wikipedia.org/wiki/Pierre-Simon_de_Laplace"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Pierre-Simon de</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> </span></span><a href="https://en.wikipedia.org/wiki/Pierre-Simon_Laplace"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Laplace</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">) This and similar
instances of interlocking resonances are likely to enhance the long-term
stability of any system architecture (Gillon et al. 2017).</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Nonetheless, the unparalleled intricacy of the architecture
of TRAPPIST-1 takes this configuration to a whole new level (<b style="mso-bidi-font-weight: normal;">Figure 3</b>), with several critical
implications for our understanding of the system. First, </span></span><a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html"><span style="color: #000099;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";">in
situ</span></i><span style="font-family: "georgia" , "serif";"> formation</span></span></span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> could
not have produced such a clockwork mechanism. These seven planets must have
formed at a greater distance from the star – probably outside the current
system ice line, which lies at a radial distance of about 0.06 AU (equivalent
to the present orbit of planet <b style="mso-bidi-font-weight: normal;">h</b>). Subsequent
interactions with the </span></span><a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">primordial nebula</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> caused
them to </span></span><a href="http://backalleyastronomy.blogspot.com/2016/11/accretion-with-migration-in-radially.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">migrate</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> inward to their present locations
very early in system history.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 6pt; page-break-after: avoid;">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 3.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> A triple transit of
TRAPPIST-1</span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-jcfFJOkoKuQ/WMzmOWzFI2I/AAAAAAAAA7k/X1IAfbo1AD8EiPU5f_ttMesWBiisLoswwCLcB/s1600/Triple%2Btransit%2Bof%2BTRAPPIST-1c%252C%2Be%2B%2526%2Bf%2B-%2BGillon%2B2017%2B-%2BNature-small.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="596" src="https://3.bp.blogspot.com/-jcfFJOkoKuQ/WMzmOWzFI2I/AAAAAAAAA7k/X1IAfbo1AD8EiPU5f_ttMesWBiisLoswwCLcB/s640/Triple%2Btransit%2Bof%2BTRAPPIST-1c%252C%2Be%2B%2526%2Bf%2B-%2BGillon%2B2017%2B-%2BNature-small.jpg" width="640" /></a></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 6pt; page-break-after: avoid;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">This figure illustrates the light curve of
TRAPPIST-1 on a day when three of its planets – <b style="mso-bidi-font-weight: normal;">c</b>, <b style="mso-bidi-font-weight: normal;">e</b>, and <b style="mso-bidi-font-weight: normal;">f</b> – transited almost simultaneously.
Given the system’s multi-resonant orbital architecture, simultaneous transits
must be frequent. Image credit: Gillon et al. 2017. </span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">----------------<o:p></o:p></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Second, these tightly packed orbits give rise to powerful
gravitational interactions that cause transit timing variations (TTVs), as in
the classic case of </span></span><a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-11</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">. The TTVs
observed in TRAPPIST-1 enable estimates of the masses of the six inner planets,
and thus of their physical compositions. Outside the Solar System, comparable
data are in extremely short supply.</span></span><br />
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Third, the dynamical history of TRAPPIST-1 must have been
extremely peaceful, with none of the perturbations or scattering events that
are theorized for the Solar System and many extrasolar systems. Such instances
of interplanetary violence would have disrupted the interlocking orbital
structure we observe today.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">planetology<o:p></o:p></span></span></span></b></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Our Sun is known to host 29 naturally spheroidal objects,
comprising 8 planets, 2 dwarf planets (Ceres and Pluto), and 19 regular moons.
Only 6 of these 29 spheres consist primarily of rock, with a minimum of volatile
content: Earth’s only Moon, Jupiter’s moon Io, and the 4 inner planets from
Mercury to Mars. Earth is the densest object among these six (1.0 in the scale
used by Gillon & colleagues, or about 5.5 g/cm-3), while our Moon is the most
rarefied (just 61% of Earth’s density).</span></span><br />
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">As shown in <b style="mso-bidi-font-weight: normal;">Table
1</b> and <b style="mso-bidi-font-weight: normal;">Figure 2</b>, the six
well-characterized planets of TRAPPIST-1 are remarkably similar in size and mass
to Earth and Venus, and remarkably similar in density to the six rocky spheres of
our Solar System. Both TRAPPIST-1c and -1g are more massive than Earth by a
factor of about one-third; the others are intermediate in mass between Venus
and Mars. TRAPPIST-1c is also a bit denser than Earth (117%), possibly because
of enrichment in iron. The densities of the other five planets range from TRAPPIST-1g
at 94% of Earth (virtually identical to Venus) to TRAPPIST-1f at 61% of Earth
(virtually identical to the Moon). Therefore, it is quite possible that all six
inner planets are rocky.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">On theoretical grounds, however, Gillon & colleagues
prefer a mixed composition of rock and ice for planets <b style="mso-bidi-font-weight: normal;">b</b> through <b style="mso-bidi-font-weight: normal;">g</b>. They argue
that these objects most likely formed in the outer system, where ices would be
more abundant than refractory elements. We can get a helpful perspective on
their potential compositions by examining the spherical icy moons in our own Solar
System (three around Jupiter, seven around Saturn, five around Uranus, and one
each around Neptune and Pluto). Their interior structure includes rock and ice
in varying proportions. The three largest – Ganymede, Titan, and Callisto – are
each about one-third ice and two-thirds rock (Hussmann et al. 2015). With the
notable exception of Europa, which is only about 7% ice, the mid-size moons in
this sample tend to be at least 40% ice (Hussmann et al. 2015). Thus, we might
expect planets as large as the TRAPPIST seven to accrete about 30%-50% of their
original mass in ice. Later, after migrating into their present orbits, they
would have endured intense irradiation over geological timescales, reducing
their overall volatile content.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">More massive planets on cooler orbits would be expected
to retain more volatiles than warmer, less massive objects. Nonetheless, even
though we see some association between mass and density in the TRAPPIST family
– for example, the two most massive planets (<b style="mso-bidi-font-weight: normal;">c</b> and <b style="mso-bidi-font-weight: normal;">g</b>) are also the
densest – we also see anomalies. Planet <b style="mso-bidi-font-weight: normal;">b</b>
is hotter and less massive than planet <b style="mso-bidi-font-weight: normal;">c</b>,
but it is also much less dense. One possible explanation is that it is a purely
rocky planet with enrichment in silicates relative to iron, like Io and our
Moon.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Luger & colleagues remind us that tidal heating is
</span></span><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">yet another consequence of the interlocking resonances that
structure the TRAPPIST-1 system. It is </span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">also likely to be a factor in the thermal evolution of these seven planets. </span></span><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Luger’s group argues that planet <b>b</b> could have a tidal flux similar to that of Io </span><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">(which also participates in a multi-resonant chain), potentially
generating intense volcanism. Thus, it might be no coincidence that the density of planet <b>b</b> (</span><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">66% Earth) is also
very similar to the density of Io (64% Earth). Planets <b>c</b> through <b>e</b> experience lesser tidal effects, but still much stronger than the heat flux caused on Earth by our </span><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">homeworld’s radioactive core. These planets might also experience
volcanism comparable to Io’s. The more distant planets <b>f</b> through <b>h</b> appear to have avoided such a sizzling history.</span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">All seven planets might still support atmospheres, even
if their original envelopes were stripped by stellar irradiation at primordial
times. Various sources of outgassing, including volcanism, would readily
regenerate a lost atmosphere around a rocky planet (see discussion in Barnes et
al. 2016).</span></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><br /></span></span></span></b>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">exotic alien life-forms?</span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">So here we have a system with not one, not two, but three
Earth-size, Earth-mass planets whose blackbody equilibrium temperatures are consistent
with surface bodies of water. Unfortunately, we have no information on the
actual surface temperatures of these worlds, nor do we know whether water in
any form is present on any of them. Our knowledge of the evolutionary history of
M dwarfs predisposes us in the direction of pessimism, as illustrated by the
excerpt from </span><i style="font-family: georgia, "times new roman", serif; font-size: x-large;">The Sun</i><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> quoted above.
Nevertheless, members of the original discovery team for TRAPPIST-1 conducted
an analysis of the thermal evolution of this system, returning a more
optimistic conclusion: “Depending on their initial water contents, [the planets
of TRAPPIST-1] could have enough water to remain habitable” (Bolmont et al.
2017).</span><br />
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">However, their conclusion did not take into account the
tidal heating that is likely generated by the interlocking resonances in which
the system’s seven planets participate. This omission is perfectly
understandable, since the analysis offered by Bolmont & colleagues was
based on the three-planet model presented in the original discovery paper
(Gillon et al. 2016). That paper simply noted, “In some cases tidal heating
could trigger a runaway greenhouse state.” The more recent analysis by Luger
& colleagues (2017) presents exactly the case in which tidal heating would
produce this unhappy outcome.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">As a result, I remain agnostic about the possibility of life-bearing
environments in the TRAPPIST-1 system. I don’t reject it out of hand,
especially given the prospect of three terrestrial planets in the system
habitable zone (not to mention the potential for various factors to lift the
temperatures of planets <b style="mso-bidi-font-weight: normal;">g</b> and <b style="mso-bidi-font-weight: normal;">h</b> into habitable levels). But so many
other hostile factors are in play that it seems inappropriate to sing a cheery song
about the inevitability and ubiquity of life.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Dead or alive, these planets remain fascinating objects
of study. Because they transit so frequently and are situated so close to home,
we can look forward to more and more conclusive data on their surface
conditions over the next few years.</span></span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">celebrity septets<o:p></o:p></span></span></span></b></div>
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><br /></span></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Given the remarkable nature of this discovery, especially
its potential to create buzz in various desirable audiences, NASA invited the
general public (or at least people with Twitter accounts) to suggest names for
the seven planets of TRAPPIST-1. This was an unusual move, since the standard
practice is to use robotic catalog numbers with alphabetic suffixes, such as Kepler-62f
and HD 95872 b. In this case, the fact that seven names are involved should
inspire creativity, since so many cultural phenomena involve groups of seven:
the seven seas, the seven deadly sins, the seven classical planets, the seven
days of the week, the seven hills of Rome, the seven swans a-swimming, the
seven stars of the Pleiades, the Seven Samurai, the Seven Against Thebes, the
Seven Sisters of Academia, the seven wonders of the world, and of course the
very first septet that popped into my head:<o:p></o:p></span></span></span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">So how about a system with planets named after Tydeus,
Hippomedon, Amphiaraos, Polyneikes, and the other ill-fated warriors in
Aeschylus’ Theban septet? Alas, those choices are probably too <i style="mso-bidi-font-style: normal;">recherché</i> and hard to pronounce for typical
consumers of modern infotainment. Sloth, Greed, and Lust would present no such
problems, but might they be too racy? Could Aventine, Viminal, Caelian, Capitoline,
and so forth strike an appropriately classical note without seeming too recondite?</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">In light of the predictable objections, I’m favoring
homier alternatives – maybe Happy, Dopey, Sleepy, and the rest of the dwarf mining
crew, or maybe an even simpler schema based on the days of the week – Monday
for tranet <b style="mso-bidi-font-weight: normal;">b</b> through Sunday for
tranet <b style="mso-bidi-font-weight: normal;">h</b>, with Thursday, Friday, and
Saturday assigned to the lucky trio in the habitable zone. (As in, “We’re
spending a month on Saturday to ski the tropical glaciers, then rocketing off
to Thursday for windsailing in the twilight zone.”)</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">With so
many rich possibilities, the response to NASA’s invitation has been even more
childish and absurd than I anticipated. I clearly overestimated the
intelligence of the Twitterverse. Respondents tweeted names like Planet
McPlanetface and Moony McMoonface, plus a set of seven taken from the principal
characters in an American sitcom, <i style="mso-bidi-font-style: normal;">Friends</i>
(which I confess I’ve never seen), plus another septet based on the titles of
the first seven movies in the <i style="mso-bidi-font-style: normal;">Fast &
Furious</i> franchise (ditto).</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Alongside
many whimsical suggestions to use the names of the Seven Dwarfs, which I
heartily endorse, a few serious options emerged. One involves the names of
seven of the eleven official Trappist breweries, from Achel to Westvleteren.
That suggestion makes sense because TRAPPIST (<b style="mso-bidi-font-weight: normal;">Tra</b>nsiting <b style="mso-bidi-font-weight: normal;">P</b>lanets and <b style="mso-bidi-font-weight: normal;">P</b>lanetes<b style="mso-bidi-font-weight: normal;">i</b>mals <b style="mso-bidi-font-weight: normal;">S</b>mall <b style="mso-bidi-font-weight: normal;">T</b>elescope), the acronym that names the
telescope as well as the planetary system, was intentionally crafted to reflect
the research group’s fondness for Trappist beer. Another suggestion was to name
the planets after the seven astronauts who died in the crash of the <i style="mso-bidi-font-style: normal;">Challenger</i>. You might still be able to
check out the March 2 issue of </span></span><a href="http://www.telegraph.co.uk/news/2017/03/02/nasa-asks-public-help-name-7-new-plants-chaotic-results/"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">The
Telegraph</span></span></i></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> and the March 3 issue of </span></span><a href="http://www.dailymail.co.uk/sciencetech/article-4273038/NASA-tweet-asking-help-naming-new-planets-backfires.html"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">The
Daily Mail</span></span></i></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> to see a selection of these candidates. </span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><br /></span></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">As it
turns out, however, NASA was just kidding. It looks like we’ll have to use
those old catalog numbers after all. Far better that outcome than Planet
McPlanetface or <i style="mso-bidi-font-style: normal;">Furious 7!</i><o:p></o:p></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><i style="mso-bidi-font-style: normal;"><br /></i></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
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<span style="font-size: large;"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099;">Click
here for a permanent index by topic of blog posts</span></span></i><i style="mso-bidi-font-style: normal;"><span style="color: blue; font-family: "georgia" , "serif";"><br />
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Back Alley Astronomy</span></span></i></a><span style="font-family: "georgia" , "serif";">
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;"><span style="font-size: small;"><span style="font-family: Arial, Helvetica, sans-serif;">REFERENCES</span></span></span></b><br />
<span style="font-family: Arial, Helvetica, sans-serif;"><span style="font-family: "times" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Barnes R</span></b><span style="color: black; mso-themecolor: text1;">, Deitrick R, Luger
R, Driscoll PE, Quinn TR, Fleming DP, Guyer B, McDonald DV, Meadows VS, Arney
G, Crisp D, Domagal-Goldman SD, Lincowski A, Lustig-Yaeger J, Schwieterman E.
(2016) The habitability of Proxima Centauri b I: Evolutionary scenarios. In
press. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2016arXiv160806919B"><span style="color: #000099; font-family: "times" , "times new roman" , serif;">2016arXiv160806919B</span></a></span></div>
<span style="font-family: Arial, Helvetica, sans-serif;"><span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="font-family: "times" , "times new roman" , serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Bolmont E</span></b><span style="color: black; mso-themecolor: text1;">, Selsis F, Owen JE,
Ribas I, Raymond SN, Leconte J, Gillon M. (2017) Water loss from Earth-sized
planets in the habitable zones of ultracool dwarfs: Implications for the
planets of TRAPPIST-1. <i style="mso-bidi-font-style: normal;">Monthly Notices of
the Royal Astronomical Society</i> 464, 3728-3741. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2017MNRAS.464.3728B"><span style="color: #000099; font-family: "times" , "times new roman" , serif;">2017MNRAS.464.3728B</span></a></span><span style="color: black; font-family: "arial" , "helvetica" , sans-serif;"><o:p></o:p></span></span><br />
<span style="font-family: Arial, Helvetica, sans-serif;"><span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="font-family: "times" , "times new roman" , serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Gillon M</span></b><span style="color: black; mso-themecolor: text1;">, Jehin E, Lederer
SM, Delrez L, de Wit J, Burdanov A, Van Grootel V, Burgasser A, Triaud A,
Opitom C, Demory B-O, Sahu DK, Bardalez-Gagliuffi D, Magain P, Queloz D. (2016)
Temperate Earth-sized planets transiting a nearby ultracool dwarf star. <i style="mso-bidi-font-style: normal;">Nature</i> 533, 221-224. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2016Natur.533..221G"><span style="color: #000099; font-family: "times" , "times new roman" , serif;">2016Natur.533..221G</span></a></span><span style="color: black; font-family: "arial" , "helvetica" , sans-serif;"><o:p></o:p></span></span><br />
<span style="font-family: Arial, Helvetica, sans-serif;"><span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="font-family: "times" , "times new roman" , serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Gillon M</span></b><span style="color: black; mso-themecolor: text1;">, Triaud A, Demory
B-O, Jehin E, Agol E, Deck KM, Lederer SM, de Wit J, Burdanov A, Ingalls JG,
Bolmont E, Leconte J, Raymond SN, Selsis F, Turbet M, Barkaoui K, Burgasser A,
Burleigh MR, Carey SJ, Chaushev A, Copperwheat CM, Delrez L, Fernandes CS,
Holdsworth DL, Kotze EJ, Van Grootel V, Almleaky Y, Benkhaldoun Z, Magain P, Queloz
D. (2017) Seven temperate terrestrial planets around the nearby ultracool dwarf
star TRAPPIST-1. <i style="mso-bidi-font-style: normal;">Nature</i> 542, 456-460.
Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2017Natur.542..456G"><span style="color: #000099; font-family: "times" , "times new roman" , serif;">2017Natur.542..456G</span></a></span><span style="color: black; font-family: "arial" , "helvetica" , sans-serif;"><o:p></o:p></span></span><br />
<span style="font-family: Arial, Helvetica, sans-serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Hamill J.</span></b><span style="color: black; mso-themecolor: text1;"> “Life, But Not As We
Know It.” <i style="mso-bidi-font-style: normal;">The Sun</i>, 24 February 2017.<o:p></o:p></span></span><br />
<span style="font-family: Arial, Helvetica, sans-serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Hussmann H</span></b><span style="color: black; mso-themecolor: text1;">, Sotin C, Lunine J.
(2015) Interiors and evolution of icy satellites. In <i style="mso-bidi-font-style: normal;">Treatise on Geophysics, Volume 10: Physics of Terrestrial Planets and
Moons</i>, ed. G. Schubert. Elsevier B.V.<o:p></o:p></span></span><br />
<span style="font-family: "times" , "times new roman" , serif;"><span style="font-family: Arial, Helvetica, sans-serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Luger R</span></b></span><span style="color: black; font-size: 10pt;"><span style="font-size: small;"><span style="font-family: Arial, Helvetica, sans-serif;">, Sestovic M, Kruse
E, Grimm SL, Demory B-O, Agol E, Bolmont E, Fabrycky D, Fernandes CS, Van
Grootel V, Burgasser A, Gillon M, et al. (2017) A terrestrial-sized exoplanet
at the snow line of TRAPPIST-1. In press.</span><span style="font-family: "times" , "times new roman" , serif;"><o:p></o:p></span></span></span></span><br />
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-20026057654138842532017-01-02T20:12:00.004-08:002017-05-31T15:25:02.807-07:002016: Backyard Bonanza<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><a href="https://1.bp.blogspot.com/-bJEqQzhzmi8/WGsmtLOoBqI/AAAAAAAAA60/ITZ3xGNPIAoiAhxF3npl3smZXpXEFXNnwCLcB/s1600/Exoplanets%2Bwithin%2B20%2Bpc-2017-rednames.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="640" src="https://1.bp.blogspot.com/-bJEqQzhzmi8/WGsmtLOoBqI/AAAAAAAAA60/ITZ3xGNPIAoiAhxF3npl3smZXpXEFXNnwCLcB/s640/Exoplanets%2Bwithin%2B20%2Bpc-2017-rednames.gif" width="640" /></a></span></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span></b><br />
<span style="font-size: small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"> A snapshot
of 79 exoplanetary systems located within 20 parsecs (65 light years), arranged
by increasing distance from our Sun. Names in red mark planets added to the
census in 2016. Spectral types are indicated by the color key at lower right.
The inner circle has a radius of 5 parsecs, and each successive ring represents
an increment of 5 parsecs. Stellar icons are arranged in 2 dimensions by right
ascension, which is marked at the edge of the outer circle; declination is
ignored. Note that this diagram shows the relative distance of planetary
systems from our Sun, but not from each other.<o:p></o:p></span></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><br />
</span></span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-size: small;">----------------------<o:p></o:p></span></span></span></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
<span style="font-family: "times new roman"; font-size: small;">
</span></span></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Without doubt, the biggest exoplanetary news of 2016 was
the announcement of a small planet orbiting Proxima Centauri, a dim red star
that happens to be our Sun’s closest neighbor. Not only did this story dominate
headlines in the popular media: a recent query of the SAO/NASA </span></span><a href="http://adsabs.harvard.edu/abstract_service.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Astrophysics Data System</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> returned a total of 28
scientific articles about Proxima Centauri (including the discovery paper) either
published or in press over the past 5 months. My blog post on the detection (</span></span><a href="http://backalleyastronomy.blogspot.com/2016/08/the-perils-of-proxima.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">The Perils of Proxima</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">) was
the biggest click magnet for <i style="mso-bidi-font-style: normal;">Back Alley
Astronomy</i> in 2016.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">No other discovery, analysis, or commentary in the field
of exoplanetary astronomy rivaled the news from Proxima. Given the imbalance,
it would be a stretch to concoct a “Top Ten” list of extrasolar news for the
year just ended. So I’ll content myself with a Top Five: <b style="mso-bidi-font-weight: normal;">1)</b> the announcement of Proxima Centauri b by Guillem Anglada-Escudé
& colleagues (blogged </span></span><a href="http://backalleyastronomy.blogspot.com/2016/08/the-perils-of-proxima.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">here</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">), <b style="mso-bidi-font-weight: normal;">2)</b> the radial velocity detection of a sixth planet in the
well-known Kepler-20 system by Lars Buchhave & colleagues (blogged </span></span><a href="http://backalleyastronomy.blogspot.com/2016/09/a-new-planet-for-kepler-20.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">here</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">), <b style="mso-bidi-font-weight: normal;">3)</b> the final data dump of the Kepler Mission, which added 1,284
mostly lonely planets to the extrasolar census in one fell swoop (blogged </span></span><a href="http://backalleyastronomy.blogspot.com/2016/05/1284-new-kepler-planets-none-like-earth.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">here</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">), <b style="mso-bidi-font-weight: normal;">4)</b> the return of Rho Coronae Borealis b, one of the first
exoplanets ever announced, which was widely rejected as a <i style="mso-bidi-font-style: normal;">bona fide</i> planet a few years ago and then restored to consensual reality
by Benjamin Fulton & colleagues, and <b style="mso-bidi-font-weight: normal;">5)</b>
the discovery by Michael Endl & colleagues of a new </span></span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Jupiter analog</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, <b style="mso-bidi-font-weight: normal;">HD 95872 b</b>, which like Proxima Centauri
b is located right in the </span></span><a href="http://backalleyastronomy.blogspot.com/2016/03/the-nearest-20-parsecs.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Sun’s back yard</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> (announced
in a preprint late in 2015 but not formally published – or noticed by me –
until 2016). Since I haven’t yet written about the last two items, I’ll devote
some words to these less heralded new arrivals, and then assess the message their
discoveries might be telling us about our immediate Galactic neighborhood. <o:p></o:p></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><br /></span></span></span>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b></span></span>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">the newest,
nearest Jupiter analog<o:p></o:p></span></span></span></b><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">HD 95872 b is a gas giant planet in a more or less
circular orbit whose estimated period of 4375 days falls within 1% of the orbital
period of Jupiter. It was reported along with another cool gas giant – the
latter following a notably eccentric orbit around a somewhat more distant star,
Psi Draconis – by Endl et al. (2016).</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">The host star, HD 95872, is located just 25 light years
away (7.56 parsecs) in the constellation of Crater the Wine Bowl. By any
definition, it is a Sun-like star, included in the respectable </span></span><a href="https://en.wikipedia.org/wiki/Henry_Draper_Catalogue"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Henry Draper Catalog</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> and
assigned a spectral type of K0 V in the </span></span><a href="http://simbad.u-strasbg.fr/simbad/sim-fid"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">SIMBAD
database</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">. However, Endl’s group reports a few stellar oddities.
First, this object was omitted from the </span></span><a href="https://en.wikipedia.org/wiki/Hipparcos"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Hipparcos
Catalog</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> of nearby stars, from which most targets monitored by
radial velocity searches have been drawn. The omission means that HD 95872 did
not receive a precise distance estimate during the Hipparcos mission, a factor
noted without comment by Endl & colleagues. Second, the star’s assigned
spectral type seems rather late (i.e., reddish) for the mass of 0.95 ±0.4 Solar
(0.95 ±0.4 Msol) determined by Endl’s group. For context, Kepler-20 has a
virtually identical mass, but its spectral type is G2. Third, at an estimated
age of 10 billion years, a star near Solar mass is likely to be evolving into a
subgiant – an expectation contradicted by the classification of HD 95872 as a
main sequence star by SIMBAD as well as Endl’s group.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">HD 95872 bears a notable resemblance to 55 Cancri, another
nearby star that also happens to host a gas giant in an orbit similar to
Jupiter’s. Both the mass (0.90 ±0.015 Msol) and the age (10.2 billion years) of
55 Cancri are similar to those of HD 95872, and both stars are also
significantly enriched in metals. According to von Braun & colleagues
(2011), the metallicity of 55 Cancri is +0.31, while Endl & colleagues
report +0.41 for HD 95872. Given these similarities, it’s illuminating to remember
that the spectral classification of 55 Cancri is subject to disagreement, with
published types ranging from G8 V to K0 IV. (Note that alternative values of
3-9 billion years are available for the age of 55 Cancri; see Teske et al. 2013.)</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">As substantial research has shown, metal-rich stars like
55 Cancri and HD 95872 have a high likelihood of supporting gas giant planets. 55
Cancri hosts two: planet <b style="mso-bidi-font-weight: normal;">b</b> with an
orbital radius of 0.11 astronomical units (AU) and a minimum mass 0.83 times
Jupiter (0.83 Mjup), and planet <b style="mso-bidi-font-weight: normal;">d</b>
with an orbital radius of 5.76 AU and a minimum mass of 3.84 Mjup. 55 Cancri
also hosts two </span><a href="http://backalleyastronomy.blogspot.com/2016/07/k2-and-tweens.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">tweens</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
(objects intermediate in mass between Neptune and Saturn) in the gap between
planets <b style="mso-bidi-font-weight: normal;">b</b> and <b style="mso-bidi-font-weight: normal;">d</b>, plus a dense, highly irradiated Super Earth interior to planet <b style="mso-bidi-font-weight: normal;">b</b> with a mass of about 8 Earth units (8
Mea) and an orbital period shorter than 24 hours. These massive progeny
illustrate the superior planet-forming potential of metal-rich </span><a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">protoplanetary
disks</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Our new friend, HD 95872, has only a single reported
planet, for which Endl & colleagues provide a minimum mass of 4.6 Mjup.
This is similar to the estimate for 55 Cancri d, but well above the median for
the full census of gas giants. For orbital period and semimajor axis, Endl’s
group reports values almost identical to those of Jupiter: respectively 4375
days and 5.2 AU. In addition, they find a relatively circular orbit, although
their results for eccentricity are understandably imprecise: 0.06 ±0.04. Within
uncertainties, this value is consistent with the well-measured eccentricity of
Jupiter: 0.048.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Endl
& colleagues confidently identify both HD 95872 b and their other new
planet, Psi Draconis b, as Jupiter analogs. They define such planets as “within
a factor of a few Jupiter-masses and in orbits longer than 8 years.” As
explained in a previous post (</span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Almost
Jupiter</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">), I find this definition incomplete, since I’m more interested in
analogs of our Solar System than in cool gas giants <i style="mso-bidi-font-style: normal;">per se</i>. My definition of a Jupiter analog is <b style="mso-bidi-font-weight: normal;">1)</b> a gas giant planet (minimum mass 0.16 Mjup/50 Mea) that is <b style="mso-bidi-font-weight: normal;">2)</b> located outside the system’s liquid
water zone <b style="mso-bidi-font-weight: normal;">3)</b> with an orbit whose
eccentricity is under 0.3 and <b style="mso-bidi-font-weight: normal;">4)</b>
with a semimajor axis that permits the survival of terrestrial planets on
habitable orbits but <b style="mso-bidi-font-weight: normal;">5)</b> without any
gas giant companions on interior orbits. A Solar System analog contains a cool
giant that meets all these criteria and is centered around a Sun-like star in
the mass range of 0.65-1.30 Msol, corresponding to spectral types between mid K
and late F.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">HD 95872
nicely satisfies all these criteria and thereby assumes the status of the
nearest Solar System analog, demoting HD 154345 to the second-nearest. But Psi
Draconis, the other system reported by Endl et al., does not meet my standard. Despite
its appealingly memorable name, this system is disqualified by the significant
orbital eccentricity (0.4) of its gas giant, Psi Draconis b. It’s a
disappointing result, since the host star is very similar to our Sun in age,
mass, and spectral type, and it’s only 22 parsecs away.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">In light
of these recent discoveries, I conducted a new review of the full sample of
exoplanets detected by radial velocity measurements (the only technique that
has managed to characterize any Solar System analogs). I based the review on a
December download of the catalog of the </span></span><a href="http://exoplanet.eu/catalog/"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Extrasolar
Planets Encyclopaedia (EPE)</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, with additional revisions based on
discovery papers. The results appear in <b style="mso-bidi-font-weight: normal;">Table
1</b>. Systems are listed by increasing semimajor axis of the Jupiter analog.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Seventeen potential
analogs of our Solar System</span></span></span><br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-DnVw7HXcuzU/WGsjmjMWO5I/AAAAAAAAA6k/2NHtF-B0vlkYQpu3sbwfB1FiHHqmJZ0gwCLcB/s1600/Solar%2BSystem%2BAnalogs%2BDecember%2B2016.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="432" src="https://3.bp.blogspot.com/-DnVw7HXcuzU/WGsjmjMWO5I/AAAAAAAAA6k/2NHtF-B0vlkYQpu3sbwfB1FiHHqmJZ0gwCLcB/s640/Solar%2BSystem%2BAnalogs%2BDecember%2B2016.gif" width="640" /></a></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<i style="mso-bidi-font-style: normal;"><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;">Tags:</span></i><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;"> Type = spectral
type; Msol = star mass in Solar units; [Fe/H] = metallicity; Dist. = distance
in parsecs (rounded); Mjup = planet mass in Jupiter units; a = semimajor axis
in Earth units; e = orbital eccentricity; Period = orbital period in years. <i style="mso-bidi-font-style: normal;">Selection criteria:</i> star mass 0.65-1.30
Msol; a > 3 AU; e < 0.3, no interior giants perturbing the system
habitable zone. <i style="mso-bidi-font-style: normal;">Note:</i> HD 30177 has a
second planet, a massive “Saturn analog” on an orbit exterior to planet b, with
an uncertain orbital period. </span></div>
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none; text-align: center;">
<span style="font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="font-size: small;">------------------------------------------<o:p></o:p></span></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">This is
the third time I’ve searched the EPE sample for Solar System analogs during the
past few years (previously blogged </span><a href="http://backalleyastronomy.blogspot.com/2012/05/how-weird-is-our-solar-system.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">here</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
and </span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">here</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">),
and each time the population has grown. HD 95872 is an especially welcome
addition. Not only is it the nearest Jupiter analog – it also has the longest
orbital period of the lot, making it Jupiter’s closest rival within this subset.</span></span><br />
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">The 17
systems summarized in Table 1 bear a strong family resemblance, at least in terms
of their gross parameters. But how much of the apparent similarity might stem
from selection biases?</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span></div>
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</div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Distance:</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> All but 2 are located at
distances between 25 and 60 parsecs; no Solar System analogs have been reported
outside that space. Their scarcity inside 25 parsecs might be an accurate
reflection of the absolute rarity of such systems, but their absence beyond 60
parsecs almost certainly results from historical limits on the sensitivity of
radial velocity searches.</span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><br /></span></span></span></div>
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</div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Star mass:</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Even given my inclusion
criterion of 0.65-1.3 Msol, the host stars fall in a narrow range of masses – 0.88-1.18
Msol – and center on a narrow range of spectral types – early to mid G. All are thus
extremely similar to our Sun (type G2), while stars less massive than 0.85 Msol
are conspicuously absent from the list. Since the entire population is based on
continuing searches that began decades ago, this outcome might simply reflect
the preference of early search programs for G-type stars. On the other hand,
the non-appearance of stars in the range of 0.65-0.85 Msol might be
significant. Maybe architectures like the Solar System are restricted to more
massive stars.</span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><br /></span></span></span></div>
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</div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Metallicity:</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Eleven
out of 17 host stars have Solar metallicity or less; only 4 have [Fe/H] greater
than +0.2, which would indicate a notable enhancement in heavy elements. My own
(not very educated) guess is that this relative indifference to metallicity
might be real. In any case, the scarcity of extremely metal-rich stars in this
population supports the premise that our Sun is a typical host of Jupiter
analogs. It also warrants a reminder that even the Sun is metal-rich compared
to the average star in our Galactic neighborhood, where typical metallicities
are about -0.10. Whether or not cool giants on circular orbits are intrinsically
rare around stars impoverished in heavy elements, no extreme metallic enrichment
seems necessary to form them.</span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><br /></span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Planet mass:</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> The
median gas giant mass in this sample is approximately 2 Mjup, and only 2 of
these “Jupiter analogs” have masses smaller than 0.99 Mjup. This outcome might
be just another reflection of the limited sensitivity of the radial velocity
method, since planets of lower mass are more difficult to detect on long-period
orbits. But it might instead be a clue that Jupiter is relatively small for a
cool giant, and that most of its analogs are more massive. <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><br /></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Orbital eccentricity:</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Most
gas giants with semimajor axes of 3 AU or more have eccentricities in excess of
0.5, and many giants with smaller eccentricities have giant companions on
interior orbits that rule out Earth-like planets. Even in Table 1, which
includes only unaccompanied giants with eccentricities < 0.3, the median value
of 0.1 is relatively high by the standards of our Solar System. Of course, it’s
possible that exoplanetary eccentricities are systematically overestimated (Shen
& Turner 2008), and that the giants in <span style="font-family: "georgia" , serif;">Table </span>1 are actually less eccentric
(and therefore friendlier) than they look on the screen. But it’s also possible
that cool giants on orbits as circular as Jupiter’s are even rarer than Hot
Jupiters. If eccentricity < 0.1 turns out to be the limiting factor for
Solar System analogs, then they are much less numerous than I project here.</span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><br /></span></span></span>
<br />
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</div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">-------------------------------<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><br /></span></span></span>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">On the
other hand, if <span style="font-family: "georgia" , serif;">Table </span>1 presents a reasonable survey of systems like our own, the
outlook for Earth-like planets is rosy. The available data now show that 2 out
of 260 star systems (0.8%) identified within 10 parsecs – including almost 3%
of the 70 FGK stars in this space – host a gas giant on a cool orbit that would
permit the survival of Earth-like planets in the habitable zone. Since one of
those two star systems (our own) is known to support such a world, we’re entitled
to conjecture that the other one (HD 95872) does, too.</span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><br /></span></span></span>
<br />
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">If we
restrict our scope to stars other than our Sun with confirmed planets, rather
than all known stars, we can say that 2% (N=2) of the 79 exoplanetary systems located
within 20 parsecs qualify as Solar System analogs. Furthermore, if Jupiter
analogs are at least as common as Hot Jupiters, as various analyses suggest,
then 3 more systems resembling our own might be awaiting discovery within the
same volume of space. That prediction rests on the fact that five Hot Jupiters
are already known in this region. If we further extend this back-of-the-envelope
approach to a radius of 60 parsecs, we can say that a dozen more Solar System
analogs await detection in the immediate Solar neighborhood, since more than 30
Hot Jupiters systems have been reported in that space, but only 17 <i style="mso-bidi-font-style: normal;">bona fide</i> analogs of our system.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">counting planets in the Sun’s back yard</span></span></span></b><br />
<span style="font-family: georgia, "times new roman", serif; font-size: large;"><br /></span>
<span style="font-family: georgia, "times new roman", serif; font-size: large;">The statistics on exoplanets within any volume-limited
sample depend sensitively on how many such planets we regard as validated.
Unfortunately, different exoplanetary catalogs offer different pictures even of
the well-studied space within 20 parsecs. Accordingly, I have to approach this
problem with a consistent set of exclusions: Alpha Centauri b, Kapteyn’s b
& c, all companions identified as brown dwarfs, and all objects reported by
direct imaging or astrometry, with the exception of Beta Pictoris b.</span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">When I apply these criteria to the full census maintained
by </span></span><a href="http://exoplanet.eu/catalog/"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">EPE</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, along
with corrections for stellar parameters found in the literature, I get the 79
systems pictured in <b style="mso-bidi-font-weight: normal;">Figure 1</b>. Given
additional exclusions discussed in a series of older blog posts (starting </span><a href="http://backalleyastronomy.blogspot.com/2013/01/year-of-signal.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">here</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">,
most recently </span><a href="http://backalleyastronomy.blogspot.com/2015/10/the-ghost-in-window.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">here</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">),
these 79 systems support a total of 141 planets.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">However, if I apply the same criteria to the </span></span><a href="http://exoplanetarchive.ipac.caltech.edu/"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">NASA
Exoplanet Archive</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, I get a notably smaller sample of 65
systems supporting only 113 planets. Much of the mismatch can be explained by
NASA’s exclusion of candidates reported in an unpublished manuscript by Michel
Mayor & colleagues (2011), as well as other candidates proposed by other
investigators on the strength of Bayesian reanalyses of existing radial
velocity data (see list of exclusions </span></span><a href="http://exoplanetarchive.ipac.caltech.edu/docs/removed_targets.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">here</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">). Thus even my relatively
skeptical approach might err on the side of optimism.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">The difficulty of establishing an accurate count of the known
exoplanetary systems, let alone the very nearest, is illustrated by the
observational history of <b style="mso-bidi-font-weight: normal;">Rho Coronae
Borealis</b>. This Sun-like star, one of the first to be identified as an exoplanetary
host, is located 17.24 parsecs (56 light years) away. In 1997, Robert Noyes
& colleagues reported an object (<b style="mso-bidi-font-weight: normal;">b</b>)
with a minimum mass of 1.1 Mjup orbiting this star in a period of 39 days with
an eccentricity of 0.03. Although their radial velocity measurements could not
rule out much higher masses, they presented theoretical arguments in favor of a
gas giant planet instead of a brown dwarf star.</span></span></div>
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</div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><br /></span></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Follow-up studies summarized by Fulton et al. (2016)
presented additional astrometric data that, according to several astronomers, unmasked the so-called exoplanet as a
very dim M dwarf or massive brown dwarf. In this argument, the original mass value reported by Noyes & colleagues was a drastic underestimate, because we happen to observe the system from a face-on viewing angle. Such a geometry minimizes the Doppler shift detectable from Earth, whereas an edge-on angle maximizes it. According to this argument, the companion must be at least 100 times more massive than Jupiter. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><br /></span></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Most radial velocity detections that are not supported by
transit observation are vulnerable to a similar challenge, but the prevailing
attitude in the exoplanet community seems to be “innocent until proven guilty.”
In the case of Rho Coronae Borealis b, the skeptical argument quietly won the
day, and the proposed planet was excluded from EPE sometime around 2011. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><br /></span></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Now Fulton’s group has conducted a concentrated observing
program to test the planetary hypothesis. Their findings confirm the existence of planet b while
ruling out even the dimmest of M dwarfs. Thus Rho Coronae Borealis, jewel in
the Northern Crown, has returned to the fold of exoplanet host stars.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><br /></span></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Fulton & colleagues also report a second planet (<b style="mso-bidi-font-weight: normal;">c</b>) with an orbital period of 102 days
and a semimajor axis of 0.41 AU, similar to Mercury’s. Depending on our viewing
angle, the planet’s minimum mass of 25 Earth units (Mea) suggests either a
large low-mass planet like GJ 436 b (a “Super Neptune” or tween) or a small gas
giant. In the first alternative, Rho Coronae Borealis would present a
relatively rare and extremely interesting architectural feature: a pair of
adjacent planets in which the inner object is a gas giant and the outer is a
low-mass planet like Neptune. Apart from Rho Coronae Borealis, just five
examples of this architecture have been observed: our Solar System (Saturn +
Uranus), GJ 876 (planets <b style="mso-bidi-font-weight: normal;">b</b> + <b style="mso-bidi-font-weight: normal;">e</b>), Kepler-87 (planets <b style="mso-bidi-font-weight: normal;">b</b> + <b style="mso-bidi-font-weight: normal;">c</b>), Kepler-89 (planets <b style="mso-bidi-font-weight: normal;">d</b>
+ <b style="mso-bidi-font-weight: normal;">e</b>), and WASP-47 (planets <b style="mso-bidi-font-weight: normal;">b</b> + <b style="mso-bidi-font-weight: normal;">d</b>). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">In the second alternative, Rho Coronae Borealis might a
scaled-down version of 55 Cancri, which presents an adjacent pair in which the
inner object is a massive gas giant (planet <b style="mso-bidi-font-weight: normal;">b</b>) and the outer is a tween (planet <b style="mso-bidi-font-weight: normal;">c</b>) of about 55 Mea.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span></div>
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none; text-align: center;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">-------------------------------<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">One of
my favorite articles of 2016 was “Challenges in Planet Formation,” by
Alessandro Morbidelli & Sean Raymond, two seasoned veterans of exoplanetary
science. The authors frankly discuss everything we don’t really know about the
evolution of planetary systems, which their opening sentence describes as “a
vast, complex, and still quite mysterious subject.” The same characterization
could easily apply to our understanding of exoplanetology, given the vast,
mysterious, and constantly expanding nature of the extrasolar census. As with
planetary evolution, when it comes to the demographics of exoplanets, “even our
most successful models are built on a shaky foundation” (Morbidelli &
Raymond 2016).</span></span><br />
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</div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><br /></span></span></span>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">So
here’s looking forward to the mysterious, terrifying, and inevitably
fascinating events and revelations to come in 2017 . . . . </span></span></span></div>
<br />
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Click
here for a permanent index by topic of blog posts</span></span></i><i style="mso-bidi-font-style: normal;"><span style="color: blue; font-family: "georgia" , "serif";"><br /><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span></span></i><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">on
Back Alley Astronomy</span></span></i></a><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;"><span style="font-family: "times" , "times new roman" , serif;">REFERENCES<o:p></o:p></span></span></b></div>
<span style="font-family: "times" , "times new roman" , serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Endl M</span></b><span style="color: black; mso-themecolor: text1;">, Brugamyer EJ,
Cochran WD, MacQueen PJ, Robertson P, Meschiari S, Ramirez I, Shetrone M,
Gullikson K, Johnson MC, Wittenmyer R, Horner J, Ciardi DR, Horch E, Simon AE,
Howell SB, Everett M, Caldwell C, Castanheira BG. (2016) Two new long-period giant
planets from the McDonald Observatory Planet Search and two stars with
long-period radial velocity signals related to stellar activity cycles. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 818, 34. Abstract:
</span></span><a href="http://adsabs.harvard.edu/abs/2016ApJ...818...34E"><span style="color: #000099; font-family: "times" , "times new roman" , serif;">2016ApJ...818...34E</span></a><span style="color: black; mso-themecolor: text1;"><o:p></o:p></span><br />
<span style="font-family: "times" , "times new roman" , serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black;">Fulton BJ</span></b><span style="color: black;">,
Howard AW, Weiss LM, Sinukoff E, Petigura EA, Isaacson H, Hirsch L, Marcy GW,
Henry GW, Grunblatt SK, Huber D, von Braun K, Boyajian TS, Kane SR, Wittrock J,
<span style="color: black; mso-themecolor: text1;">Horch E, Ciardi DR, </span>Howell
SB, Wright JT, Ford EB. (2016) Three temperate Neptunes orbiting nearby stars. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 830, 46. Abstract:</span>
</span><a href="http://adsabs.harvard.edu/abs/2016ApJ...830...46F"><span style="color: #000099; font-family: "times" , "times new roman" , serif;">2016ApJ...830...46F</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p></o:p></span><br />
<span style="font-family: "times" , "times new roman" , serif;">
<span style="color: black;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Mayor M</span></b><span style="color: black; mso-themecolor: text1;">, Marmier M, Lovis C,
Udry S, Ségransan D, Pepe F, Benz W, Bertaux J-L, Bouchy F, Dumusque X, Lo
Curto G, Mordasini C, Queloz D, Santos NC. (2011) The HARPS search for southern
extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties
of super-Earths and Neptune-mass planets. Unpublished; abstract at </span></span></span><a href="https://arxiv.org/abs/1109.2497"><span style="color: #000099; font-family: "times" , "times new roman" , serif;">https://arxiv.org/abs/1109.2497</span></a><span style="color: black; mso-themecolor: text1;"><span style="font-family: "times" , "times new roman" , serif;"> <o:p></o:p></span></span><br />
<span style="font-family: "times" , "times new roman" , serif;">
<span style="color: black;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Morbidelli A, Raymond
S.</span></b><span style="color: black; mso-themecolor: text1;">
(2016) Challenges in planet formation. Invited review; in press. Abstract:</span></span> </span><a href="http://adsabs.harvard.edu/abs/2016arXiv161007202M"><span style="text-decoration: none; text-underline: none;"><span style="color: #000099; font-family: "times" , "times new roman" , serif;">2016arXiv161007202M</span></span></a><o:p></o:p><br />
<span style="font-family: "times" , "times new roman" , serif;">
<span style="color: black;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Noyes R</span></b><span style="color: black; mso-themecolor: text1;">, Jha S, Korzennik SG,
Krockenberger M, Nisenson P, Brown TM, et al. (1997) A planet orbiting the star
Rho Coronae Borealis. <i style="mso-bidi-font-style: normal;">Astrophysical
Journal</i> 483, L111-L114. <o:p></o:p></span></span></span><br />
<span style="font-family: "times" , "times new roman" , serif;"><span style="color: black;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Shen Y, Turner EL.</span></b><span style="color: black; mso-themecolor: text1;"> (2008) On the
eccentricity distribution of exoplanets from radial velocity surveys. </span><i style="mso-bidi-font-style: normal;">Astrophysical
Journal</i> <span style="color: black; mso-themecolor: text1;">685, 553-559.<o:p></o:p></span></span></span><br />
<span style="font-family: "times" , "times new roman" , serif;"><span style="color: black;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Teske JK</span></b><span style="color: black; mso-themecolor: text1;">, Cunha K, Schuler SC,
Griffith CA, Smith VV. (2013) Carbon and oxygen abundances in cool metal-rich
exoplanet hosts: A case study of the C/O ratio of 55 Cancri. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 778, 132.<o:p></o:p></span></span></span><br />
<span style="font-family: "times" , "times new roman" , serif;"><span style="color: black;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">von Braun K</span></b><span style="color: black; mso-themecolor: text1;">, </span><span style="font-size: 10pt;"><span style="font-size: small;">Boyajian
TS, ten Brummelaar TA, Kane SR, van Belle GT, <span style="color: black; mso-themecolor: text1;">Ciardi DR,</span> Raymond SN, Lopez-Morales M, McAlister
HA, Schaefer G, Ridgway ST, Sturmann L, Sturmann J, White R, Turner NH,
Farrington C, Goldfinger PJ. (2011) 55 Cancri: Stellar astrophysical parameters,
a planet in the habitable zone, and implications for the radius of a transiting
Super-Earth. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i>
740, 49.<span style="color: black; mso-themecolor: text1;"><o:p></o:p></span></span></span></span></span><br />
<br />
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-42602512494120155772016-11-27T22:35:00.000-08:002016-12-08T16:57:52.454-08:00Accretion with Migration in Radially Structured Disks<br />
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<a href="https://1.bp.blogspot.com/-QlSCHbFWb0E/WDvNEP25n9I/AAAAAAAAA5w/6JiTQ5eNli05y-C4YBXNZfqdb1LfFiP5wCLcB/s1600/TW%2BHydrae%2B-%2BSAndrews-ALMA--ESO-NAOJ-NRAO--rota.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="468" src="https://1.bp.blogspot.com/-QlSCHbFWb0E/WDvNEP25n9I/AAAAAAAAA5w/6JiTQ5eNli05y-C4YBXNZfqdb1LfFiP5wCLcB/s640/TW%2BHydrae%2B-%2BSAndrews-ALMA--ESO-NAOJ-NRAO--rota.jpg" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> The
innermost regions of the protoplanetary disk surrounding TW Hydrae are unveiled
in this composite of images captured by the </span><a href="http://www.almaobservatory.org/en/home"><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="color: #000099;">ALMA instrument</span></span></a><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">. TW Hydrae is a newborn Sun-like star located 176 light
years away. Sean Andrews & al. (2016) propose that the gap and ring
structure visible in the inset image has been carved by a rocky planet
accreting at a circumstellar radius of about 1 astronomical unit (1 AU),
equivalent to the distance of Earth from the Sun. However, Barbara Ercolano &
al. (2016) argue that the dust-free gap results from photoevaporation of the
inner disk by X-ray flux from the central star. Both studies suggest that the
gaps in the outer disk represent the condensation fronts of various chemical
species, such as carbon monoxide and molecular nitrogen. Current theory
identifies these structures as potential <b style="mso-bidi-font-weight: normal;">planet
traps</b>.</span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------<o:p></o:p></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Despite recent interest in the theory of <i style="mso-bidi-font-style: normal;"><a href="http://backalleyastronomy.blogspot.com/2016/10/where-do-baby-planets-come-from.html"><span style="color: #000099;">in
situ<span style="font-style: normal;"> assembly</span></span></a></i>, most astronomers
over the past two decades have relied on models involving planet migration to
explain the origin of planetary systems. These models are directly informed by
our understanding of the primordial nebulae that surround newborn stars. The </span><a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">previous
installment</span></a><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> of this three-part series offered a basic picture of <b style="mso-bidi-font-weight: normal;">protoplanetary disks</b> (the preferred
term for planet-forming nebulae) and reviewed the pros and cons of <i style="mso-bidi-font-style: normal;">in situ</i> formation. This installment
explores the recent explosion of theoretical studies that invoke gas-driven
migration in radially structured protoplanetary disks as the principal mechanism
underlying planet formation and system architecture. <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Intensive discussion of radially structured
disks dates back at least 10 years, to the 2006 publication of a study led by
Frederic Masset: “Disk surface density transitions as protoplanet traps.” Since
then, the term <b style="mso-bidi-font-weight: normal;">planet trap</b> has
appeared regularly in articles on system evolution, with a currency even wider
than the extended circle of researchers who collaborate with Masset. Such traps
are a key example of the structures invoked in this emerging field.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">A major shortcoming of existing migration models
provided the jumping-off point for the current approach: growing recognition that
the combined effects of aerodynamic drag and gas-driven migration, as modeled
in theory, would likely sweep all solid particles in the nebula into the
central star before they had time to form <b style="mso-bidi-font-weight: normal;">protoplanets</b>.
Without those building blocks, planetary systems could never evolve.</span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Yet obviously they do. Accordingly, theorists
have searched for some mechanism that could stall migration and enable solid
mass to accumulate at various radial locations. These efforts have coincided
with increasingly precise measurement and imaging of nearby protoplanetary
disks, which are beginning to offer evidence of disk structures that are
consistent with planet traps (<b style="mso-bidi-font-weight: normal;">Figure 1</b>).
<o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
</span></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">So this posting will look at 1) the
basic elements of migration theory, 2) a sample of the disk structures proposed
by current scenarios, 3) <b style="mso-bidi-font-weight: normal;">condensation
fronts</b> and their evolution, 4) the formation of the first planetesimals, 5)
the late stages of disk evolution, and 6) the nativity and composition of the
known exoplanets.<o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Protoplanetary disk model</span></span></span><br />
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<a href="https://4.bp.blogspot.com/-MjbtEXRCA4E/WDvNTxLgKmI/AAAAAAAAA50/PKopJJ1vwusmIyVYD1_dQHfTYzLw2TxRACLcB/s1600/Protoplanetary%2Bdisk%2B-%2Bmassive%2Bsettled%2Bdisk.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="380" src="https://4.bp.blogspot.com/-MjbtEXRCA4E/WDvNTxLgKmI/AAAAAAAAA50/PKopJJ1vwusmIyVYD1_dQHfTYzLw2TxRACLcB/s640/Protoplanetary%2Bdisk%2B-%2Bmassive%2Bsettled%2Bdisk.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Schematic view of a protoplanetary disk
surrounding a Sun-like star, seen edge-on. The large lavender shape represents the
gas nebula, which is made of hydrogen & helium in an approximate ratio of
3:1. The vertical profile of the gas flares in the outer region of the disk,
beyond 10 AU. Despite a gas-free central cavity inside 0.05 AU, gas molecules
flow continually through the disk to the star and deposit mass through the
stellar magnetic poles. Suspended throughout the disk are solid particles of
rock and frozen volatiles, the building blocks of planets. Rocky particles
dominate the hot inner disk; icy particles dominate the cold outer disk. As the
disk evolves, solids settle at its midplane, which is likely aligned with the
star’s rotational axis. Accumulation of solids favors the assembly of pebbles
and planetesimals, which collide to form protoplanets (also known as embryos or
cores).</span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------<o:p></o:p></span></div>
<br />
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">1) migration modes<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">In a protoplanetary disk, nothing stands
still. Gas molecules orbit the central star and dust grains are carried along
with the flow of hydrogen and helium (H/He) as it streams into the gravity well
created by the star’s mass (<b style="mso-bidi-font-weight: normal;">Figure 2</b>).
Current models tell us that the process of accretion is driven by turbulence
within the nebula. The calmest region of the gas disk is the midplane, where dust
settles and aggregates, encouraging collisions. Micron-sized grains are
lightweight enough to travel at the same speed as the gas, but aggregations
measurable in centimeters – that is, <b style="mso-bidi-font-weight: normal;">pebbles</b>
– experience aerodynamic drag that causes their orbits to decay. <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The problem of orbital decay gets worse as
aggregates grow. When a protoplanet approaches the mass of Mars (~0.1 Earth
masses or Mea), it becomes subject to <b style="mso-bidi-font-weight: normal;">Type
I migration</b>. Objects ranging up to and beyond the mass of Neptune are
similarly affected. Type I migration is caused by the interaction between the
flow of gases and a forming protoplanet, such that the object is simultaneously
subject to positive and negative torques. When the negative torque exceeds the
positive torque, as is usually the case, the object migrates inward. When the
reverse is true, the object migrates outward. In some models, Type I migration
can deliver an Earth-mass protoplanet originally traveling on an Earth-like
orbit to the threshold of its parent star in about 100,000 years (Baillie et
al. 2015). More massive objects travel even faster, such that on object of 10
Mea originally orbiting at 5 astronomical units (AU) can migrate to the inner
edge of the disk in less than 30,000 years (Chambers 2006).<o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 3.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Type II migration of a baby gas giant<o:p></o:p></span></span></span></div>
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<a href="https://1.bp.blogspot.com/-ih102eNu9hM/WDvNlpa-9CI/AAAAAAAAA54/att-HQayR3w_uCQxFuY8ZVEOEqMzj0NyACLcB/s1600/Type%2BII%2BMigration%2B-%2BFig%2B32%2Bof%2BArmitage%2B2010.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="640" src="https://1.bp.blogspot.com/-ih102eNu9hM/WDvNlpa-9CI/AAAAAAAAA54/att-HQayR3w_uCQxFuY8ZVEOEqMzj0NyACLcB/s640/Type%2BII%2BMigration%2B-%2BFig%2B32%2Bof%2BArmitage%2B2010.jpg" width="636" /></a></div>
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span><br />
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">The blob in the dark ring is the forming
planet, which is being carried inward by the flow of accretion as it orbits in
a dust-free lane. Spiral streams of gas link the planet to the inner and outer
disk. (Source: Figure 32 of Armitage 2010)</span><br />
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<br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Still larger objects can escape this process.
Depending on the vertical extent of the gas disk at the site of formation, a
protoplanet approaching the mass of a gas giant (e.g., 50-150 Mea) can open a
gap in the disk, meaning that it clears all the gas from its vicinity and
orbits alone in an empty lane around the star. The exact mass required for gap clearing
depends on the scale height of the disk, which changes over time. Disks are at
their thickest and most massive in extreme youth, attenuating incrementally as
they age. Only a massive object can open a gap in a thick disk, whereas a
protoplanet as lightweight as Neptune might be able to open a gap in a
sufficiently attenuated disk. Once ensconced in its gap, the planet undergoes <b style="mso-bidi-font-weight: normal;">Type II migration</b> (<b style="mso-bidi-font-weight: normal;">Figure 3</b>), traveling inward with the flow of gas at a more
leisurely rate than in the Type I regime. Nevertheless, it still migrates, so
that the orbit of a young gas giant in this mode might shrink from 5 AU to 0.1
AU in half a million years (Chambers 2006). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"></span></span></span></b><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">2) barriers and traps</span></span></span></b><br />
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b> </div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">For as long as the various modes of drag,
drift, and migration have been understood, astronomers have realized that some
opposing mechanism must be available to ensure that pebbles don’t fall into
stars before they accrete into protoplanets, and that protoplanets don’t get
stranded at the inner edges of disks before they can establish cooler orbits. In
recent years, theorists seem to be converging on a scenario in which the gas
dynamics in specific regions of a protoplanetary disk counterbalances negative
torque with positive torque, for a net of zero. In this way, solid particles can
remain stranded in one place long enough to accrete into planetesimals,
protoplanets, and planets. Such regions have been identified as the sites of “transitions”
(Masset et al. 2006), “inhomogeneities” (Hasegawa & Pudritz 2011), or “irregularities”
(Baillie et al. 2016).</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Several candidates have been proposed as the
agents of this trapping or focusing process. Collectively, they are known as
planet traps. Among the most consistent choices are condensation fronts (or
sublimation lines) in the evolving disk – locations where specific chemical
species condense (or sublime) out of the disk gases (<b style="mso-bidi-font-weight: normal;">Figures 4 & 5</b>). The most widely discussed of these fronts is
the ice line or <b style="mso-bidi-font-weight: normal;">snow line</b>: the
minimum radius at which water freezes instead of sublimating. Another is the <b style="mso-bidi-font-weight: normal;">silicate condensation line</b>, where
silicate dust vaporizes, creating an inner edge for the solid component of the
disk (Morbidelli et al. 2016). In many models, this edge casts a long shadow on
the midplane of the disk, contributing to a reduction in temperature.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Condensation fronts act as planet traps
because they create a bump in the local surface density of solids (Masset et
al. 2006). However, other candidates for this role are available. In a series
of publications, Yasuhiro Hasegawa and collaborators have detailed a total of
three potential planet traps (2010, 2011, 2014). In addition to <b style="mso-bidi-font-weight: normal;">condensation fronts</b> – to which they
apply the generic term ice line, regardless of chemical species – they include <b style="mso-bidi-font-weight: normal;">dead zones</b> and <b style="mso-bidi-font-weight: normal;">heat transitions</b>.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Many theoretical treatments of planet traps
specify that they are intended to apply only in cases of Type I migration, with
some even defining a relatively narrow range of masses for affected objects.
Others, however, discuss structures capable of capturing a broad distribution
of masses, including gas giants undergoing Type II migration. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 4.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Water condensation front around V883 Orionis<o:p></o:p></span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-YMOnmPiKY_w/WDvN3JjkCkI/AAAAAAAAA58/gH45OIce9wA2kOa2jZGRE2H_hULKnzJbwCLcB/s1600/V883%2BOrionis%2B-%2BALMA-tweaked.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="460" src="https://3.bp.blogspot.com/-YMOnmPiKY_w/WDvN3JjkCkI/AAAAAAAAA58/gH45OIce9wA2kOa2jZGRE2H_hULKnzJbwCLcB/s640/V883%2BOrionis%2B-%2BALMA-tweaked.jpg" width="640" /></a></div>
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span><br />
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">This image by the </span><a href="http://www.almaobservatory.org/en/home"><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="color: #000099;">ALMA instrument</span></span></a><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> reveals a protoplanetary disk of about 0.3 Msol with a
radius of about 125 AU surrounding the protostar </span><a href="http://www.almaobservatory.org/en/press-room/press-releases/989-alma-observes-first-protoplanetary-water-snow-line-thanks-to-stellar-outburst"><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="color: #000099;">V883 Orionis</span></span></a><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">, which masses about 1.3 Msol and resides 415 parsecs (1350
light years) away in the Orion Nebula Cluster (Cieza & al. 2016). The dark
ring corresponds to the system ice line or <b style="mso-bidi-font-weight: normal;">water
condensation front</b>, currently located at a circumstellar distance of about
42 AU. Normally the ice line would be much closer to a star of this age and
mass (in the range of 5-10 AU), but an abrupt accretion event in which the star
ingested a substantial amount of hydrogen from the disk caused an <b style="mso-bidi-font-weight: normal;">outburst</b>. The result was a temperature
spike throughout the disk that evaporated water inside a radius similar to the
orbit of Pluto in our Solar System. Image credit: ALMA (ESO/NAOJ/NRAO) / L.
Cieza. </span><br />
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">-----------------------------<o:p></o:p></span></div>
<br />
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">2a) dead zones<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">A dead zone is a region in the nebula where
turbulence and viscosity are suppressed and the inward flow of gases slows down.
This concept has been discussed for at least 20 years as the possible source of
several phenomena associated with protoplanetary disks. (I have to wonder if
its coinage was related to Stephen King’s 1979 horror novel,</span><span style="font-family: "georgia" , "serif";"> </span></span></span><a href="https://en.wikipedia.org/wiki/The_Dead_Zone_(novel)"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">The Dead Zone</span></span></i></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">.) Dead
zones have been proposed to explain outbursts like those observed in newborn
stars such as </span></span><a href="https://en.wikipedia.org/wiki/FU_Orionis_star"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">FU Orionis</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">, where
the sudden accretion of a large quantity of trapped mass stimulates a burst of stellar
radiation (Gammie 1996). They have been described as a mechanism for trapping
dust grains at specific locations in the disk, promoting the rapid collapse of
solids into planetary embryos the size of Mars (Lyra et al. 2008). They have even
been anointed as the saviors of planetary systems through their ability to halt
Type II migration, which would otherwise deliver forming gas giants to
star-hugging orbits (Matsumura et al. 2007). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">In the classic model, the magnetic field of a
newborn star induces magnetorotational instability in the protoplanetary
nebula. This instability creates turbulence, which drives the flow of H/ He
into the central star. Although the existence of magnetorotational instability
has recently been questioned (Morbidelli & Raymond 2016), theorists still
seem to agree that turbulence is real, regardless of the source. Current models
still indicate that its suppression – by whatever means – will reduce viscosity
and halt inward migration. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Some discussions appear to confine the dead
zone to the midplane of the protoplanetary disk, without emphasizing its radial
location (Martin & Livio 2012). However, Hasegawa and Pudritz (2011)
describe their dead zone as a high-density region confined to the inner disk,
where turbulence is calmed and the accumulation of dust at the midplane is
enhanced. They note that an inner dead zone can coexist with other kinds of
planet traps, including ice lines (which they also describe as opacity
transitions) and the heat transition. They even characterize ice lines as a
species of “self-regulated, localized dead zone.”<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The <b style="mso-bidi-font-weight: normal;">heat
transition</b> proposed by these authors and others refers to the radius in a
protoplanetary disk where the source of heating shifts from friction caused by
the flow of gases (viscous heating) to energy radiated by the central star
(irradiative heating). Models of disks around Sun-like stars place this transition
around 10-12 AU (Baillie et al. 2016), where the disk begins to flare. Flaring
enables the outer disk to escape the shadow of the inner dust wall, so that surface
gases heat up and ionize. At the midplane, however, icy solids still accumulate.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt; mso-layout-grid-align: none;">
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Hasegawa & Pudritz (2011) argue that the placement of
the various planet</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Roman; mso-themecolor: text1;"> traps defines the initial
orbital distribution of protoplanets. Although all traps tend to move inward
over time as the rate of gas flow through the disk declines, each type of trap moves
differently, resulting in different outcomes for its complement of protoplanets.
</span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Hasegawa & Pudritz</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Roman; mso-themecolor: text1;"> also argue
that the positioning of planet traps depends strongly on the mass of the
central star and its rate of gas accretion. Through this dependency, they
conclude, “host stars establish preferred scales [for] their planetary systems.</span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">”<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 5.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Carbon monoxide condensation front around TW
Hydrae<o:p></o:p></span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://4.bp.blogspot.com/-FCY_spT5Y84/WDvOx1PkcCI/AAAAAAAAA6A/o4uxaZ4DSYca-G_IK8mv3HRTGG7TZUtjwCLcB/s1600/TW%2BHydrae%2B-%2BCO%2Bcondensation%2B-%2BALMA-KarinOberg-HarvardU-UVirginia-yellowsun.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="640" src="https://4.bp.blogspot.com/-FCY_spT5Y84/WDvOx1PkcCI/AAAAAAAAA6A/o4uxaZ4DSYca-G_IK8mv3HRTGG7TZUtjwCLcB/s640/TW%2BHydrae%2B-%2BCO%2Bcondensation%2B-%2BALMA-KarinOberg-HarvardU-UVirginia-yellowsun.jpg" width="638" /></a></div>
<span style="font-family: "arial" , "sans-serif"; font-size: 10pt;"></span><br />
<span style="font-family: "arial" , "sans-serif"; font-size: 10pt;">A
doughnut made of carbon monoxide surrounds TW Hydrae, a young Sun-like star, as
captured in this image by the </span><a href="http://www.almaobservatory.org/en/press-room/press-releases/619-snow-in-an-infant-planetary-system"><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="color: #000099;">ALMA instrument</span></span></a><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;">. Qi & al. (2013)
estimate its inner radius at 30 AU. (Note that the yellow dot in the center of
this figure is an artificial marker of the position of the host star – it is
not present in the original image.)</span><br />
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<span style="font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------<o:p></o:p></span></div>
<br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">2b) condensation fronts<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Kevin Baillie & colleagues (2016) follow
Hasegawa & Pudritz in emphasizing the role of condensation fronts (which
they call sublimation lines) as well as the heat transition barrier. Although
their terminology foregrounds “lines,” they explain that each line actually marks
the center of a broad zone or plateau that extends over a radial distance of about
1 AU. In addition to water and silicates, they model the sublimation of
volatile and refractory organic molecules, along with troilite, an iron sulfide
mineral. These three types of molecules sublime at temperatures between those
of silicates and water, creating a series of tightly spaced traps between 0.1
and 1 AU. However, the authors find that the traps created by troilite and
volatile organics are less effective than the others, while the silicate
condensation trap is less enduring than the water trap.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Like other theorists (Pollack et al. 1996,
Masset et al. 2006, Hasegawa & Hirashita 2014, Bitsch et al. 2015), Baillie
& colleagues also foreground the effects of opacity on disk evolution. Local
opacity is determined by the size of ambient dust grains, with micron-sized
particles increasing opacity and millimeter to centimeter sizes (pebbles) reducing
it. They note that condensation fronts are associated with <b style="mso-bidi-font-weight: normal;">opacity transitions</b>, since solids accrete more readily outside these
fronts than inside them, causing an increase in opacity. Again, however,
troilite and volatile organics make a smaller contribution to opacity than do
water and silicates. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">While Baillie & colleagues do not
discount the potential of dead zones for trapping planets, they have not
explicitly integrated this type of structure in their extant models. Instead,
they focus on the behavior of migrating objects that arrive at planet traps,
paying special attention to mass. They argue that condensation fronts can trap
low-mass planets, but not gas giants, for which the heat transition is the only
effective barrier. They also find that the condensation fronts and heat
transition zone create <b style="mso-bidi-font-weight: normal;">planet deserts</b>
about 1 AU beyond the inner edge of each structure. Thus, each plateau is
bounded by an outer wasteland where no worlds can grow. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">2c) outward migration<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Another emphasis of their work is the
existence of zones of outward migration associated with the heat transition,
the silicate condensation front, and the water condensation front. These
structures can confer a positive torque on low-mass planets, adding further
complexity to an evolving system’s orbital architecture. Nevertheless, all
planet traps move inward over time with the cooling and attenuation of disk
gases. Thus, even if a forming planet is stalled in a trap, its orbit will
still shrink as the disk ages.<span style="mso-spacerun: yes;"> </span><o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Like Baillie’s group, Bitsch & colleagues
(2016) omit dead zones from their modeling to focus on opacity transitions
associated with thermal discontinuities, disk geometry, and local surface
densities of solids. Similarly, they explore the zones of outward migration
created by temperature changes. Unlike Baillie’s group, however, they find that
more massive planets are more likely to migrate outward than low-mass planets.
They are particularly interested in <b style="mso-bidi-font-weight: normal;">metallicity</b>,
which they propose as a key determinant of the temperature profile of a
protoplanetary disk. They find that higher metallicity is associated with wider
zones of outward migration in more distant regions of the disk. These factors
can help to explain the well-known correlation between stellar metallicity and
the presence of gas giant planets. (See Cossou et al. 2014 for an analogous treatment
of metallicity and migration.) <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt; page-break-after: avoid;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">2d) giants at the gate<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">One final, widely discussed barrier to
drifting dust and migrating planets is the presence of a growing gas giant,
which depletes the gas and dust from its vicinity and opens a gap in the
nebula. A migrating planet that approaches such a radial gap will either stall
or be scattered, usually into a wider orbit. Izidoro & colleagues (2015) argue
that this behavior can explain the orbital architecture of our Solar System,
where the accretion of Jupiter outside the water condensation front created a
barrier to the inward migration of Saturn, Uranus, and Neptune. That may well
be so: the emergence of one or more gas giants might effectively segment the
disk into inner and outer domains. But the authors go on to predict an
anti-correlation between systems with gas giants and those with compact collections
of warm gas dwarfs. The architectures of a half-dozen systems – HD 10180, HD
219134, Kepler-48, Kepler-90, Kepler-167, and WASP-47 – demonstrate that this
prediction cannot be universally valid (see </span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Almost
Jupiter</span></a><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Benitez-Llambay & colleagues (2015) use
the example of Jupiter to frame a different argument about the migration of
growing giants. They consider the case of a protoplanet of 3 Mea that has
already formed beyond the water condensation front. This hypothetical object
could easily stand in for baby Jupiter. Over a period of 100,000 years, it
doubles in mass by accreting a continual flux of colliding bodies. At the same
time, it responds to a negative torque that tends to shrink its orbit. But the
high temperatures produced by collisional accretion induce a <b style="mso-bidi-font-weight: normal;">heating torque</b> that “constitutes a
robust trap against inward migration.” Depending on various factors, the
heating torque can cause the forming planet to decelerate its inward trajectory,
stall completely, or migrate outward. More massive planets are more likely to
experience outward migration, with a correlation between the object’s mass and
the radial distance it travels. Like Cossou’s group, Benitez-Llambay &
colleagues find that outward migration and full-blown gas giants are also more
likely to result from disks enhanced in metals. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt; page-break-after: avoid;">
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 6.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Potential planet traps in
a young protoplanetary disk</span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://4.bp.blogspot.com/-f2b4y8Cj4us/WDvPK1dkH5I/AAAAAAAAA6E/N_CBF2CZQTI8KL8uje0Pc3XraS7fF7egQCLcB/s1600/Potential%2BPlanet%2BTraps.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="284" src="https://4.bp.blogspot.com/-f2b4y8Cj4us/WDvPK1dkH5I/AAAAAAAAA6E/N_CBF2CZQTI8KL8uje0Pc3XraS7fF7egQCLcB/s640/Potential%2BPlanet%2BTraps.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 6pt; page-break-after: avoid;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Several sources of structure have been
proposed for evolving disks. A series of <b style="mso-bidi-font-weight: normal;">condensation
fronts</b> (or snow lines or sublimation zones) arises in regions where local
temperatures cause common chemical species to condense. These molecules include
silicates, water, and carbon monoxide. In addition, the transition from viscous
heating in the inner disk to irradiative heating in the outer disk creates a
discontinuity known as the <b style="mso-bidi-font-weight: normal;">heat
transition</b>. The radial location of these structures changes over time,
moving from larger to smaller radii along with a general cooling trend
throughout the disk.</span></div>
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">-------------------<o:p></o:p></span></div>
<br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">3) condensation fronts in space & time<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Again and again, theoretical models converge
on condensation fronts as a primary source of disk structure as well as
exemplary planet traps. Many different chemical species have been nominated for
leading roles, with three standing out: silicates, water, and carbon monoxide.
Each one condenses around a specific temperature: silicates below 1500 K, water
below 170 K, and carbon monoxide below 70 K. However, the exact location in the
disk where each temperature obtains will vary widely, not only from system to
system but also from time to time within the same disk. This is because local
temperatures depend on irradiation, viscosity, and opacity. All these factors
vary over time, such that the location of each condensation front changes substantially
during disk evolution. Although the general trend is for each front to move
inward as the disk attenuates and cools, stellar flares and outbursts can cause
abrupt increases in temperature that evaporate dust grains and temporarily shift
all fronts outward (Cieza et al. 2016; see <b style="mso-bidi-font-weight: normal;">Figure
4</b>). <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Baillie & colleagues (2015) emphasize
that condensation fronts are regions rather than sharply defined lines,
preferring to describe them as plateaux. In addition, even their <b style="mso-bidi-font-weight: normal;">silicate condensation front</b> is
relatively distant from the star during the earliest evolutionary phases.
Baillie’s group identifies an initial plateau extending from 0.3 to 1.3 AU, which
contracts to about 0.05-0.1 AU after “a few million years.” Similarly, Morbidelli
& colleagues (2016) propose that the inner edge of the silicate front in
the early Solar System was located at 0.7 AU, near the present orbit of Venus. Given
the extreme temperatures prevailing in the vicinity of the star, no solids can
survive in the inner disk and no accretion can happen there until the silicate front
shrinks substantially. Presumably this situation would complicate the models of
<i style="mso-bidi-font-style: normal;">in situ</i> accretion discussed in the </span><a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">previous
installment</span></a><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> of this series, especially for exoplanets orbiting inside ~0.1-0.3
AU. It is conceivable that most objects on orbits shorter than about 100 days (i.e.,
the vast majority of Kepler planets) formed outside the original front and
arrived at their present location by inward migration after the inner system
cooled. <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">The plateau in surface density and pressure
created by the silicate front appears to coincide or at least overlap with the
dead zones proposed by Hasegawa, Lyra, and others. However, I haven’t noticed
any published treatments of their resemblance or lack thereof.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The <b style="mso-bidi-font-weight: normal;">water
condensation front</b> is by far the most extensively discussed of any radial
structure, typically under the name of the snow line (or recently, snow region).
Since water is abundant in protoplanetary disks around Sun-like stars, the
local surface density of solids increases by a factor of 2 to 4 on orbits
outside the water condensation front (Kenyon & Kennedy 2008, Martin &
Livio 2012). This enhancement increases the likelihood that planetesimals and
protoplanets will form in this region. <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The vagaries and excursions of the snow line have
been traced over the past few decades. Older literature typically defined 2.7
AU as the classical snow line in our Solar System, given the sharp transition in
the composition of small bodies observed at that location in the contemporary
Asteroid Belt. Purely rocky objects, like Vesta (semimajor axis = 2.36 AU),
orbit inside that radius, while objects that include a substantial proportion
of water ice, like Ceres (semimajor axis = 2.77 AU), orbit outside it. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">However, more recent research argues that
this transition is simply the fossilized imprint of the position of the snow
line during the first few million years of system evolution, since the
present-day snow line is actually located inside Earth’s orbit (Ida &
Guillot 2016, Morbidelli et al. 2016). Thus, Kennedy & Kenyon (2008) have proposed
that the snow line migrates from about 6 AU to 1 AU during the first few
million years of disk evolution around a G-type star. Using a different
approach emphasizing dust opacity, Mulders & colleagues (2016) recently argued
for a primordial dispersion in the location of the snow line, with values
ranging from 1.4 AU to 8 AU for a G-type star. All these studies concur that
the snow line moves inward over time, and that its radial location at early
times leaves an indelible imprint on planetesimal accretion and ultimately
system architecture at later times.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The <b style="mso-bidi-font-weight: normal;">carbon
monoxide condensation front</b> has received growing attention in the past few
years, especially since Qi & colleagues (2013) reported the detection of
this structure around TW Hydrae with the ALMA instrument (<b style="mso-bidi-font-weight: normal;">Figure 5</b>). Since carbon monoxide freezes at much lower temperatures
than water, this condensation front is located much farther from a Sun-like
star than the other two major fronts. Qi & colleagues propose a radial
location of about 30 AU for stars of Solar mass, making the carbon monoxide
region the most accessible to direct observation. This structure might be
implicated in the formation of planets on very wide orbits (Dodson-Robinson et
al. 2008), and its effects might be especially significant for disks with radii
substantially larger than the original Solar nebula.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt; page-break-after: avoid;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">4) first planetesimals<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Classic studies of planet formation by
accretion assume that planetesimals will congeal throughout an infant disk. Indeed,
many simulation studies of system evolution have started with a numerical set-up
in which protoplanets are packed in a series of concentric orbits separated by several
</span><span style="color: black; mso-themecolor: text1;"><a href="https://en.wikipedia.org/wiki/Hill_sphere"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Hill radii</span></a></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> (e.g., Raymond
et al. 2006, Ida & Lin 2010). Yet the ensemble of research discussed here
supports a different picture of accretion. It seems that planetesimals, the
essential precursors of protoplanets and planets, are born only in favored
locations: the planet traps and filters detailed above. System architectures
are thus erected on chemical and physical foundations established early in disk
evolution. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Dust, defined as particles smaller than one
millimeter, is evidently abundant throughout a newborn protoplanetary disk.
Dust grains can settle in the midplane and clump into pebbles, defined as
centimeter-sized particles. But further aggregation into planetesimals, defined
as particles measured in tens of meters, appears to be possible only in refuges
where gas drag and Type I migration can be overcome. Some models propose that
pebbles cluster in multitudes in these refuges, eventually thronging so densely
that they collapse into planetesimals 100 km in diameter – much larger than the
boulder-sized constituents invoked by older models. Objects this size can
readily accrete pebbles and grow even bigger. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">As Alessandro Morbidelli & Sean Raymond (2016)
recently observed, “All this may be suggestive that the first planetesimals
formed only at select locations (near the disk’s inner edge or beyond the
snowline).” In this way, they say, the first generation of planetesimals “may serve
as a planetary system’s blueprint.”<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt; page-break-after: avoid;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">5) dissipation & relaxation<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">A forming planetary system can be regarded as
a closed system engaged in feverish, time-limited activity. The processes of accretion,
migration, and orbital evolution are determined by factors originating within
the system: stellar irradiation, stellar outbursts, gas flow through the disk, planetesimal/disk
interactions, planetesimal/planetesimal interactions, planet/disk interactions,
and eventually planet/planet interactions. All these factors are sensitive to
the passage of time. For example, as gas flow slackens, migration slows, and as
gas dissipates, opportunities close down for young planets to accrete H/He
atmospheres. At the same time, the radial location of planet traps moves inward,
while their ability to trap solids diminishes. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The emergence of a gas giant planet anywhere
in the disk has far-reaching consequences. A gas giant clears gas and solids
from its orbit, cutting off the flow of volatiles and the migration of planetesimals
and protoplanets originating at wider radii. This development starves the inner
disk of replenishment for the gases, dust, and pebbles that it continually loses
to stellar accretion. The inner disk therefore drains rapidly into the star,
creating a central hole. Its removal exposes the outer disk to photoevaporation
by stellar flux. The process of dissipation is quite fast, measured in
thousands rather than millions of years. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Although not all disks give birth to giants,
most of them evidently evaporate on a similar schedule in a similar sequence: drainage
of the inner disk followed by photoevaporation of the outer disk. This might be
a universal process that is simply accelerated by the assembly of one or more
giant planets. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Disk dispersion is a watershed moment in
system evolution. Not only is gas accretion quenched for young planets; the
dissipation of H/He removes the mechanism by which orbital eccentricities are
damped, exposing planets to mutual perturbations. These interactions can happen
even in the presence of nebular gas, but their likelihood is significantly
amplified at this evolutionary phase. Orbit crossings, planetary collisions,
and planetary ejections are all possible, potentially leading to major
revisions in the “blueprint” created by primordial disk structures. Planet/planet
interactions after gas removal probably cause the orbital eccentricities
observed in multiplanet systems, while orbital sculpting and dynamical upsets
are especially likely in systems with gas giants outside 1 AU (Matsumura et al.
2013, Mustill et al. 2016). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">External influences on system evolution are
also possible. Most stars shining today, including our Sun, formed in clusters
containing thousands of protostars packed within an area just a few parsecs in
radius. Under these conditions, stellar flybys are inevitable. Close encounters
in stellar nurseries can strip the outer disk while a star is accreting gases,
or scatter outer planets at any point during a system’s lifetime. Nevertheless,
studies disagree on the frequency of flybys (e.g., Malmberg et al. 2011, Li
& Adams 2015), so it is unclear how common or rare an extreme disruption
might be. In any case, given the short lifetimes of protoplanetary disks, flybys
are more likely to happen after the gas dissipates, when orbital architectures are
beginning to stabilize.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">6) where do baby planets come from?<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">At the beginning of this series I asked the simple
question shown above. According to the body of research discussed here, the
simple answer seems to be: from <b style="mso-bidi-font-weight: normal;">farther
out</b> in the protoplanetary disk. Baby planets originate in cooler zones
where the preservation and accretion of solids is supported at primordial times.
They achieve their mature compositions and orbits by interacting with local
solids and gases, typically engaging in orbital migration over some distance
and likely incorporating pebbles and planetesimals that formed in planet traps.
<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 6pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The disk models discussed here are unfriendly
to the accretion of protoplanets on short-period orbits during the earliest
phases of evolution. Instead, rocky objects assemble outside the silicate
condensation front and icy objects assemble in the snow region. As these fronts
move inward, their clutches of planetesimals and protoplanets are conveyed into
the inner system, contributing to the composition of planets with various
ratios of solids and volatiles. We can expect warm, low-mass planets that are
purely rocky, like the four terrestrial planets in the Solar System; part rock
and part ice, like the larger satellites of Jupiter and Saturn; or composed
primarily of rock and ice but supporting H/He envelopes, like Uranus and
Neptune. Gas giant planets, which seem constrained to form on still wider
orbits (in the snow region, near the heat transition, or even beyond the zone
where carbon monoxide freezes) often have opportunities to intrude on these
families of low-mass planets, with the odds of inner-system derangement
increasing along with the mass of the intruders. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Given these widely endorsed views, it surprises me that theoretical
models of planet composition in Kepler systems still limit their focus to rocky
objects with H/He envelopes but no water or other volatile content (e.g., Lopez
& Fortney 2014, Rogers 2015, Dorn et al. 2015, Wolfgang et al. 2016). I
look forward to studies that widen their scope to accommodate more diverse
internal structures. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Click
here for a permanent index by topic of blog posts</span></span></i><i style="mso-bidi-font-style: normal;"><span style="color: blue; font-family: "georgia" , "serif";"><br /><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span></span></i><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">on
Back Alley Astronomy</span></span></i></a><span style="font-family: "georgia" , "serif";">
<o:p></o:p></span></div>
<br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p><span style="font-family: "times new roman";"> </span></o:p></span></div>
<br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">REFERENCES<o:p></o:p></span></b></div>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Andrews SM</span></b><span style="color: black; font-size: 10pt;">, Wilner DJ, Zhu Z, Birnstiel T, Carpenter
JM, Perez LM, Bai XN, Oberg K, Hughes AM, Isella A, Ricci L. (2016) Ringed
substructure and a gap at 1 AU in the nearest protoplanetary disk. <i style="mso-bidi-font-style: normal;">Astrophysical Journal Letters</i> 820, L40.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Armitage PJ</span></b><span style="color: black; font-size: 10pt;">. (2010) Lecture notes on the formation and
early evolution of planetary systems. </span><a href="https://arxiv.org/abs/astro-ph/0701485"><span style="font-size: 10pt; text-decoration: none; text-underline: none;"><span style="color: #000099;">https://arxiv.org/abs/astro-ph/0701485</span></span></a><span style="color: black; font-size: 10pt;"> <o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Baillie K</span></b><span style="color: black; font-size: 10pt;">, Charnoz S, Pantin
E. (2015) Time evolution of snow regions and planet traps in an evolving
protoplanetary disk. <i style="mso-bidi-font-style: normal;">Astronomy &
Astrophysics</i> 577, A65.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Baillie K</span></b><span style="color: black; font-size: 10pt;">, Charnoz S, Pantin
E. (2016) Trapping planets in an evolving protoplanetary disk: preferred time,
locations, and planet mass. <i style="mso-bidi-font-style: normal;">Astronomy
& Astrophysics</i> 590, A60.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Baruteau C</span></b><span style="color: black; font-size: 10pt;">, Bai X, Mordasini C,
Molliere P. (2016) Formation, orbital and internal evolutions of young
planetary systems. <i style="mso-bidi-font-style: normal;">Space Science Reviews</i>.
<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Benitez-Llambay P</span></b><span style="color: black; font-size: 10pt;">, Masset F,
Koenigsberger G, Szulagyi J. (2015) Planet heating prevents inward migration of
planetary cores. <i style="mso-bidi-font-style: normal;">Nature</i> 520, 63-65.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Bitsch B</span></b><span style="color: black; font-size: 10pt;">, Johansen A,
Lambrechts M, Morbidelli A. (2015) The structure of protoplanetary discs around
evolving young stars. <i style="mso-bidi-font-style: normal;">Astronomy &
Astrophysics</i> 575, A28.<o:p></o:p></span><br />
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(2016) Probabilistic mass-radius relationship for sub-Neptune-sized planets. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 825, 19.<o:p></o:p></span>Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-68601965146451128092016-11-05T22:34:00.002-07:002017-05-22T18:28:04.113-07:00Protoplanetary Disks and In Situ Formation<br />
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<a href="https://1.bp.blogspot.com/-309fUaQN2TA/WB7AyIaXhVI/AAAAAAAAA5M/wgvicsvesKs9yNw-aHc9WNsD885QdRq1wCLcB/s1600/TW-Hydrae-protoplanetary-straight-on-2016.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="478" src="https://1.bp.blogspot.com/-309fUaQN2TA/WB7AyIaXhVI/AAAAAAAAA5M/wgvicsvesKs9yNw-aHc9WNsD885QdRq1wCLcB/s640/TW-Hydrae-protoplanetary-straight-on-2016.jpg" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"> TW Hydrae,
the nearest protoplanetary disk observed at high resolution by the Atacama
Large Millimeter/submillimeter Array (</span><a href="http://www.almaobservatory.org/en/home"><span style="font-family: "arial" , "sans-serif";"><span style="color: #000099;">ALMA</span></span></a><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="font-size: small;">). The disk
has an approximate radius of 80 AU, which is twice the semimajor axis of Pluto
in our Solar System. For other physical parameters of TW Hydrae, see <b style="mso-bidi-font-weight: normal;">Table 1</b> below.</span> </span><span style="color: black; font-size: 10pt;"><o:p></o:p></span></div>
<br />
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">-----------------------------<o:p></o:p></span></div>
<br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">This is the second installment in a series about current
theories of the formation of planetary systems. </span></span><a href="http://backalleyastronomy.blogspot.com/2016/10/where-do-baby-planets-come-from.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Part One</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">
provides background on planetology and system architecture. This installment begins
with an overview of <b style="mso-bidi-font-weight: normal;">protoplanetary disks</b>,
which are the site of all planet formation, and then proceeds to outline one
popular theory: <i style="mso-bidi-font-style: normal;">in situ</i> accretion. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Astronomical discoveries over the last two decades have established
that planetary systems are an ordinary outcome of star formation. The formation
process begins in the depths of dark, cold, massive clouds of gas and dust, typically
located in the spiral arms of their parent galaxies. Individual clumps of gas
collapse under their own gravity to fuel the ignition of stars, which are born
as hot, bloated spheres. Baby stars arrive swaddled in remnants of their native
cocoons: vaporous leftovers known as protoplanetary nebulae (<b style="mso-bidi-font-weight: normal;">Figures 1-4</b>). As a star spins, its surrounding
nebula flattens into a disk-like structure through which molecules of
hydrogen/helium (H/He) and dust swirl in a ratio of about 100:1 (Williams &
Cieza 2011, hereafter WC11). The ambient “dust” consists of ices of various compositions
mixed with refractory particles of metals and silicates. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">The primordial disk is a kind of pancake made of clouds, except
that it puffs up around the edges and eventually evaporates instead of turning
crispy. Its basic ingredients determine the nature of the planetary residue left
behind when the cloud disperses. Among the most important characteristics of the
protoplanetary disk are its overall mass and chemical composition, especially
the proportion of heavy elements (particularly iron) to hydrogen. This
proportion is known as <b style="mso-bidi-font-weight: normal;">metallicity</b>
and expressed as [Fe/H], on a scale where zero equals the metallicity of our
Sun. Another potential contributor to gestating planets is the ratio of water
to silicates in the composition of the ambient dust (Bitsch & Johansen
2016), although this topic has not been studied as well as metallicity and mass.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">High metallicity is robustly associated with the presence
of <b style="mso-bidi-font-weight: normal;">gas giant planets</b> – that is,
objects of at least 50 Earth masses (50 Mea) but less than 13 Jupiter masses
(13 Mjup) – whereas <b style="mso-bidi-font-weight: normal;">low-mass planets</b>
occur around stars of all chemical compositions (Buchhave & al. 2012). Stellar
mass is also a predictor of gas giant companions. At constant metallicity,
stars more massive than the Sun are more likely to host gas giants than stars
of Solar mass or less (Johnson & al. 2010). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Extensive observations of star-forming regions have
demonstrated that the mass of a protoplanetary disk scales roughly with the
mass of its parent star. This relationship suggests a further correlation
between disk mass and planet mass (WC11, Andrews et al. 2013). Despite a
significant dispersion at any given value, a typical disk is probably 1% or
less of the mass of its parent star, possibly in the range of 0.2% to 0.6%. The
median mass for disks around Sun-like stars might be as low as 5 Mjup (WC11),
equivalent to 0.48% of the Sun’s mass (o.0048 Msol). <o:p></o:p></span></span></span></div>
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;"></span></b></span></span><br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;">Figure 2. </span></b><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;">The
ALMA Three</span></span></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p></o:p></span><br />
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<a href="https://2.bp.blogspot.com/-L_8uUsvoM7M/WB7AjXK4YfI/AAAAAAAAA5I/uySUI0N2ceo7VWKazt_FAEqVTUBRBu5lgCLcB/s1600/ALMA%2BThree.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="254" src="https://2.bp.blogspot.com/-L_8uUsvoM7M/WB7AjXK4YfI/AAAAAAAAA5I/uySUI0N2ceo7VWKazt_FAEqVTUBRBu5lgCLcB/s640/ALMA%2BThree.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">These spectacular photographs by the </span></span><a href="http://www.almaobservatory.org/"><span style="font-family: "arial" , "sans-serif";"><span style="color: #000099; font-family: "arial" , "helvetica" , sans-serif;">ALMA instrument</span></span></a><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;"> captured three nearby protoplanetary disks – HL Tauri,
TW Hydrae, and V883 Orionis – that differ significantly in radius, mass, and
age. See <b style="mso-bidi-font-weight: normal;">Table 1</b> for individual
parameters. <o:p></o:p></span></span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="font-size: small;"><span style="font-family: "arial" , "helvetica" , sans-serif;">-------------------------<o:p></o:p></span></span></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">The observed radial extent of protoplanetary disks shows at
least as much variation as their masses. Disk radii in nearby star-forming
regions range from about 15 to 200 astronomical units (AU), with outliers in
the range of 400 to 600 AU (WC11). A survey of the Orion Nebula estimated that
three-quarters of all disks have radii smaller than 75 AU (WC11). By comparison,
the radius of Neptune’s orbit is about 30 AU, while the aphelion of Pluto (i.e.,
its widest separation from the Sun) is about 50 AU. Current theory places the
outer boundary of the original Solar nebula at Neptune’s orbit. If that
estimate is accurate, our system’s protoplanetary disk would fall near the low
end of the distribution of disk radii. <o:p></o:p></span></span></span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Disk mass has no necessary correspondence with disk
radius, as evidenced by the largest protoplanetary disk observed in the Orion
Nebula (Bally & al. 2015). Known as 114-426, this object is observed almost
edge-on, with a radius of 475 AU. Dust is concentrated inside 175 AU. Despite
these impressive dimensions, the central star is evidently an early M or late K
dwarf with a mass in the range of 0.4-0.7 Msol. The estimated disk mass is only
3.1 Mjup (0.003 Msol). Nevertheless, recent observations indicate that 114-426
has undergone substantial evolution, even though its age is only 1-2 million
years. Much of its original solid mass has likely already congealed into
planetesimals and protoplanets. <o:p></o:p></span></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">
Parameters of the ALMA Three (see <b style="mso-bidi-font-weight: normal;">Figure
2</b>)<o:p></o:p></span></span></span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">Abbreviations: <strong>AU</strong> = astronomical unit (93
million miles/150 million km); <strong>Msol</strong> = Solar mass; <strong>Myr</strong> = millions of years; <strong>Pc</strong> =
parsec (3.26 light years). Sources: Nomura et al. 2016, Andrews et al. 2016, ALMA
Partnership 2015, Cieza et al. 2016.<o:p></o:p></span></span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="font-size: small;"><span style="font-family: "arial" , "helvetica" , sans-serif;">-----------------------------<o:p></o:p></span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">The recent high-resolution imaging of three active
protoplanetary disks suggests that this new sample is somewhat atypical. As summarized
in Table 1, the masses of these structures exceed the proposed means. At an age
of half a million years, the disk around V883 Orionis has a radius of about 125
AU and an imposing mass of 0.3 Msol, about 20% of the mass of its F-type host
star. At an age of 1 million years, the HL Tauri disk is about 100 AU in extent
with a poorly constrained mass of 0.03-0.14 Msol. At an age approaching 10
million years, the TW Hydrae disk has a radius of 75 AU or more and a mass of 0.05
Msol. It may be that these earliest image captures are outliers within the full
sample of nearby protoplanetary disks. If so, we would have a parallel between
extrasolar disks and extrasolar planets: Although Hot Jupiters dominated the
exoplanetary census in the late 1990s, this planetary type comprises less than
1% of the de-biased sample (Wright et al. 2012). <o:p></o:p></span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">In any event, the overall life expectancy of
protoplanetary disks appears to be better constrained than their typical masses
and radii. Disks are born at the same time as their parent stars, and few (with
the notable exception of TW Hydrae) have been observed at ages of 10 million
years or more. For disks around stars of Solar mass or less, surveys regularly
find a median age of 2 to 3 million years. A recent theoretical study arrived
at a slightly higher mean age of 3.7 million years (Kimura et al. 2016). Shorter
lifetimes are observed for hotter, more massive stars, as well as for stars
with binary companions (WC11). <o:p></o:p></span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">All these data on the mass, extent, and lifespan of protoplanetary
disks provide inescapable limits for theories of planet formation. Most
critically, any object with an atmosphere containing more than 1%-2% of its
bulk composition in H/He must have formed in the presence of a gas disk, since
no other source of light gases would be available to forming planets. That bulk
composition describes the vast majority of exoplanets detected by any method.
In sharp contrast, Earth and Venus evidently accreted many millions of years
after the dispersal of our system’s protoplanetary nebula (Hansen 2009). <span style="mso-spacerun: yes;"> </span><o:p></o:p></span></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 3.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Protoplanetary disks in the Orion Nebula</span></span></span></div>
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<o:p> </o:p><br />
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<a href="https://4.bp.blogspot.com/-QZus8WSgZDw/WB7AX57IO1I/AAAAAAAAA5E/ykDj5tYBfscaFeU8s2J6FNA2w8MFoEukwCLcB/s1600/Proplyds%2Bin%2BOrion%2B-%2B2016%2Bselection.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="382" src="https://4.bp.blogspot.com/-QZus8WSgZDw/WB7AX57IO1I/AAAAAAAAA5E/ykDj5tYBfscaFeU8s2J6FNA2w8MFoEukwCLcB/s640/Proplyds%2Bin%2BOrion%2B-%2B2016%2Bselection.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">With likely ages between 1 and 2 million
years, all these young stellar objects are still embedded in their primordial
nebulae. The five photos in the center of this selection depict systems that have
evolved to the stage of flared protoplanetary disks, as revealed by their
silhouettes. Image credit: NASA/ESA/Luca Ricci.</span></span><br />
<span style="font-family: "arial";"></span> </div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="font-size: small;"><span style="font-family: "arial" , "helvetica" , sans-serif;">--------------------------<o:p></o:p></span></span></span></div>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"></span></span></span></b><br />
<div style="text-align: left;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">minimum
mass solar nebula</span></span></span></b></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Even before the availability of instruments capable of studying
nearby stellar nurseries, astronomers based inferences about the Sun’s original
nebula on the structure of the present-day Solar System. The construct of the
“minimum mass Solar nebula” (MMSN) was developed in the 1970s to estimate the
approximate mass and density of our long-vanished native cloud. Stuart
Weidenschilling (1977) provided a detailed exposition of the assumptions
underlying this model. First, the area occupied by the orbits of the eight
major planets (excluding Pluto, despite its recognition as a planet before 2006)
and the asteroid belt was divided into nine zones. Next, the bulk mass of each
governing planet or debris field was distributed evenly throughout its zone,
augmented with enough H/He to bring the zonal composition in line with the
ratio of elements in the Sun. Mercury’s zone was assumed to extend inward as
far as 0.23 AU, while Neptune’s extended outward to 41 AU.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">This model enabled a rough estimate of the mass of the
original Solar nebula. Depending on assumptions about the quantity of solid
mass in the gas giant planets, estimates of the MMSN ranged from 0.01 to 0.07
Msol (Weidenschilling 1977). The same approach also enabled an estimate of the <b style="mso-bidi-font-weight: normal;">surface density</b> of the original nebula
(i.e., its two-dimensional distribution of mass). Mercury was excluded from the
latter calculation, which indicated a peak in surface density around the orbit
of Venus and then a steady depletion out to the orbit of Neptune. Despite the
high surface density of solids inside 1 AU, the broad radial extent of the MMSN
guaranteed that most of its volatile and refractory mass lay outside 4 AU.</span></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Weidenschilling noted that the model was intended to
represent the nebula at the time of planet formation, not in its primordial
state. This clarification opened the possibility that unknown evolutionary
processes had transformed the original cloud. In particular, Weidenschilling emphasized
the depletion of mass in Mercury’s zone (0.23-0.56 AU) as well as in the more
extensive region occupied by Mars and the asteroids (1.2-4.2 AU). These notable
gaps suggested a “preferential removal” of “several Earth masses of solid
matter.” (For more about the missing mass problem, see</span><span style="font-family: "georgia" , "serif";"> </span></span></span><a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-i.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Solar System Archaeology</span></span></a><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, </span></span><a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Jupiter Descending</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, and </span></span><a href="http://backalleyastronomy.blogspot.com/2016/07/jupiter-re-ascending.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Jupiter Re-Ascending</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">.)</span></span></span></div>
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<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Four decades later, the MMSN is still regularly invoked
and critiqued in theoretical studies, and investigators are still debating the mass
and surface density of Solar and extrasolar nebulae. Williams & Cieza favor
the low end of Weidenschilling’s mass estimate (0.01 Msol), which is equivalent
to approximately 10 Mjup. They estimate that only 15% of disks around stars
like our Sun harbor this much mass within radii of 50 AU, and they infer that
disks harboring substantially more mass must be rare (WC11). Yet many theorists
have invoked disks as much as an order of magnitude more massive than 0.01 Msol
to explain specific system architectures.</span></span></span></div>
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<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">As for surface density, the consensus view is that it
peaks in the innermost region of the disk and trails into vacuum at the outer
limits, but the shape of the slope remains controversial. Many studies have adopted
a smooth power-law decline, with disagreements from author to author on the
relative steepness of the slope. Meanwhile, a growing number of studies argue against
a smooth slope, favoring bumps and valleys instead. This view appears to be supported
by recent imaging of HL Tauri, TW Hydrae, and V883 Orionis (<b style="mso-bidi-font-weight: normal;">Figures 1 & 2</b>).</span></span></span></div>
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<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">The rings and gaps in these images, as well as the peaks
and valleys in surface density predicted by theory, imply that protoplanetary
disks are <b style="mso-bidi-font-weight: normal;">radially structured</b>. Among
the potential sources of this structure, two candidates are especially popular.
The first is growing planets, which might carve out rings in the disk by
clearing dust and potentially gas along their orbits. The second is discontinuities
in heat, pressure, and density located at specific radii in the disk. These two
explanations are not mutually exclusive, and indeed might be interdependent.
Radial structure is the focus of <a href="http://backalleyastronomy.blogspot.com/2016/11/accretion-with-migration-in-radially.html" target="_blank">Part Three</a> in this series, which will look at
such constructs as planet traps, snow lines, sublimation radii, planet deserts,
sweet spots, and dead zones.</span></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 4.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Artist’s view of a young protoplanetary disk<o:p></o:p></span></span></span></div>
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<a href="https://4.bp.blogspot.com/-wMLEPeKLx8U/WB7AE8HVVGI/AAAAAAAAA5A/DvsoGuqvAWgzwVb1Yf4p8SSThDjBsRjQACLcB/s1600/Protoplanetary%2Bdisk%2B-%2BNASA-JPLCaltech%2B-%2Bcrop.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="468" src="https://4.bp.blogspot.com/-wMLEPeKLx8U/WB7AE8HVVGI/AAAAAAAAA5A/DvsoGuqvAWgzwVb1Yf4p8SSThDjBsRjQACLcB/s640/Protoplanetary%2Bdisk%2B-%2BNASA-JPLCaltech%2B-%2Bcrop.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">Image credit: Reported as NASA</span></span></div>
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</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">----------------<o:p></o:p></span></span></div>
<br />
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">cloud dynamics</span></span></span></b><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">I’ve always preferred “nebula” over “disk” to designate
the environment where baby planets are born. “Nebula” makes me think of
swirling gases, whereas “disk” suggests a rigid, brittle object like a CD or an
LP. Although I concede that the gaps and rings revealed by </span></span><a href="http://www.almaobservatory.org/"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">ALMA</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
actually do resemble the </span></span><a href="https://en.wikipedia.org/wiki/Gramophone_record#/media/File:Vinyl_groove_macro.jpg"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">tracks</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"> on a vinyl LP, I still
think “disk” is a dubious metaphor for the objects pictured in <b style="mso-bidi-font-weight: normal;">Figures 1-4</b>. These clouds are fluid and
highly dynamic. Their constituent gas molecules travel inward in a continuous flow
to feed the central star, carrying dust along with them. At the same time, the far
reaches of the nebula spread outward, such that the cloud is attenuating
simultaneously at its inner and outer boundaries. It's like a candle burning at both ends.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">The steady outward diffusion of gas and dust into the
interstellar medium means that protoplanetary disks lack a sharp outer edge –
another way in which they differ from vinyl or plastic disks. The photograph of
TW Hydrae (<b style="mso-bidi-font-weight: normal;">Figure 1</b>) illustrates
this diffuse border zone. By contrast, disks have a distinct inner cavity
created by the interaction between the central star’s magnetic field and the disk’s
constituent gases. While accreting H/He can jump this gap to accrete through
the star’s north and south magnetic poles, the cavity creates a distinct inner
wall in the nebula, inside which gas dynamics cease.</span></span></span></div>
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<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">When we remember that more than half of protoplanetary
disks vanish within 4 million years, we get a sense of the rapid pace of their
evolution. “Feverish” seems a more appropriate descriptor than “glacial.” To
put the timescale in perspective, here are some big numbers: Our Solar System
orbits the core of the Milky Way Galaxy in a period of about 250 million years,
known as the <b style="mso-bidi-font-weight: normal;">galactic year</b>. By that
measure, our Sun is about 18 galactic years old. The Cryogenian Glaciation on
our planet, often described as Snowball Earth, lasted 85 million years (from 720
to 635 million years ago). Although it was merely a single brief winter in the
galactic year, this icy interval was still much longer than the span of 7
million years needed for <i style="mso-bidi-font-style: normal;">Pan prior</i>, the
last common ancestor of apes and humans, to evolve into <i style="mso-bidi-font-style: normal;">Homo sapiens</i>, our own species. The accretion of mighty Jupiter,
which could swallow Earth and a convoy of planets like it, probably took only
half as long as that. Planet formation is a fleeting process in which small
changes have big consequences, and time is of the essence.</span></span></span></div>
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<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure </span></b><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;">5</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">. Distribution of mass in selected systems
with at least 3 planets</span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-wkiqxD-lSa0/WB6_1q0CnTI/AAAAAAAAA48/M8XJGvvqPak9v780ZasJaErbIJATmne7gCLcB/s1600/Mass%2BDistribution%2Bin%2BInner%2BSystems%2BII.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="393" src="https://3.bp.blogspot.com/-wkiqxD-lSa0/WB6_1q0CnTI/AAAAAAAAA48/M8XJGvvqPak9v780ZasJaErbIJATmne7gCLcB/s640/Mass%2BDistribution%2Bin%2BInner%2BSystems%2BII.gif" width="640" /></a></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p></o:p></span> </div>
<br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">in situ</span></i></b><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> formation</span></b></span></span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Until a few decades ago, astronomers believed that the
Sun’s eight planets formed more or less where they are now observed. Then came
the first discoveries of extrasolar gas giants (so-called “Hot Jupiters”) orbiting
their host stars in periods of a few days or weeks. These momentous findings
coincided with a rapid expansion of our understanding of protoplanetary disks,
especially those in the </span></span><a href="http://apod.nasa.gov/apod/ap160718.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Orion Nebula</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">. Theorists
responded to the new extrasolar data with a steadily growing body of research
on the multifarious ways in which forming planets can change their orbits. In
the most popular scenarios, they do so by navigating the flux of evolving
nebulae. In recent years this work has resulted in a novel explanation for the
apparent anomalies in our Solar System’ architecture. Known as the </span></span><a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Grand Tack</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">, the
new model indicates that all four of the outer planets have undergone orbital migration
and scattering, sometimes inward and sometimes outward, so that none of these
planets are exactly where they started.</span></span></span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">But just as cosmologists were beginning to think that the
evolution of the Solar System was more like that of a typical exoplanetary
system, with various regimes of migration and numerous opportunities for planet
scattering and collisions, a radical new model emerged for the genesis of
low-mass planets like those on the left side of <b style="mso-bidi-font-weight: normal;">Figure 5</b>: coalescence in place, better known as <i style="mso-bidi-font-style: normal;">in situ</i> formation. Apart from a few
earlier, more limited inquiries (Raymond et al. 2008, Montgomery & Laughlin
2009), this theory emerged in 2012-2013 as a fully-fledged paradigm.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">In this model, planets of several Earth masses readily
form in place on short-period orbits, tracing the distribution of solids in the
protoplanetary disk. Eugene Chiang & Greg Laughlin (2013) presented a
“strict” version (their word) of this hypothesis. Arguing that “disk-driven
migration seems too poorly understood to connect meaningfully with
observations,” they deprecated models that require migration as both
“premature” and “naïve.” In their place they proposed a <b style="mso-bidi-font-weight: normal;">minimum mass extrasolar nebula (MMEN)</b>. This construct was obtained by
plotting the masses and orbits of the small Kepler planets then known and
constructing a disk in which the concentration of solids matched the location
of the planets. Results highlighted the pile-up of planetary mass inside 0.25
AU, which is correlated with the rapidly declining likelihood of detecting
transiting planets on periods longer than 10 days.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Contrary to standard models of the MMSN, which feature a
gap with a radius of about 0.5 AU at the center of the protoplanetary disk, Chiang
& Laughlin preferred an inner disk edge around 0.05 AU. They associate this
boundary with the <b style="mso-bidi-font-weight: normal;">dust sublimation
radius</b> around a mature Sun-like star – the radius within which silicate
particles transition to gas, removing the possibility of accretion. The surface
density of their MMEN peaks at this location and declines smoothly with
increasing distance.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Given this structure, the MMEN could support a much
larger concentration of solids at small semimajor axes than previous models. After
boosting its overall mass by a factor of 5 to 10, Chiang & Laughlin
proposed that the MMEN could even support an <i style="mso-bidi-font-style: normal;">in situ</i> origin for the planets of </span></span><a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-11</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> and </span></span><a href="http://backalleyastronomy.blogspot.com/2016/09/a-new-planet-for-kepler-20.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">other high-multiplicity systems</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Meanwhile,
Brad Hansen & Norm Murray (2012) had already presented a “less strict”
variation on this approach (according to Chiang & Laughlin). They proposed
that 30-100 Mea of solids could migrate from the outer regions of the
protoplanetary disk to clump within 1 AU, where planetesimals would congregate
and rapidly condense into an ensemble of Super Earths (which I would call gas
dwarfs). Perhaps to distinguish their approach from one that relies exclusively
on solids of local origin, they used the term “<i style="mso-bidi-font-style: normal;">in situ</i> assembly” to describe it. (The questions raised by this
migration + assembly scenario differ from the ones posed by the MMEN, as we’ll
see below.)</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Critiques
of <i style="mso-bidi-font-style: normal;">in situ</i> theory came pretty
quickly. The principal objection was that the MMEN is inconsistent with
observations of actual protoplanetary disks. The high mass required for <i style="mso-bidi-font-style: normal;">in situ</i> formation inside 1 AU can be
obtained only with extreme fine-tuning: Either the overall disk mass must be
many times larger than the MMSN, or the surface density profile of the disk
must be far steeper than that of the MMSN (Raymond et al. 2014). Sean Raymond
& colleagues offered the example of a disk with a radius of 50 AU and an
overall mass of 0.05 Msol – that is, 5 to 25 times the typical disk mass
estimated by CW11 and Andrews et al. 2013. With a slope in surface density
similar to the MMSN, such a disk would harbor only 0.4 to 3 Mea inside 1 AU,
hardly enough to build a system like Kepler-11 or Kepler-20 (see <b style="mso-bidi-font-weight: normal;">Figure 5</b>). Only a substantially steeper
decline in surface density would supply as much as 40 Mea in the inner region
of the disk.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Now, we
know that some stars harbor protoplanetary disks on the order of 0.1 to 0.3
Msol, and it’s possible that some fraction of them also support central
concentrations of mass consistent with the quantities needed for <i style="mso-bidi-font-style: normal;">in situ </i>assembly of hot gas dwarfs. But
the whole point of <i style="mso-bidi-font-style: normal;">in situ</i> models was
to capture “a major if not the dominant mode of planet formation in the Galaxy”
(Chiang & Laughlin 2013), not to showcase a rare and unlikely pathway to a widespread
system architecture.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Another major
objection was that the MMEN implies a universal disk structure that is
incompatible with the diversity of known multiplanet systems (Raymond &
Cossou 2014). Sean Raymond & Christophe Cossou attempted to construct their
own version of the MMEN, following the approach of Chiang & Laughlin. They
took a sample of exoplanetary systems with at least three low-mass planets
inside 1 AU, smoothly distributed all masses in each system in a series of concentric
rings (as Weidenschilling did to construct the MMSN), and attempted to build a
single disk model that could reproduce their sample. It was an impossible task.
Not only would it require an unrealistically large disk mass (two to three
times larger than the value proposed by Chiang & Laughlin for their MMEN) –
no single profile could accommodate the diverse sample of available systems. In
some of them, the mass profile had to increase with radial distance, which is implausible.
In others, the slope was so steep that it produced unphysical distributions.
The authors were able to demonstrate that “a universal disc profile is
statistically excluded at high confidence.”</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">As they
concluded: “The known systems of hot super-Earths must therefore not represent
the structure of their parent gas discs and cannot have predominantly formed <i style="mso-bidi-font-style: normal;">in situ</i>. We instead interpret the
diversity of disc slopes as the imprint of a process that re-arranged the
solids relative to the gas in the inner parts of protoplanetary discs.”
(Raymond & Cossou 2014).</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">In other
words, the observed architectures of planetary systems do not reflect their
primordial distribution of solid mass. Yet the “strict” <i style="mso-bidi-font-style: normal;">in situ</i> model favored by Chiang & Laughlin ruled out migration,
the likeliest mechanism for concentrating solid mass at small semimajor axes.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Other critiques
focused specifically on this anti-migrationist assumption. André Izidoro &
colleagues (2014) argued, “</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Roman; mso-themecolor: text1;">Assuming
that no migration occurs essentially ignores 30 years of disk-planet studies
that show the inevitability of orbital migration.</span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">” Similarly, Kevin Schlaufman
(2014) found that, without migration, the observed Kepler system architectures
would be impossible.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Yet
another flaw in the model was identified by one of its proponents, Eugene
Chiang, in collaboration with first author Eve Lee and co-author Chris Ormel. As
Lee & colleagues (2014) noted, the same conditions that favor the assembly
of gas dwarfs in the range of 5-10 Mea at small semimajor axes are equally
favorable to the formation of gas giants at the same locations. Since the latter
outcome is not observed – gas giants are much less common than low-mass planets
inside 1 AU – <i style="mso-bidi-font-style: normal;">in situ</i> theory needs
adjustment. They proposed that the formation of planets capable of accreting
hydrogen atmospheres must be delayed until shortly before the dissipation of
the nebular gas. Otherwise, planets of a few Earth masses would rapidly balloon
into warm and hot gas giants. Although this looks like a case of fine-tuning,
the authors did not suggest a mechanism to account for the delay. Thus we might
have another example of a scenario that would apply only to a small fraction of
extant planetary systems, rather than representing a characteristic and widely
encountered mode of planet formation.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">immobility
versus drift</span></span></span></b></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">All these
critiques have resulted in revisions in more recent <i style="mso-bidi-font-style: normal;">in situ</i> models, with some studies discarding the MMEN altogether and
invoking in its place either a) drift of solids or even b) migration of
fully-formed planets from cooler regions of the protoplanetary disk into
short-period orbits (e.g., Lee & Chiang 2016). In fact, one of the earliest
self-described <i style="mso-bidi-font-style: normal;">in situ</i> scenarios (Hansen
& Murray 2012) conceded that locally available solids inside 1 AU would
probably be insufficient to form systems like Kepler 11 or Kepler 20. Therefore,
in a series of articles published 2012-2015, lead author Brad Hansen proposed
either migration or radial drift to explain the accumulation of 20-100 Mea of
solids in the inner regions of protoplanetary disks.</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid; text-align: left;">
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">But does
such an approach still qualify as an <i style="mso-bidi-font-style: normal;">in
situ</i> model? In an extended footnote to their recent study of the formation
of Jupiter, Sean Raymond & colleagues say no:</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">“This
model should not be confused with the strict ‘<i style="mso-bidi-font-style: normal;">in-situ</i> accretion’ model, which conjectures that hot super-Earths
form locally from locally-condensed solids (proposed and subsequently rejected
by </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">Raymond et al. 2008</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">, then re-proposed by </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">Chiang
& Laughlin </span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">(</span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">2013</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">)). <i style="mso-bidi-font-style: normal;">In-situ</i>
accretion requires extremely massive disks very close to their stars (</span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">Chiang
& Laughlin 2013</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">). There are many arguments against the
strict <i style="mso-bidi-font-style: normal;">in-situ</i> accretion model (see </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">Raymond
et al. 2008</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">, </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">2014</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">; </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">Schlichting 2014</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">; </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">Schlaufman
2014</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">; </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">Raymond & Cossou 2014</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">; </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">Inamdar
& Schlichting 2015</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">; </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8;">Ogihara et al. 2015</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8; mso-themecolor: text1;">). We propose that models that invoke the inward drift of
solids followed by accretion at close-in orbital radii should be referred to as
a separate category of ‘drift’ models rather than being lumped together with <i style="mso-bidi-font-style: normal;">in situ</i> accretion.” (Raymond et al.
2016)</span></span></span></div>
<span style="font-family: "georgia"; font-size: large;"></span><br />
<div style="text-align: left;">
<div style="text-align: center;">
<span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">------------------</span></span></div>
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Indeed, Hansen himself has noted various criticisms of <i style="mso-bidi-font-style: normal;">in situ</i> approaches, even expressing a
degree of support for models that depend heavily on migration:</span></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Roman; mso-themecolor: text1;">“. . . the speed and even direction of
migration depend on the imperfect cancellation between the torques exerted by
the disk interior and exterior to the planet. One possibility, suggested by
several authors (e.g., Masset et al.</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Roman;"> </span><span style="color: blue; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Roman;">2006</span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Roman; mso-themecolor: text1;">), is that the torque may reverse sign at
particular locations in the disk, leading to “planet traps”—preferred locations
where planets assemble. It is possible that our power-law surface density
profile may simply represent an averaged version of a disk with several such
preferred locations, and that some disks may have more localized distributions
which could contribute to the overabundance of low-multiplicity systems.
Indeed, this might help to place our solar system in the </span><i><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Italic; mso-themecolor: text1;">Kepler </span></i><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Roman; mso-themecolor: text1;">context, as our own terrestrial planet system is potentially the outcome
of a disk with a single localized planet trap.” (Hansen & Murray 2013)</span></span></span></div>
<span style="font-family: "georgia"; font-size: large;"></span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Times-Roman; mso-themecolor: text1;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: CMR8; mso-themecolor: text1;">------------------</span></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p></o:p></span></span></span></div>
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<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">all accretion is local</span></span></span></b><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">From my
back alley perspective it looks like the exoplanet community’s recent
flirtation with <i style="mso-bidi-font-style: normal;">in situ</i> hypotheses has
cooled off. The diverse problems with this approach have now been widely
discussed. Meanwhile, no study to date has presented a planetary system whose
architecture can be explained only by strict <i style="mso-bidi-font-style: normal;">in situ</i> assembly. Instead, any system architecture that might
appear consistent with <i style="mso-bidi-font-style: normal;">in situ</i>
formation (e.g., Proxima Centauri or Kepler-11) can be explained even more plausibly
by a scenario involving migration (Coleman et al. 2016, D’Angelo &
Bodenheimer 2016).</span></span></span></div>
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<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">I don’t
mean to suggest that <i style="mso-bidi-font-style: normal;">in situ</i> models have
been a waste of time. Their elaboration in the literature has inspired many
researchers to look more closely at the ways in which solids and gas can be incorporated
by planets forming in the inner regions of a protoplanetary disk. The result
has been in a net gain in our understanding of accretion. Just as important, <i style="mso-bidi-font-style: normal;">in situ</i> theorists have successfully
retired models in which solids are necessarily depleted from protoplanetary
nebulae inside 0.5 AU. This move represents a further advance in the Copernican
paradigm, which displaces our Solar System from its position as the reference
case for evolutionary models. We now understand that a system of four or five
planets, with masses ranging from 0.5 Mea to about 2 Mea, might form <i style="mso-bidi-font-style: normal;">in situ</i> on orbits ranging from 10 to 500
days – without invoking some special mechanism to boost the local mass in
solids. Instead, it’s clear that the depletion of mass in the ancient Solar
System is the oddity that needs explanation. </span></span></span></div>
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<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Whether
solids are present <i style="mso-bidi-font-style: normal;">ab initio</i> in the
feeding zone of a protoplanet, or whether they migrate there in the form of
pebbles or planetesimals from elsewhere in the disk, all accretion is local.
The universe has evolved manifold pathways to move mass from one region of a
dusty nebula to another. These paths will be explored in the <a href="http://backalleyastronomy.blogspot.com/2016/11/accretion-with-migration-in-radially.html" target="_blank">final installment</a>
of the series, which reviews current variations on the theme of disk-driven
migration. </span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> </span></o:p></span></div>
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<a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Click
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Back Alley Astronomy</span></span></i></a><span style="font-family: "georgia" , "serif";">
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Chiang E, Laughlin G.</span></b><span style="color: black; mso-themecolor: text1;"> (2013) The
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Cieza L</span></b><span style="color: black; mso-themecolor: text1;">, Casassus S, Tobin
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Coleman GAL</span></b><span style="color: black; mso-themecolor: text1;">, Nelson RP,
Paardekooper SJ, Dreizler S, Giesers B, Anglada-Escude G. (2016) Exploring
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">D’Angelo G,
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Hansen B, Murray N.</span></b><span style="color: black; mso-themecolor: text1;"> (2012) Migration
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circulation of dust in protoplanetary discs and the initial conditions of
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Johnson JA</span></b><span style="color: black; mso-themecolor: text1;">, Aller KM, Howard
AW, Crepp JR. (2010) Giant planet occurrence in the stellar mass-metallicity
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Lee EJ, </span></b><span style="color: black; mso-themecolor: text1;">Chiang E, Ormel CW.
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Nomura H</span></b><span style="color: black; mso-themecolor: text1;">, Tsugakoshi T,
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Raymond SN</span></b><span style="color: black; mso-themecolor: text1;">, Kokubo E,
Morbidelli A, Morishima R, Walsh KJ. (2014) Terrestrial planet formation at
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Raymond SN, Cossou C.</span></b><span style="color: black; mso-bidi-font-weight: bold; mso-themecolor: text1;"> (2014) No
universal minimum-mass extrasolar nebula: evidence against <i>in situ </i>accretion
of systems of hot super-Earths. </span><i style="mso-bidi-font-style: normal;"><span style="color: black; mso-themecolor: text1;">Monthly Notices of
the Royal Astronomical Society</span></i><span style="color: black; mso-bidi-font-weight: bold; mso-themecolor: text1;"> </span><span style="color: black; mso-themecolor: text1;">440, L11-L15.<o:p></o:p></span></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Raymond SN</span></b><span style="color: black; mso-themecolor: text1;">, Izidoro A, Bitsch B, Jacobson SA. (2016)
Did Jupiter’s core form in the innermost parts of the Sun’s protoplanetary
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Schlaufman KC.</span></b><span style="color: black; mso-themecolor: text1;"> (2014) Tests of <i style="mso-bidi-font-style: normal;">in situ</i> formation scenarios for compact multiplanet systems. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 790, 91.<o:p></o:p></span></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Schlichting HE.</span></b><span style="color: black; mso-themecolor: text1;"> (2014) Formation of close in Super-Earths
& Mini-Neptunes: Required disk masses & their implications. <i style="mso-bidi-font-style: normal;">Astrophysical Journal Letters</i> 795, L15.
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Swanson-Hysell NL</span></b><span style="color: black; mso-themecolor: text1;">, Rose CV, Calmet CC,
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Wright JT</span></b><span style="color: black; mso-themecolor: text1;">, Marcy GW, Howard AW, Johnson JA, Morton TD,
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160.<o:p></o:p></span></div>
<div style="text-align: left;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Weidenschilling JS.</span></b><span style="color: black; mso-themecolor: text1;"> (1977) The distribution of mass in
the planetary system and solar nebula. <i style="mso-bidi-font-style: normal;">Astrophysics
and Space Science</i> 51, 153-158.<o:p></o:p></span></div>
<div style="text-align: left;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Williams JP, Cieza
LC.</span></b><span style="color: black; mso-themecolor: text1;">
(2011) Protoplanetary disks and their evolution. <i style="mso-bidi-font-style: normal;">Annual Review of Astronomy and Astrophysics</i> 49, 67-117. Abstract:</span> <a href="http://adsabs.harvard.edu/abs/2011ARA%26A..49...67W"><span style="color: #000099;">2011ARA&A..49...67W</span></a><o:p></o:p></div>
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-31910806046575927282016-10-30T01:02:00.002-07:002016-11-06T10:41:16.351-08:00Where Do Baby Planets Come From?<br />
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<a href="https://3.bp.blogspot.com/-EFVkRgfqTds/WBWonBYi33I/AAAAAAAAA4Y/5doPgzZFDVYppZTHMki0Rc1waSh44bs-wCLcB/s1600/IC%2B348.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="640" src="https://3.bp.blogspot.com/-EFVkRgfqTds/WBWonBYi33I/AAAAAAAAA4Y/5doPgzZFDVYppZTHMki0Rc1waSh44bs-wCLcB/s640/IC%2B348.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"> A
stellar nursery:</span><span style="font-family: "arial" , "sans-serif";">
</span></span><a href="http://annesastronomynews.com/burst-of-light-discovered-in-a-suspected-binary-protostar/ic-348/"><span style="font-family: "arial" , "sans-serif";"><span style="color: #000099; font-family: "arial" , "helvetica" , sans-serif;">IC 348</span></span></a><span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">, the nearest rich star cluster still embedded in its
primordial cloud of gaseous hydrogen and dust. Located in the constellation
Perseus at a distance of 320 parsecs (1040 light years), IC 348 occupies one
end of</span><span style="font-family: "arial" , "sans-serif";"> </span></span><a href="http://apod.nasa.gov/apod/ap151010.html"><span style="font-family: "arial" , "sans-serif";"><span style="color: #000099; font-family: "arial" , "helvetica" , sans-serif;">an extensive complex</span></span></a><span style="font-family: "arial" , "sans-serif";"><span style="font-family: "arial" , "helvetica" , sans-serif;"> <span style="color: black; mso-themecolor: text1;">of molecular hydrogen clouds that
includes</span> </span></span><a href="http://apod.nasa.gov/apod/ap140306.html"><span style="font-family: "arial" , "sans-serif";"><span style="color: #000099; font-family: "arial" , "helvetica" , sans-serif;">NGC 1333</span></span></a><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">. At an age of 2 to 3 million years, IC 348 contains about
300 infant stars, ranging from dim red dwarfs to hot blue stars of spectral
class B. Credit: NASA/Lucas Cieza</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">----------------<o:p></o:p></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Back when the only planets ever discovered were the ones in
our Solar System, astronomers thought they knew how planetary systems formed. Then
came the news about extrasolar planets – first a trickle of oddballs and
outliers, eventually a flood of rank and file candidates from the Kepler
Mission and its successors. Theoreticians are still struggling to produce
models that can explain the full range of system architectures and planetary
types implied by observations.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The most widely accepted theory of planet formation is <b style="mso-bidi-font-weight: normal;">core accretion</b>, which descends from the
so-called nebular hypothesis proposed in the 18th century by Immanuel Kant and
Pierre-Simon de Laplace. In different languages, these two philosophers argued
that the young Sun was surrounded by a thin cloud of dust grains, which
coagulated in vortices to form larger units called <b style="mso-bidi-font-weight: normal;">planetesimals </b>(Planetesimale, planétésimaux), which in turn collided
to form the known <b style="mso-bidi-font-weight: normal;">planets</b>.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The 21st century variation on this hypothesis begins with
a similar circumsolar nebula, now specified as a blend of hydrogen and helium
(H/He) supporting a sprinkling of dust. The ambient cloud is typically called a
<b style="mso-bidi-font-weight: normal;">protoplanetary disk</b> or primordial
nebula or some variation on those terms. Figure 2 is a photograph of such a structure
at an approximate age of 1 million years.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Inside these clouds, sticky collisions among dust grains,
pebbles, and planetesimals form larger objects known as <b style="mso-bidi-font-weight: normal;">protoplanets</b> (or planetary cores or planetary embryos). These
objects have masses ranging from Moon-like to Mars-like. Under appropriate
conditions, protoplanets can continue to accrete mass by colliding and merging with
other protoplanets. Beyond some threshold in the range of 1 to 5 Earth masses
(Mea), a growing planet will acquire a H/He envelope, forming an object like Uranus
or the</span><span style="font-family: "georgia" , "serif";"> </span></span><a href="http://backalleyastronomy.blogspot.com/2013/11/puffy-planets.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-size: large;">puffy Super Earths</span></span></a><span style="font-family: "georgia" , "serif";"><span style="font-size: large;"> <span style="color: black; mso-themecolor: text1;">discovered by</span> </span></span><a href="https://en.wikipedia.org/wiki/Kepler_(spacecraft)"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-size: large;">Kepler</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">. If an object continues
stockpiling gas until its envelope rivals the mass of its solid core, runaway
accretion will ensue, resulting in a massive gas-dominated planet like Jupiter.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;">
</span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">The protoplanetary nebula around HL Tauri<o:p></o:p></span></span></div>
<br />
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<a href="https://1.bp.blogspot.com/-iz6b4Dl1d2I/WBWozPgQN5I/AAAAAAAAA4c/SxaFfNz3UZgnAIbtJY8KO1lg7W92NE0xgCLcB/s1600/HL%2BTauri%2B-%2B%2BALMA%2B-ESO-NAOJ-NRAO-%2BNSF.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="640" src="https://1.bp.blogspot.com/-iz6b4Dl1d2I/WBWozPgQN5I/AAAAAAAAA4c/SxaFfNz3UZgnAIbtJY8KO1lg7W92NE0xgCLcB/s640/HL%2BTauri%2B-%2B%2BALMA%2B-ESO-NAOJ-NRAO-%2BNSF.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">This photograph by the</span><span style="font-family: "arial" , "sans-serif";"> Atacama Large
Millimeter/submillimeter Array (</span></span><a href="http://www.almaobservatory.org/"><span style="font-family: "arial" , "sans-serif";"><span style="color: #000099; font-family: "arial" , "helvetica" , sans-serif;">ALMA</span></span></a><span style="font-family: "arial" , "sans-serif";"><span style="font-family: "arial" , "helvetica" , sans-serif;">) captures a series
of bright rings and dark gaps in the protoplanetary disk surrounding HL Tauri,
a newborn Sun-like star located at a distance of about 140 parsecs (456 light
years). Various sources provide radii in the range of 90-120 astronomical units
(AU) for the disk, which is about 1 million years old. These values are larger
by a factor of 3 to 4 than the radius proposed for our Sun’s protoplanetary
disk. </span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">---------------------------------<o:p></o:p></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Most contemporary research on planet formation is based
on the accretion scenario, which is currently available in two basic models. In
one, disk-driven <b style="mso-bidi-font-weight: normal;">migration</b> is a
fundamental process governing system evolution. Pebbles, protoplanets, and planets
change their radial position in the nebula over time, migrating either inward
or outward by interacting with the flow of gases, and in turn interacting with
each other, to produce planetary systems. The other approach favors <b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;">in
situ</i> formation</b>, arguing that growing planetary cores stay where they
are and simply accrete material from their immediate surroundings. Subsequent
posts in this series will explore each model.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span> </div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">the extrasolar
bestiary<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">We know that planets come in a limited number of species,
as determined by their mass and composition (Figure 3). The easiest to discover
are the <b style="mso-bidi-font-weight: normal;">high-mass</b> or <b style="mso-bidi-font-weight: normal;">gas giant planets</b>, which are objects
more massive than about 50 Mea with bulk compositions that are more than 50%
H/He. Our Solar System has two beauties: Jupiter and Saturn.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Then come the <b style="mso-bidi-font-weight: normal;">low-mass
planets</b>, which can be divided into two sub-populations. <b style="mso-bidi-font-weight: normal;">Gas dwarfs</b> are generally at least 2 Mea
but less than 50 Mea. Their defining characteristic is a bulk composition that
is less than 50% but at least 0.1% H/He. <b style="mso-bidi-font-weight: normal;">Terrestrial
planets</b> are devoid of gaseous H/He and generally less massive than 10 Mea. Our
Solar System has two gas dwarfs (Uranus and Neptune) and four terrestrial
planets (Mercury, Venus, Earth, and Mars).</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Despite their diminutive size and difficulty of
observation in other star systems, most humans are most interested in the terrestrials.
The reason is simple. As far as anyone knows, life – or at least life that we
would recognize as such – can evolve only on </span><span class="MsoHyperlink"><span style="font-family: "georgia" , "serif";"><a href="http://backalleyastronomy.blogspot.com/2015/01/much-ado-about-earth-2.html"><span style="color: #000099;">small
rocky worlds</span></a></span></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> like Earth.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
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<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure </span></b><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;">3</span></b><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">. </span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Planets across
five orders of magnitude in mass<o:p></o:p></span></span></span></div>
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<a href="https://1.bp.blogspot.com/-ygwQDoUrkfg/WBWo8o0SWsI/AAAAAAAAA4g/tfsqY28AMJk5eAejvqGZpxUBijNebxOdwCLcB/s1600/Planets%2BAcross%2BFive%2BMagnitudes%2Bin%2BMass.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="336" src="https://1.bp.blogspot.com/-ygwQDoUrkfg/WBWo8o0SWsI/AAAAAAAAA4g/tfsqY28AMJk5eAejvqGZpxUBijNebxOdwCLcB/s640/Planets%2BAcross%2BFive%2BMagnitudes%2Bin%2BMass.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Planets are shown at their relative sizes. All
have well-measured masses and radii. The numbers above the red line indicate the
objects’ approximate mass in Earth units (Mea). In more exact terms, Mars is
0.11 Mea, Uranus is 14.5 Mea, and Saturn is 95 Mea. The numbers below the red
line indicate the objects’ mass in Jupiter units or </span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-bidi-font-family: "Times New Roman"; mso-bidi-font-size: 12.0pt; mso-themecolor: text1;">Mj</span></i><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">. The image of WASP-10b
is an artist’s impression; the other images are photographs or photo composites.
</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">--------------------------------------<o:p></o:p></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Data from the most successful search methods – radial
velocity and transit observations – demonstrate that low-mass planets are the
most numerous species orbiting within a few astronomical units of main sequence
stars (astronomical unit = AU; 1 AU = distance between Earth and Sun). They
occur preferentially in multiplanet systems and are observed in a variety of
architectures. Most interesting are the <b style="mso-bidi-font-weight: normal;">high-multiplicity
systems</b>, meaning those with at least three planets. A representative sample
of high-multiplicity architectures centered on Sun-like stars appears in Figure
4.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure </span></b><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;">4</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">. Distribution of mass in selected systems
with at least 3 planets <o:p></o:p></span></span></div>
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<a href="https://4.bp.blogspot.com/-nfdi5o_D6l8/WBWpE167zxI/AAAAAAAAA4k/Args4jBK6ckeXO58CUKwSb6OSKe0kTRJQCLcB/s1600/Diversity%2Bof%2BSystem%2BArchitectures%2B-%2B2016.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://4.bp.blogspot.com/-nfdi5o_D6l8/WBWpE167zxI/AAAAAAAAA4k/Args4jBK6ckeXO58CUKwSb6OSKe0kTRJQCLcB/s640/Diversity%2Bof%2BSystem%2BArchitectures%2B-%2B2016.gif" width="464" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Star masses are indicated in Solar units. Planet masses
are indicated in Earth units</span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="font-size: small;">-----------------------------<o:p></o:p></span></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Within the high-multiplicity sample, we now know well
over a hundred examples of <b style="mso-bidi-font-weight: normal;">compact low-mass
systems</b> consisting of three to six low-mass planets orbiting within 1 AU of
stars with spectral types ranging from M to F (e.g., </span><a href="http://backalleyastronomy.blogspot.com/2016_09_01_archive.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-20</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">,
HD 69830, and </span><a href="http://backalleyastronomy.blogspot.com/2013/04/holy-grail-earthman.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-62</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
in Figure 4). We even know of several <b style="mso-bidi-font-weight: normal;">compact
mixed-mass systems</b> that include gas giants orbiting alongside two or more low-mass
planets (e.g., Kepler-90, </span><a href="http://backalleyastronomy.blogspot.com/2015/08/the-blazing-wasp-47s.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">WASP-47</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">,
Kepler-48, and HD 10180 in Figure 4). Both types of systems contain a much
higher concentration of mass on short-period orbits than does our Solar System.
Next to these robust ensembles, the inner Solar System seems anemic, with just
four pint-size spheres and a swarm of rocky debris inside 5 AU, collectively
totaling only 2 Mea.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">In fact, after two decades in which the extrasolar census
grew from a few dozen planets to a few thousand, we can confidently assert that
our system is odd. Although many systems support a clutch of small planets on
short-period orbits, and many others have gas giant planets orbiting outside 1
AU, relatively few support both configurations. Virtually none of these bear a
significant resemblance to the Solar System. In a recent study, </span></span><a href="http://adsabs.harvard.edu/abs/2016arXiv161007202M"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Morbidelli & Raymond</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> (2016) estimated
that only 1% of G-type stars like our Sun are accompanied by a gas giant on a
circular orbit outside 1 AU. Planets like our Jupiter might be as rare as Hot
Jupiters.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Another remarkable feature of our system is the
hierarchical distribution of planetary mass, such that each planet exceeds the
sum of the masses of all smaller planets. It’s unclear how common this
architecture might be, since relatively few systems with four or more planets
have well-constrained masses. Although mass hierarchies analogous to the Solar
System are attested for 55 Cancri and Gliese 876, many other systems have
flatter mass distributions. Among them, Figure 4 depicts Kepler-20, HD 10180, and
Mu Arae. Another well-known example is </span><a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-11</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">.
Compact multiplanet systems in particular appear to favor collections of
similar-mass planets over hierarchical arrangements. </span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">How do all these varied system architectures arise? And almost
as important, why should we care?</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Before addressing the difficult question of <b style="mso-bidi-font-weight: normal;">how</b>, here are some reasons <b style="mso-bidi-font-weight: normal;">why</b>:</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<ul>
<li><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Existing data on exoplanetary systems are sparse, incomplete, and likely to remain so for decades. If we had a working model of the conditions and processes that produce the full range of system architectures, we could use available data to predict or rule out unseen companions of known objects.</span></span></li>
</ul>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span><br />
<ul>
<li><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Knowing how and especially where any given planet formed provides an invaluable clue to its likely composition. Planets assembled from material available near the central star will be rocky, while planets formed from material with a more distant origin will contain a significant fraction of volatiles.</span></span></li>
</ul>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<ul>
<li><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Composition is a critical factor in</span><span style="font-family: "georgia" , "serif";"> </span></span></span><a href="http://backalleyastronomy.blogspot.com/2015/12/quantifying-earth-like.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">habitability</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">, since a planet’s structure and bulk constituents are associated with atmospheric properties, surface temperatures, and the potential for open bodies of water.</span></span></li>
</ul>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">In sum: knowing how and where planets of various types
form will help us predict the distribution of Earth-like planets in the</span><span style="font-family: "georgia" , "serif";"> </span></span></span><a href="http://backalleyastronomy.blogspot.com/2016/03/the-nearest-20-parsecs.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Solar neighborhood</span></span></a><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> <span style="color: black; mso-themecolor: text1;">and throughout our Galaxy. <o:p></o:p></span></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">(This discussion will continue with <b style="mso-bidi-font-weight: normal;"><a href="http://backalleyastronomy.blogspot.com/2016/11/protoplanetary-disks-and-in-situ.html" target="_blank">Protoplanetary Disks and In Situ Formation</a></b> and conclude with <b style="mso-bidi-font-weight: normal;">Accretion
with Migration in Radially Structured Disks</b>.) <o:p></o:p></span></span></span></div>
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-20150166971821774142016-09-17T12:05:00.001-07:002016-12-26T19:44:17.215-08:00A New Planet for Kepler-20<br />
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<a href="https://4.bp.blogspot.com/-hditIiguyP8/V92TJ42eTNI/AAAAAAAAA3w/7NIUiDGorYoyqxplyb26amSGuwL-4BosACEw/s1600/Kepler-20-system-architecture.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="590" src="https://4.bp.blogspot.com/-hditIiguyP8/V92TJ42eTNI/AAAAAAAAA3w/7NIUiDGorYoyqxplyb26amSGuwL-4BosACEw/s640/Kepler-20-system-architecture.gif" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> All
announced planets of Kepler-20 at their relative sizes, with colors
corresponding to the densities provided by Buchhave et al. 2016 (see Figure 2
for the color key; redder hues indicate higher densities). Since planet <b style="mso-bidi-font-weight: normal;">g</b> does not transit, the radius and
composition shown here are informed guesses, as indicated by the striped fill. Planets
<b style="mso-bidi-font-weight: normal;">e</b> and <b style="mso-bidi-font-weight: normal;">f</b> are about the same size as Earth, but none of the planets in this
system are cool enough to support Earth-like conditions.</span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">----------<o:p></o:p></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Lars Buchhave and colleagues just reported new radial
velocity data on <b style="mso-bidi-font-weight: normal;">Kepler-20</b>, one of
the best-known systems revealed by the Kepler Mission. The new results include
more precise mass estimates for three of the system’s five known planets (<b style="mso-bidi-font-weight: normal;">Kepler-20b, c, d</b>) and robust evidence
for a previously unknown sixth candidate (<b style="mso-bidi-font-weight: normal;">Kepler-20g</b>), whose orbit is apparently misaligned with the others.
The new planet is notably more massive than its five companions, but despite its
short orbital period (35 days), it was not observed in transit by the Kepler
Telescope.<o:p></o:p></span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">When preliminary Kepler data started circulating in 2011,
we learned that a highly specific orbital architecture is more common than
anyone ever dreamed: compact systems with three or more planets visible in
transit (Lissauer et al. 2011b). Lissauer & colleagues reported 55 in 2011.
By May 2016, the </span></span><a href="http://exoplanet.eu/"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Extrasolar Planets Encyclopaedia</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> listed
154. <o:p></o:p></span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Two systems have always stood out from the pack. One is
Kepler-20 (Gautier et al. 2012), which harbors five transiting planets inside a
semimajor axis of 0.4 astronomical units (AU). The other is Kepler-11 (Lissauer
et al. 2011a), with six transiting planets inside a semimajor axis of 0.5 AU.
Both systems center on mature G-type stars like our Sun, yet each one sustains a
rich multiplanet architecture confined within a radius similar to the semimajor
axis of Mercury (0.39 AU). The corresponding region of our Solar System, of
course, </span></span><a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">is empty</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">From the beginning, Kepler-11 has received the lion’s
share of attention, for one simple reason. Five of its six planets exhibit <b style="mso-bidi-font-weight: normal;">transit timing variations (TTVs)</b>, such
that their orbital motion periodically speeds up or slows down to avoid awkward
encounters with neighboring planets. Although this behavior had previously been
theorized, Kepler-11 and another early discovery, Kepler-9, were the first cases
ever actually confirmed (Ford et al. 2011). <o:p></o:p></span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">For Kepler-11, analysis of TTVs enabled estimates of the masses
of the five inner planets, which presented yet another surprise (Lissauer et
al. 2011a). Although their radii were </span></span><a href="http://backalleyastronomy.blogspot.com/2012/05/kepler-11-as-testbed.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">originally estimated</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> in the
range of 2 to 4.5 Earth units (Rea), all five turned out to be substantially
less massive than Neptune, whose radius of 3.9 Rea contains a mass of 17.2 Earth
units (Mea). Like Neptune, Lissauer & colleagues concluded, the Kepler-11 planets
must be enveloped in extended atmospheres of hydrogen and helium (H/He), even
though their individual masses are smaller than 10 Mea. With that inference began
the </span></span><a href="http://backalleyastronomy.blogspot.com/2012/04/between-earth-and-uranus-part-ii.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">modern era</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> of planetology.
<o:p></o:p></span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The planets of Kepler-20 are packed almost as tightly as
those of Kepler-11, but no TTVs are available to provide mass estimates.
Fortunately, Kepler-20 is substantially closer to our Solar System than is Kepler-11.
Its distance is estimated at 290 parsecs (945 light years), versus 613 parsecs
(1998 light years) for Kepler-11. This proximity enabled the collection of ground-based
radial velocity observations in 2009-2011 by the Keck/HIRES spectrograph, which
placed rough constraints on the masses of Kepler-20b, c, and d (Gautier et al.
2012). These constraints indicated that, despite the broad similarities between
Kepler-20 and Kepler-11, the planets of the former star are both denser and
more massive than those of the latter. <o:p></o:p></span></span></span></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Kepler-20 and Kepler-11 Compared</span></span></span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p></o:p></span> </div>
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<a href="https://2.bp.blogspot.com/-jskdYsd10I0/V92TJrN8rQI/AAAAAAAAA3s/1_sKE-pjYjwO2Jn5uo21uQM4BEIw2rtggCLcB/s1600/Kepler-20%2Band%2BKepler-11%2BCompared.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="344" src="https://2.bp.blogspot.com/-jskdYsd10I0/V92TJrN8rQI/AAAAAAAAA3s/1_sKE-pjYjwO2Jn5uo21uQM4BEIw2rtggCLcB/s640/Kepler-20%2Band%2BKepler-11%2BCompared.gif" width="640" /></a></div>
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span><br />
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Planets are rendered at their relative sizes
on the same orbital scale, with semimajor axes in astronomical units (AU).
Numbers in red indicate approximate planet masses in Earth units, rounded to
the nearest integer. Planet colors indicate approximate densities, with values
taken from Buchhave et al. (2016) for Kepler-20 and from Lissauer et al. (2013)
for Kepler-11. Buchhave et al. propose that Kepler-20e and -20f have Earth-like
compositions, given their similarity in size to Venus and Earth. The radius and
composition of Kepler-20g are informed guesses, as are the mass and composition
of Kepler-11g.</span><br />
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">---------------------<o:p></o:p></span></div>
<br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Observations of these two benchmark systems have
continued since their discovery. A follow-up study using additional quarters of
Kepler data revised the masses and radii of the Kepler-11 planets, mostly
downward, resulting in even puffier planets (Lissauer et al. 2013). Another follow-up
study reported radial velocity data on Kepler-11 obtained by Keck/HIRES in
2014 (Weiss et al. 2015), placing an upper limit of twice the mass values derived
by Lissauer & colleagues. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Now Buchhave & colleagues (2016) have refined the
masses of the known planets around Kepler-20 and validated a sixth planet by
analyzing archival HIRES data and new HARPS-N data. <b style="mso-bidi-font-weight: normal;">Figure 2</b>, above, is a graphic comparison of the two systems based
on current findings; <b style="mso-bidi-font-weight: normal;">Table 1</b>, below,
provides comparative numbers.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The most striking difference between the planetary
systems of Kepler-11 and Kepler-20 appears in the bulk compositions of their
planets. The estimated density of Kepler-20b substantially exceeds that of Earth,
the densest planet in our system. Its composition might be explained by an
enhancement in iron. Likewise, despite their similarity in mass to Uranus,
Kepler-20c and -20d are much denser, indicating a smaller bulk percentage of
H/He and an enrichment in metal, rock, and potentially ice. Conclusive data are
lacking for the other three planets of Kepler-20, but planet <b style="mso-bidi-font-weight: normal;">g</b> is likely to be similar in
composition to planets <b style="mso-bidi-font-weight: normal;">c</b> and <b style="mso-bidi-font-weight: normal;">d</b>, while planets <b style="mso-bidi-font-weight: normal;">e</b> and <b style="mso-bidi-font-weight: normal;">f</b> might resemble
our system’s terrestrial planets. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">All the planets of Kepler-11 are less massive than
Uranus, but two of them (<b style="mso-bidi-font-weight: normal;">b, d</b>) have
densities comparable to Neptune and Uranus, respectively. The other three
planets with estimated densities (<b style="mso-bidi-font-weight: normal;">c, e,
f</b>) are comparable to Saturn, the most rarefied planet in our system,
despite masses in the range of 2 to 8 Mea. For context, Saturn is 95 Mea, and
if our ringed planet fell into an ocean large enough, it would float. Half the
Kepler-11 planets could float along with it, bobbing like rubber ducklings after
a colossal rubber duck. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">kepler-20: six-planet architecture<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The new study by Buchhave & colleagues (hereafter B16)
begins by reexamining the properties of the host star. For Kepler-20, they
report a higher mass (0.948 Solar masses or Msol), a slightly higher
metallicity (+0.07 ±0.08), and an earlier spectral type (G2) than did earlier
sources. These new parameters imply an even closer resemblance among Kepler-20,
Kepler-11, and our own Sun than previously indicated. Kepler-20 and Kepler-11 are
now assigned virtually identical masses and metallicities, while both appear older
and slightly less massive than our Sun. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The most significant contribution from B16 is their
discovery of the new planet, Kepler-20g. Notably, Hansen & Murray (2013) previously found that a stable orbit might be available between planets <strong>f</strong> and <strong>d</strong>, and the HARPS-N radial velocity
data have confirmed their hypothesis. In addition, B16
report a more precise mass for Kepler-20d, which remains the outermost of the
known planets. Its new mass value (10 Mea) is just half the upper limit
reported in the discovery paper (Gautier et al. 2012), although the precision of
that estimate is still inferior to those for planets <b style="mso-bidi-font-weight: normal;">b</b> and <b style="mso-bidi-font-weight: normal;">c</b>. <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Other major results from B16 support the basic picture
unveiled by the discovery paper, including null results for TTVs. As before, we
see a distinctive mass distribution in which smaller, more lightweight planets
alternate with larger, more massive planets inside 0.25 AU. The two smallest
planets (<b style="mso-bidi-font-weight: normal;">e, f</b>) are remarkably
similar in radius – and probably in mass and composition – to the terrestrial
planets of our Solar System. Their diminutive profiles are inconsistent with
H/He envelopes, whereas such envelopes are essential to explain the radii of
the two largest planets (<b style="mso-bidi-font-weight: normal;">c, d</b>). We
are also safe in assuming an H/He atmosphere for the most massive planet (<b style="mso-bidi-font-weight: normal;">g</b>). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The innermost planet (<b style="mso-bidi-font-weight: normal;">b</b>), however, appears to be rocky, without any contribution from water
or H/He. Kepler-20b is therefore the most massive rocky planet discovered to
date. All other objects with similar radii and well-constrained masses – with
the possible exception of <b style="mso-bidi-font-weight: normal;">55 Cancri e</b>
– require substantial volatile content to explain their profiles. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Unfortunately, B16 did not present stability limits for
any additional planets that might orbit outside 0.4 AU, in the cooler region where
the system’s habitable zone is located. As a result, the potential of Kepler-20
to support habitable planets remains unexplored. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Kepler-20 and Kepler-11 System Parameters<o:p></o:p></span></span></span></div>
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://4.bp.blogspot.com/-YexCVzICalA/V92SssHCKeI/AAAAAAAAA3o/IIHJKKugCnkke8JwOZuKXkqdtyFs3QvDACLcB/s1600/Kepler-20%252BKepler-11-system-parameters.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="276" src="https://4.bp.blogspot.com/-YexCVzICalA/V92SssHCKeI/AAAAAAAAA3o/IIHJKKugCnkke8JwOZuKXkqdtyFs3QvDACLcB/s640/Kepler-20%252BKepler-11-system-parameters.gif" width="640" /></a></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Masses and radii are expressed in Earth
units. <b style="mso-bidi-font-weight: normal;">Mass</b> is rounded to a single
decimal place, with uncertainties omitted. <b style="mso-bidi-font-weight: normal;">Radius</b>
is rounded to two decimal places. <b style="mso-bidi-font-weight: normal;">a</b>
= semimajor axis, expressed in astronomical units (AU), where 1 AU = separation
between Earth and Sun. <b style="mso-bidi-font-weight: normal;">Period</b> =
days. <b style="mso-bidi-font-weight: normal;">Teq</b> = equilibrium temperature,
expressed in Kelvin (K); for context, the Teq of Earth is 255 K. <b style="mso-bidi-font-weight: normal;">Density</b> = grams/cc. Data on Kepler-20
are from Buchhave et al. 2016, except for Teq values in parentheses, which are
older estimates from the Kepler Table based on a lower stellar effective
temperature. Data on Kepler-11 are from Lissauer et al. 2013. </span></div>
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">----------<o:p></o:p></span></div>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">kepler-11
and kepler-20: formation scenarios<o:p></o:p></span></span></span></b><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Although they do not discuss any specific formation
models for the planets of Kepler-20, B16 opine that all six planets assembled shortly
before the dissipation of the primordial circumstellar nebula. This timing
enabled all six to accrete small amounts of H/He from the nebula without
initiating a runaway process that would turn them into gas giants. After the nebula
dispersed, stellar flux likely ablated the H/He envelopes from planets <b style="mso-bidi-font-weight: normal;">b, e,</b> and <b style="mso-bidi-font-weight: normal;">f</b> – the former because of its star-hugging orbit, the latter two
because of their small masses. The cooler and heavier planets (<b style="mso-bidi-font-weight: normal;">c, d, g</b>) were able to retain their
lightweight atmospheres, consistent with current models of atmospheric
evolution (Lopez & Fortney 2014, Erkaev et al. 2016).<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The release of B16 was almost simultaneous with the
publication of a new study of possible evolutionary scenarios for Kepler-11
(D’Angelo & Bodenheimer 2016). Given the similarities between the two host
stars, a scenario that can explain one system architecture should also tell us
something about the other. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">In their
new study, Gennaro D’Angelo and Peter Bodenheimer (hereafter DB16) test the two
most popular formation processes for close-in planets: 1) <i style="mso-bidi-font-style: normal;">in situ</i> accretion of solids locally available in the inner nebula,
and 2) accretion of solids over a broad radial distance by a planetary core
migrating from the outer nebula to the vicinity of the central star. For
brevity they call the latter process “<i style="mso-bidi-font-style: normal;">ex
situ</i> formation.” For each scenario they seek initial conditions that permit
the formation of the known planets within currently observed parameters, and in
both cases they achieve some degree of success. Accordingly, they conclude that
“it is not possible to distinguish between the two modes of formation from [the
planets’] final properties.”<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">However,
a review of their models suggests that <i style="mso-bidi-font-style: normal;">ex
situ</i> is more plausible than <i style="mso-bidi-font-style: normal;">in situ</i>,
given two major flaws in the <i style="mso-bidi-font-style: normal;">in situ</i> scenario.
First, DB16 find that a protoplanetary nebula massing 0.18 Msol inside 70 AU
would be required to achieve the concentration of solid mass needed for <i style="mso-bidi-font-style: normal;">in situ</i> formation of the Kepler-11 planets
inside 0.5 AU. This total is equivalent to the mass of a mid to late M dwarf
star. Yet observations of protoplanetary nebulae in nearby star-forming regions
indicate that most have masses in the range of 0.002 to 0.01 Msol (Williams
& Cieza 2011, Andrews et al. 2013). Such findings argue that the <i style="mso-bidi-font-style: normal;">in situ</i> nebula invoked by DB16 is
unrealistic.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">In the second
place, even assuming the existence of a protoplanetary nebula more massive than
</span></span><a href="http://backalleyastronomy.blogspot.com/2016/08/the-perils-of-proxima.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Proxima Centauri</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">, the <i style="mso-bidi-font-style: normal;">in situ</i> model still could not produce a
good analog of Kepler-11b. Since the inner nebula is completely dry in this model,
the six planets originally form as rock/metal cores surrounded by H/He
atmospheres. No water or other volatile materials are available to enhance
their composition. Given the low core mass and tight semimajor axis of
Kepler-11b, DB16 find that the planet’s primordial atmosphere would be stripped
by stellar flux within 40 million years after the evaporation of the ambient
nebula. To achieve the puffy radius observed today, some 8 billion years later,
the planet would have to outgas very large quantities of H/He from its interior
over a period lasting a few hundred million years – first to replace its
original envelope, and later to replenish the outgassed atmosphere, which would
remain vulnerable to stripping during the time it takes for a G star to settle into
maturity (Erkaev et al. 2016). DB16 acknowledge the difficulties involved in
this outcome. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Their <i style="mso-bidi-font-style: normal;">ex situ</i> or migratory model avoids both pitfalls.
They begin with a protoplanetary nebula of 0.03 Msol inside a radius of 60 AU.
Although this value is larger than a typical nebular mass, it still falls within
an order of magnitude of observations, and is therefore substantially more
realistic than the <i style="mso-bidi-font-style: normal;">in situ</i> nebula.
Within this structure DB16 insert newly formed planetary cores of approximately
Martian mass (0.1 Mea) at orbital radii ranging from 2.1 AU to 5.35 AU, in the region
of the nebula where ices are abundant. All cores begin accreting mass and
migrating inward. The timing of their insertion in the nebula is just as
important as their radial placement, since cores that achieve smaller final
masses need to begin accreting later than those that achieve larger masses in
order to avoid orbit crossings during migration. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Because
they originate in a well-hydrated region, all six forming planets accrete
abundant water along with their H/He envelopes, and all are subject to loss of
atmosphere once they reach their final destinations and the nebula dissipates.
With this model, DB16 succeed in reproducing the masses and radii of all six
planets of Kepler-11. In particular, they find that Kepler-11b retains a steam
atmosphere after the loss of its lightweight H/He envelope, and that this
heavier atmosphere can survive until the present age of the system, consistent
with the planet’s current radius. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">DB16
underscore the difference in planetary compositions produced by their two
contrasting approaches to system evolution. Although they do not apply the
term, the water-rich bodies resulting from their <i style="mso-bidi-font-style: normal;">ex situ</i> model are consistent with the Ocean Planets predicted by
Leger et al. (2004). <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">The
approach taken by DB16 is paralleled in some of the studies that accompanied
the recent announcement of </span></span><a href="http://backalleyastronomy.blogspot.com/2016/08/the-perils-of-proxima.html"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Proxima Centauri b</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> (Barnes
et al. 2016, Coleman et al. 2016), as discussed in my previous post. Like DB16,
Coleman & colleagues explored several scenarios for planet formation,
including <i style="mso-bidi-font-style: normal;">in situ</i> accretion in an
implausibly massive protoplanetary disk, as well as long-distance migration in
a more realistic disk. As with DB16, the migration scenario provided a better
fit with observations than did <i style="mso-bidi-font-style: normal;">in situ</i>
formation. Notably, Coleman’s group explicitly invoked Ocean Planets to describe
the objects produced by migration. <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">I look
forward to a study that applies similar models to the dual evolutionary
histories of Kepler-20 and Kepler-11. I suspect that <i style="mso-bidi-font-style: normal;">in situ</i> scenarios will continue to face difficult challenges.</span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">the
high-multiplicity sample<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">With the confirmation of Kepler-20g, Kepler-20 moves from
one exclusive club – systems with at least five known planets – to the even
smaller elite – systems with at least six planets. The current exoplanetary
census offers only four examples of this rare architecture: HD 10180,
Kepler-11, Kepler-90, and now Kepler-20. Their orbital arrangements provide
invaluable data that will continue to inform our understanding of planet
formation and the distribution of specific planetary types (including temperate
rocky planets) in our region of the Galaxy. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="font-family: "georgia" , "serif";"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i style="mso-bidi-font-style: normal;"><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">Click here for a permanent index by topic of
blog posts</span></i><i style="mso-bidi-font-style: normal;"><span style="color: blue;"><br /><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span></span><span style="color: #000099; font-family: "georgia" , "times new roman" , serif; font-size: large;">on Back Alley Astronomy</span></i></a> </span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p> </o:p></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<span style="font-family: "times" , "times new roman" , serif;"><br />
<span style="font-size: xx-small;"><br /><span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="font-family: "times" , "times new roman" , serif;"><br />
<span style="font-size: 10pt;"><span style="font-family: "times new roman"; font-size: small;">
</span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;"><span style="font-family: "times new roman";">REFERENCES<o:p></o:p></span></span></b><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Andrews SM</span></b><span style="color: black; font-size: 10pt;">, Rosenfeld KA, Kraus AL, Wilner DJ. (2013)
The mass dependence between protoplanetary disks and their stellar hosts. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 771, 129.<o:p></o:p></span></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Barnes R</span></b><span style="color: black; font-size: 10pt;">, Deitrick R, Luger
R, Driscoll PE, Quinn TR, Fleming DP, Guyer B, McDonald DV, Meadows VS, Arney
G, Crisp D, Domagal-Goldman SD, Lincowski A, Lustig-Yaeger J, Schwieterman E.
(2016) The habitability of Proxima Centauri b I: Evolutionary scenarios. In
press. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2016arXiv160806919B"><span style="color: #3778cd; font-size: 10pt; text-decoration: none;"><span style="font-family: "times new roman";">2016arXiv160806919B</span></span></a><span style="font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Buchhave LA</span></b><span style="color: black; font-size: 10pt;">, Dressing CD,
Dumusque X, Rice K, Vanderburg A, Mortier A, Lopez-Morales M, Lopez E, et al.
(2016) A 1.9 Earth radius rocky planet and the discovery of a non-transiting
planet in the Kepler-20 system. In press. </span></span><a href="http://adsabs.harvard.edu/abs/2016arXiv160806836B"><span style="color: #3778cd; font-size: 10pt; text-decoration: none;"><span style="font-family: "times new roman";">2016arXiv160806836B</span></span></a><span style="color: black; font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Coleman GAL</span></b><span style="color: black; font-size: 10pt;">, Nelson RP,
Paardekooper SJ, Dreizler S, Giesers B, Anglada-Escude G. (2016) Exploring
plausible formation scenarios for the planet candidate orbiting Proxima
Centauri. <i style="mso-bidi-font-style: normal;">Monthly Notices of the Royal
Astronomical Society</i>, in press. Abstract:</span><span style="font-size: 10pt;"> </span></span><a href="http://adsabs.harvard.edu/abs/2016arXiv160806908C"><span style="color: #3778cd; font-size: 10pt; text-decoration: none;"><span style="font-family: "times new roman";">2016arXiv160806908C</span></span></a><span style="font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">D’Angelo G,
Bodenheimer P.</span></b><span style="color: black; font-size: 10pt;"> (2016) <i style="mso-bidi-font-style: normal;">In situ</i> and <i style="mso-bidi-font-style: normal;">ex situ</i> formation models of Kepler 11
planets. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 828,
33. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2016ApJ...828...33D"><span style="color: #3778cd; font-size: 10pt; text-decoration: none;"><span style="font-family: "times new roman";">2016ApJ...828...33D</span></span></a><span style="color: black; font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Erkaev NV</span></b><span style="color: black; font-size: 10pt;">, Lammer H, Odert P,
Kislyakova KG, Johnstone CP, Gudel M, Khodachenko ML. (2016) Thermal mass loss
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Notices of the Royal Astronomical Society</i> 460, 1300-1309.<o:p></o:p></span></span><br />
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</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Ford EB</span></b><span style="color: black; font-size: 10pt;">, Rowe JF, Fabrycky
DC, Carter JA, Holman MJ, Lissauer JJ, et al. (2011) Transit timing
observations from Kepler. I. Statistical analysis of the first four months. <i style="mso-bidi-font-style: normal;">Astrophysical Journal Supplement Series</i>
197, 2.<o:p></o:p></span></span><br />
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</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Fressin F</span></b><span style="color: black; font-size: 10pt;">, Torres G, Rowe JF,
Charbonneau D, Rogers LA, Ballard S, Batalha NM, Borucki WJ, Bryson ST,
Buchhave LA, et al. (2012) Two Earth-sized planets orbiting Kepler-20. <i style="mso-bidi-font-style: normal;">Nature</i> 482, 195-198. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2012Natur.482..195F"><span style="color: #3778cd; font-size: 10pt; text-decoration: none;"><span style="font-family: "times new roman";">2012Natur.482..195F</span></span></a><span style="color: black; font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Gautier TN</span></b><span style="color: black; font-size: 10pt;">, Charbonneau D, Rowe
JF, Marcy GW, Isaacson H, Torres G, Fressin F, Rogers LA, Desert J-M, Buchhave
LA, et al. (2012) Kepler-20: A Sun-like star with three sub-Neptune exoplanets
and two Earth-size candidates. <i style="mso-bidi-font-style: normal;">Astrophysical
Journal</i> 749, 15. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2012ApJ...749...15G"><span style="color: #3778cd; font-size: 10pt; text-decoration: none;"><span style="font-family: "times new roman";">2012ApJ...749...15G</span></span></a><span style="color: black; font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Hansen B, Murray N.</span></b><span style="color: black; font-size: 10pt;"> (2013) Testing <i style="mso-bidi-font-style: normal;">in situ</i> assembly with the Kepler planet
candidate sample. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i>
775, 53. Abstract:</span><span style="font-size: 10pt;"> </span></span><a href="http://adsabs.harvard.edu/abs/2013ApJ...775...53H"><span style="font-size: 10pt;"><span style="color: #000099; font-family: "times new roman";">2013ApJ...775...53H</span></span></a><span style="font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Leger A</span></b><span style="color: black; font-size: 10pt;">, Selsis F, Sotin
C,<span style="mso-spacerun: yes;"> </span>Guillot T, Despois D, Mawet D,
Ollivier M, Labeque A, Valette C, Brachet F, Chazelas B, Lammer H. (2004b) A
new family of planets? “Ocean Planets.” <i style="mso-bidi-font-style: normal;">Icarus</i>
169, 499-504. <b style="mso-bidi-font-weight: normal;"><o:p></o:p></b></span></span><br />
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</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Lissauer JJ</span></b><span style="color: black; font-size: 10pt;">, Fabrycky DC, Ford
EB, Borucki WJ, Fressin F, Marcy GW, et al. (2011a) A closely packed system of
low-mass, low-density planets transiting Kepler-11. <i style="mso-bidi-font-style: normal;">Nature</i> 470, 53-58. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2011Natur.470...53L"><span style="color: #3778cd; font-size: 10pt; text-decoration: none;"><span style="font-family: "times new roman";">2011Natur.470...53L</span></span></a><span style="color: #444444; font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Lissauer JJ</span></b><span style="color: black; font-size: 10pt;">, Ragozzine D,
Fabrycky DC, Steffen JH, Ford EB, Jenkins JM, et al. (2011b) Architecture and
dynamics of Kepler’s candidate multiple transiting planet systems. <i style="mso-bidi-font-style: normal;">Astrophysical Journal Supplement Series</i>
197, 8.<o:p></o:p></span></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Lissauer JJ</span></b><span style="color: black; font-size: 10pt;">, Jontof-Hutter D,
Rowe JF, Fabrycky DC, Lopez ED, Agol E, et al. (2013) All six planets known to
orbit Kepler-11 have low densities. <i style="mso-bidi-font-style: normal;">Astrophysical
Journal</i> 770, 131. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2013ApJ...770..131L"><span style="color: #3778cd; font-size: 10pt; text-decoration: none;"><span style="font-family: "times new roman";">2013ApJ...770..131L</span></span></a><span style="color: black; font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Lopez E, Fortney J.</span></b><span style="color: black; font-size: 10pt;"> (2014) Understanding
the mass-radius relation for sub-Neptunes: Radius as a proxy for composition. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 792, 1.<o:p></o:p></span></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Weiss LM</span></b><span style="color: black; font-size: 10pt;">, Marcy GW, Isaacson
H, Deck KM, Lissauer JJ, Jontof-Hutter D. (2015) Constraining the masses of the
Kepler-11 planets with radial velocities. Abstract presented at <i style="mso-bidi-font-style: normal;">Physics of Exoplanets: From Earth-sized to
Mini-Neptunes</i>, February 23-27, 2015. <o:p></o:p></span></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span><span style="font-family: "times new roman";"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Williams JP, Cieza
LC.</span></b><span style="color: black; font-size: 10pt;">
(2011) Protoplanetary disks and their evolution. <i style="mso-bidi-font-style: normal;">Annual Review of Astronomy and Astrophysics</i> 49, 67-117. Abstract:</span><span style="font-size: 10pt;"> </span></span><a href="http://adsabs.harvard.edu/abs/2011ARA%26A..49...67W"><span style="color: #3778cd; font-size: 10pt; text-decoration: none;"><span style="font-family: "times new roman";">2011ARA&A..49...67W</span></span></a><span style="font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-family: "times new roman"; font-size: small;">
</span></span> </span> </span> </span> </span> </div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-36588304381911112322016-08-31T19:02:00.000-07:002016-10-25T19:08:32.642-07:00The Perils of Proxima<br />
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<a href="https://4.bp.blogspot.com/-0YfwAvfsyM0/V8eLosgG19I/AAAAAAAAA3E/24q2TJVpUK4zrKrtduq_JWA937rb4R9uQCLcB/s1600/Proxima%2BCentauri%2Bplanet%2Bdownside%2Bup%2B-%2BRicardo%2BRamirez.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="308" src="https://4.bp.blogspot.com/-0YfwAvfsyM0/V8eLosgG19I/AAAAAAAAA3E/24q2TJVpUK4zrKrtduq_JWA937rb4R9uQCLcB/s640/Proxima%2BCentauri%2Bplanet%2Bdownside%2Bup%2B-%2BRicardo%2BRamirez.jpg" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Guillem
Anglada-Escude and colleagues have announced a planet candidate with a minimum
mass of 1.27 Earth units on a temperate orbit around Proxima Centauri, a tiny
red dwarf that happens to be the nearest star to our Sun. This artist’s view
shows the planet alongside its red host star, with the binary system of Alpha
Centauri visible in the distance. Image credit: Ricardo Ramirez.</span><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> <o:p></o:p></span></div>
<div style="text-align: center;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">----------<o:p></o:p></span></div>
<br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">By now you’ve probably heard the news. Last week, a team
led by Guillem Anglada-Escude reported radial velocity data from the HARPS
spectrograph supporting the presence of a terrestrial planet orbiting <b style="mso-bidi-font-weight: normal;">Proxima Centauri</b>, our Sun’s nearest
neighbor. Proxima is an especially tiny red star of spectral type M5.5. Its
mass is only 12% Solar and its luminosity is less than 1% Solar. <b style="mso-bidi-font-weight: normal;">Proxima b</b>, as the new object is known,
is also quite small for an exoplanet: its minimum mass is estimated at only
1.27 Earth units (1.27 Mea). That finding has inspired journalistic catchphrases
such as “second Earth” and “Earth twin” (McKiernan 2016).<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Just as newsworthy is the planet’s likely temperature.
Even though Proxima b has an orbital period of only 11.2 days and a semimajor
axis of only 0.049 astronomical units (AU), its host star is so dim that the
planet receives just 65% of the irradiance that bathes Earth. This results in a
blackbody equilibrium temperature (Teq) of 235 Kelvin (K) – which is actually
cooler than Earth’s Teq of 255 K, and a bit warmer than that of Mars, at 210 K.
Proxima b is located squarely in its system’s habitable zone. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Anglada-Escude and colleagues were unable to determine
the eccentricity of the new planet’s orbit, offering only an upper limit of
0.35. This is a notable gap in our understanding of the overall system
architecture. Nevertheless, the discovery team collected data suggesting a
possible second planet orbiting Proxima Centauri with an orbital period longer
than 100 days. This result meets expectations, given abundant evidence that
small planets like Earth and Proxima b often have companions of similar mass. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Another limitation in our knowledge stems from the fact
that Proxima b was detected by radial velocity measurements instead of transit
observations. Radial velocity data can provide only a minimum mass, not a true
mass, and in the absence of a transit, we have no idea of the radius of Proxima
b. Thus, we cannot calculate the planet’s bulk composition, a fundamental
determinant of surface conditions.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Nevertheless, despite the sparseness of the available
data, and despite the excessive hype that often surrounds announcements of
small planets, this is truly big news. Unlike the case of the </span><span style="font-family: "georgia" , "serif";"><a href="http://backalleyastronomy.blogspot.com/2015/10/the-ghost-in-window.html"><span style="color: #000099;">phantom
planet</span></a> <span style="color: black; mso-themecolor: text1;">formerly claimed for </span><a href="http://backalleyastronomy.blogspot.com/2012/11/red-hot-planet.html"><span style="color: #000099;">Alpha
Centauri B</span></a> <span style="color: black; mso-themecolor: text1;">(Proxima’s
next-door neighbor), all commentators seem satisfied with the reality of
Proxima b. And if its host star were a G dwarf like our Sun or a K dwarf like </span><a href="http://backalleyastronomy.blogspot.com/2016/04/hd-219134-take-three.html"><span style="color: #000099;">HD
219134</span></a><span style="color: black; mso-themecolor: text1;">, I wouldn’t hesitate
to identify Proxima b as an extrasolar</span> <a href="http://backalleyastronomy.blogspot.com/2013/04/holy-grail-earthman.html"><span style="color: #000099;">Holy
Grail</span></a><span style="color: black; mso-themecolor: text1;">: a potentially
habitable Earth-like planet. <o:p></o:p></span></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">But Proxima Centauri is an M dwarf, and a very puny one at
that. It lies near the bottom of the mass range for this spectral type, right
above the cut-off for brown dwarfs (which aren’t stars). Given the findings of Luger
& Barnes (2015) and Owen & Mohanty (2016) on the evolutionary and
energetic characteristics of M dwarfs, my instinct is to discount the
possibility that Proxima b could support oceans or complex life. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Then again, I’m not a professional astronomer whose research
depends on funding from government agencies and whose career benefits from
media attention. Governments and media evidently determined long ago that
taxpayers/consumers have no interest in exoplanets unless they resemble Earth,</span><span style="font-family: "georgia" , "serif";"> <a href="https://en.wikipedia.org/wiki/Fictional_universe_of_Avatar"><span style="color: #000099;">Pandora</span></a><span style="color: black; mso-themecolor: text1;">, or </span><a href="https://en.wikipedia.org/wiki/Tatooine"><span style="color: #000099;">Tatooine</span></a><span style="color: black; mso-themecolor: text1;">. Accordingly, the publication of
Proxima’s detection last week in <i style="mso-bidi-font-style: normal;">Nature</i>
was accompanied by a posse of preprints on its potential formation history and
present habitability. All tried very hard to find scenarios that would yield a habitable
Earth-like planet, despite the unfavorable conditions predicted for the Proxima
Centauri system and others like it. </span></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">dim
stars are risky bets for life <o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Before looking at a selection of those preprints, let’s
briefly review the unfavorables, which are straightforward and stubborn:</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">First, an evolving M dwarf spends hundreds of millions of
years with temperatures far higher and stellar activity more energetic than it
will experience when it finally enters maturity on the main sequence (the phase
in stellar evolution when hydrogen fusion occurs). This developmental history
means that any planet orbiting in a red star’s mature habitable zone will likely
experience runaway greenhouse conditions for hundreds of millions of years. Loss
of atmosphere and water is probable, forestalling the emergence of life (Luger
& Barnes 2015). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Second, during much of their long lifetimes, M dwarfs are
subject to frequent flaring events and coronal mass ejections (CMEs), and they
emit high levels of extreme ultraviolet radiation. This behavior subsides very
slowly with age. Flares and CMEs are likely to erode the atmospheres and
volatile contents of any planets orbiting in the inner systems of M dwarfs,
where their habitable zones are found. Thus, even if an Earth-size planet
survived an intense greenhouse in its infancy, it would still be vulnerable to evacuation
of volatiles in its maturity (Luger & Barnes 2015). Again, a sterile desert
is more likely than a garden. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Most of the brand-new studies on Proxima b acknowledge
these two challenges. <o:p></o:p></span></span></span></div>
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p> </o:p></span></span></span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Desert Sunset<o:p></o:p></span></span></span></div>
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-VM4itkVgEuk/V8eL4e2RhbI/AAAAAAAAA3I/F1zNUwuXSGAcsmyhz3rfwnmtWzSUdhzoQCLcB/s1600/Sahara-Desert-Sunset-redder.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="400" src="https://3.bp.blogspot.com/-VM4itkVgEuk/V8eL4e2RhbI/AAAAAAAAA3I/F1zNUwuXSGAcsmyhz3rfwnmtWzSUdhzoQCLcB/s640/Sahara-Desert-Sunset-redder.jpg" width="640" /></a></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="font-family: "arial";">Landscapes on red dwarf planets are more likely to resemble this photograph of sunset over the Sahara than the watery locales pictures by many optimistic space artists. Source: Wikimedia, with a red filter over the Sun. </span></div>
<br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">proxima
centauri <i style="mso-bidi-font-style: normal;">sub specie aeternitatis</i><o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">A team of scientists led by James Davenport, none of them
associated with the Proxima discovery team, reported recent observations by the
MOST satellite that underscore the harsh conditions sketched above. They found
that Proxima Centauri emits flares at a tempo of at least 63 per Earth day,
with superflares occurring about 8 times per Earth year (approximately once every
4 orbits of Proxima b). As they remark:<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">“If these flares regularly impacted Proxima b, the
atmosphere would never fully recover. While this is not known to be a ‘show-stopper’
for habitability, it clearly necessitates a more detailed investigation of
atmospheric response […] and photoevaporation […] for Proxima b” (Davenport et
al. 2016).<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Another group led by Gavin Coleman, consisting of a
subset of the discovery team, conducted a set of numerical simulations to
investigate four potential formation scenarios for the new planet. Each was
intended to produce an analog of Proxima b within a specified range of planet
masses and orbital periods. One problem with their approach, from my
perspective, is that all their simulations assume an implausibly large mass for
the host star’s protoplanetary disk, amounting to 4.5% of the stellar mass. By
contrast, a large and growing body of observations indicates that a typical protoplanetary
disk contains about 1% or less of the mass of its parent star (Williams &
Cieza 2011, Andrews et al. 2013). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">The first scenario explored by Coleman & colleagues
was <i style="mso-bidi-font-style: normal;">in situ</i> accretion from a swarm of
embryos and planetesimals after the dissipation of the gaseous component of the
protoplanetary disk. This is similar to the process invoked to produce the four
inner planets of the Solar System. Numerical simulations tended to produce </span><span style="font-family: "georgia" , "serif";"><a href="http://backalleyastronomy.blogspot.com/2014/06/dwarfs-versus-giants-round-two.html"><span style="color: #000099;">compact
multiplanet systems</span></a> <span style="color: black; mso-themecolor: text1;">with one
or more Earth-mass planets like Proxima or Venus, often with a few lower-mass
planets alongside them, smaller than Venus but bigger than Mars. Across
simulations, their compositions ranged from water-rich to dry, and their orbits were
somewhat eccentric. <o:p></o:p></span></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">The second scenario followed the migration of several <b style="mso-bidi-font-weight: normal;">embryos</b> with initial masses ranging
from 0.05 to 0.2 Earth masses (Mea) – roughly similar to the mass of Mars –
within icy and dusty regions of a gaseous protoplanetary disk extending outward
to 9 AU – equivalent to the orbit of Saturn in our system. This scenario started
earlier in system history than the first one, so the simulations modeled
interactions between the growing embryos and the gas disk. Both inward and
outward migration were enabled, and forming planets could accrete hydrogen from the
disk. This scenario tended to yield one or two Earth-mass planets accompanied by
a few smaller planets, as in the first scenario. However, all these planets were
rich in volatiles, prompting the designation of “Ocean Planets” (Coleman et al.
2016). Their orbital eccentricities were generally smaller than those in the
first scenario. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">The third scenario was similar to the second, except that
it featured a single migrating embryo that formed at a distance of several AU.
The object was accompanied on its inward journey by a swarm of planetesimals. Each
simulation in this set produced just one volatile-rich planet, whose mass varied
from one to several Earth masses. Many simulation runs failed to produce
planets with orbits as tight as Proxima b. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">The fourth scenario also featured a single embryo growing
in a gas disk, except that it was accompanied by pebbles instead of
planetesimals. This scenario had the most difficulty forming an analog of
Proxima b, even though the embryo tended to migrate over long distances. The
authors observed that the formation of “a true Proxima analog” in this scenario
would require a substantially larger disk mass than they assumed (even though
they were already using an unrealistically massive disk). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Coleman & colleagues conclude by suggesting
observational tests for each of their four scenarios. These tests hinge on
obtaining a precise estimate of the orbital eccentricity of Proxima b;
establishing the presence or absence of additional planets in the system; and
determining Proxima’s bulk composition (dry versus watery, hydrogen versus
heavier atmospheric gases). Apart from pointing out the difficulties involved
in the single-embryo scenarios, the authors do not attempt to rank their models
according to likelihood. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Although I admire Gavin Coleman’s work, I see two limitations
in this study. First, as noted earlier, the investigators used an
unrealistically large disk mass, and second, they did not consider potential
planet masses in excess of 2 Mea, even though Proxima b’s true mass could
easily be 4 or 5 Mea instead of 1.3 Mea. How would their results change if they
broadened their simulation parameters to include these possibilities?<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Two other new studies focused on the habitability of
Proxima b. One included members of the discovery team joined by several other
distinguished researchers, mostly affiliated with European institutions (Ribas
et al. 2016). The other was conducted by astronomers who were not involved with
the discovery; all are affiliated with U.S. institutions (Barnes et al. 2016).
Notably, the latter group includes Rodrigo Luger and Rory Barnes, who wrote
that widely cited study on the likelihood of extreme water loss for M dwarf planets
(2015). <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">proxima centauri as a habitable planet: barnes & colleagues<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Barnes & colleagues (2016) begin with an expansive
review of our knowledge of Proxima Centauri. The first question they consider
is whether Proxima is the third member of a triple star system centered on
Alpha Centauri A and B, which are both Sun-like stars chemically enriched in metals.
In the present epoch of our Galaxy, Proxima is separated from our Sun by 1.3
parsecs, but its distance from Alpha Centauri AB is only 15,000 AU. The chances
are one in a million that Proxima could be found so close to the binary unless all
three stars are physically associated. Indeed, observations over the past 100
years show that Proxima shares a common motion through space with its brighter
neighbors. Yet not even the latest measurements, using the most sophisticated
instruments and analytic methods, have been able to establish any curvature in
Proxima’s trajectory. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Thus, this ruddy little twinkler might simply be part of
a <b style="mso-bidi-font-weight: normal;">moving group</b> that includes the
Alpha Centauri binary as well as other star systems. As such, Proxima might or
might not have formed in the same molecular cloud as Alpha Centauri AB. If it
did, then we can assume that its age and metallicity are similar to those of
the dazzling binary: age about 3 to 6 billion years (preferred value 4.8
billion years) and [Fe/H] about +0.25. These numbers imply a star that is
slightly older and far richer in heavy elements than our Sun. Barnes &
colleagues argue that stellar enrichment in metals is evidence that Alpha
Centauri formed substantially closer to the Galactic Core than did our Sun,
since the local concentration of metals increases with proximity to the Core. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Given the possibility that Proxima is bound to Alpha
Centauri AB as a third member of the star system, the authors point to the
recent work of Kaib & colleagues (2013) on the consequences of wide stellar
orbits. Barnes & colleagues consider it likely that Proxima’s planetary
system has been disrupted by a close encounter with the bright binary at some
point during its history, especially if the hypothetical Alpha Centauri trinary
migrated to its present Galactic orbit from the inner Milky Way. If it did,
Proxima b might once have followed a wider orbit around Proxima Centauri, but
was driven closer to the star by perturbations induced by Alpha Centauri AB.
Such perturbations might have excited the planet’s orbital eccentricity and
inclination. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Therefore, Barnes & colleagues find that Proxima b
could exist in two different orbital regimes: either 1) it is</span><span style="font-family: "georgia" , "serif";"> <a href="http://backalleyastronomy.blogspot.com/2016/04/daydream-destinations-part-2.html"><span style="color: #000099;">tidally
locked</span></a><span style="color: black; mso-themecolor: text1;">, with one hemisphere
perpetually assaulted by flares and CMEs and the other in endless night, or 2) it
is engaged in a 3:2 spin-orbit resonance like the planet Mercury, spinning 3
times for every 2 orbits around the host star. The former regime would be
consistent with a circular orbit, the latter with a moderately eccentric orbit.
<o:p></o:p></span></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Having
reviewed the planet’s motion through space, the authors explore several
additional factors, including atmospheric escape, tidal evolution, and
radiogenic heating, before they get down to business. As they tell us, their
aim is to investigate “plausible evolutionary scenarios, focusing on cases that
allow the planet to be habitable.” Their bias in favor of habitable outcomes is
explicit. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Their
approach is analytic, based on an original software package called VPLANET. They
consider two different formation histories for Proxima b. In one, it is a
water-rich planet with a hydrogen envelope amounting to 1% or less of its total
mass. In the other, it is a water-rich planet without any gaseous hydrogen. Their
primary analysis assumes a default planetary mass of 1.27 Mea, but they also
consider more massive cases. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Barnes
& colleagues conclude that, if Proxima b achieved its present orbit at the
time of its formation, it would have to support at least 10 Earth oceans in
order to retain 1 Earth ocean today. If the planet’s water content were any
smaller, given the intense stellar flux, “it is likely desiccated today.” They
also find that a hydrogen envelope any smaller than 1% would readily dissipate,
but that larger concentrations would linger, with negative consequences for
water and life. Larger planet masses would be an additional factor preventing
the escape of a primordial hydrogen envelope. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">They
discuss seven possible atmospheric states for Proxima b in the present epoch.
In the <b style="mso-bidi-font-weight: normal;">“habitable but dry”</b> case, the
planet avoids a hydrogen greenhouse and retains a small quantity of water, but
it is much dryer than Earth – more like </span><span style="font-family: "georgia" , "serif";"><a href="https://en.wikipedia.org/wiki/Dune_(novel)"><span style="color: #000099;">Dune</span></a> <span style="color: black; mso-themecolor: text1;">or </span><a href="https://en.wikipedia.org/wiki/Barsoom"><span style="color: #000099;">Barsoom</span></a> <span style="color: black; mso-themecolor: text1;">than a </span><a href="https://en.wikipedia.org/wiki/Cryogenian"><span style="color: #000099;">Cryogenian</span></a> <span style="color: black; mso-themecolor: text1;">snowball or a </span><a href="https://en.wikipedia.org/wiki/Carboniferous"><span style="color: #000099;">Carboniferous</span></a><span style="color: black; mso-themecolor: text1;"> jungle. In the <b style="mso-bidi-font-weight: normal;">“Venus-like”</b> case, it retains a thick CO2 atmosphere but is
completely desiccated and uninhabitable. In the <b style="mso-bidi-font-weight: normal;">“Neptune-like”</b> case, it is similarly desiccated and hellishly hot,
but the culprit is an extensive hydrogen envelope that failed to dissipate,
either because the planet is more massive than 1.27 Mea or because the original
envelope exceeded 1% of the bulk composition. In the <b style="mso-bidi-font-weight: normal;">“abiotic oxygen”</b> case, photolysis of the planet’s original water
content led to a complete loss of hydrogen, while the liberated oxygen accumulated
in the atmosphere and saturated global geochemistry. In this case, they argue,
the intensely oxidized environment would prevent the emergence of life, even if
quantities of water escaped photolysis (with that possibility defining the <b style="mso-bidi-font-weight: normal;">“water and oxygen, but uninhabitable”</b>
scenario). <o:p></o:p></span></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">They
also discuss an outcome they consider especially unlikely: the <b style="mso-bidi-font-weight: normal;">“no atmosphere”</b> case. While they
concede that the host star’s intense flaring activity is capable of stripping
an Earth-like atmosphere from Proxima b, they argue that such a catastrophe
would be followed by outgassing from the planet’s interior, which could
re-establish an atmosphere. Only if the planet’s core has solidified, quenching
planetary magnetism, or if the star is a few billion years older than their preferred
estimate of 4.8 billion years, providing enough time for the mantle to
completely devolatilize, would its atmosphere be permanently destroyed. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">The
authors devote the most time and space to the <b style="mso-bidi-font-weight: normal;">“Earth-like”</b> case, consistent with their stated aims. Within this
case they find three pathways to a happy ending. In one, Proxima b originally
formed as an Earth-like planet on an orbit well outside the system’s mature
habitable zone, neatly avoiding ablation of volatiles. After the star settled
down on the main sequence, a close encounter between Proxima and Alpha Centauri
AB scattered the planet into its present orbit, where it managed to maintain
its atmosphere and water despite continuing flares and CMEs. In the second
pathway, Proxima b achieved its present orbit in primordial times and suffered
desiccation as in the “Venus-like” case, but then a close passage involving
Proxima and the bright binary launched icy asteroids and comets on a collision
course with the desert planet. This bombardment re-hydrated the environment and
enabled oceans and life. In the third pathway, Proxima b started its existence
as a gas dwarf with a hydrogen envelope comprising about 0.1% of its bulk
composition, plus a water inventory amounting to 4.5 Earth oceans. The host
star then blew off the envelope and evaporated most of the water, and thus
unveiled, the planet brought forth life. This is the <b style="mso-bidi-font-weight: normal;">“habitable evaporated core”</b> scenario, which Barnes & colleagues
consider the likeliest of the three pathways to Eden. <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">In their
closing remarks, nonetheless, they concede that life-friendly outcomes
represent a small subset of the possible scenarios for our new neighbor. Having
reviewed the many mechanisms by which the planet can end up a blasted, lifeless
desert, they “identify the retention of water as the biggest obstacle for
Proxima b to support life.” <o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">proxima centauri as a habitable planet: ribas & colleagues<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Ignasi Ribas led another group including Reiners, Morin,
and Anglada-Escude from the discovery team, as well as Sean Raymond, Jeremy Leconte,
Franck Selsis, Emeline Bolmont, and others. Their study is substantially
briefer and less expansive than that of Barnes & colleagues, but they cover
much of the same ground, while reaching somewhat rosier conclusions. <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">They begin with formation scenarios, considering various possibilities
also discussed by Coleman & colleagues and Barnes & colleagues:
formation <i style="mso-bidi-font-style: normal;">in situ</i>, formation <i style="mso-bidi-font-style: normal;">in situ</i> with late water delivery by
bombardment, and formation by accretion with long-distance migration. They also
consider a subset of the evolutionary outcomes explored by those studies,
including Ocean Planets, completely desiccated planets, and of course
Earth-like planets. Among likely orbital states, they agree with Barnes &
colleagues regarding the possibility of two regimes: a “synchronous” or tidally
locked case and a 3:2 spin-orbit resonance.<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">In their exploration of stellar irradiation and the
potential for erosion of atmosphere and water, Ribas & colleagues offer a
broad range of outcomes. Although they recognize that the host star’s
troublesome behavior could desiccate a temperate planet even if it originally
supported 21 Earth oceans, they also find cases where the planet could lose
less than a single Earth ocean. It’s no surprise that this special case assumes
special prominence in their overall findings. As they put it, the “general
conclusion from our study is that Proxima b could have liquid water on its
surface today and thus can be considered a viable candidate habitable planet.”
I’d say the fix is in.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">wrapping
it up<o:p></o:p></span></span></span></b></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">After typing these thousands of characters,
I’ve only scratched the top layers of the new literature on Proxima b. Members
of Ribas’ group have also produced a study of the planet’s potential climate (Turbet
et al. 2016), members of Barnes’ group just circulated a lengthy treatment of its
potential environments and their observational signatures (Meadows et al.
2016), while a solo author offers a study of heat distribution (Goldblatt
2016). So many experts contributing so much brain power to that pale red dot!<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Here’s what I think. Given the limited available data,
it’s premature to speculate about the possibility of Earth-like conditions
on Proxima b. Given my understanding of human behavior under the regime of
desiring-production enforced by terminal commodity capitalism, however, such
speculations are inevitable. We want excitement, and we want it now!<o:p></o:p></span></span></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Before Proxima b was announced, we already knew that M
dwarfs readily supported Earth-size planets in their classical habitable zones,
and we already knew that the likelihood of life on such planets was far lower
than on their counterparts orbiting Sun-like stars. The announcement of Proxima
b hasn’t changed any of that. But it has inspired a lot of cogitation and
calculation, and fortunately, that’s not likely to stop.</span> <o:p></o:p></span></div>
<br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p> </o:p></span></div>
<br />
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<a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-size: large;">Click
here for a permanent index by topic of blog posts</span></span></i><i style="mso-bidi-font-style: normal;"><span style="color: blue; font-family: "georgia" , "serif";"><br /><span style="font-size: large;">
</span></span></i><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: #000099; font-size: large;">on
Back Alley Astronomy</span></span></i></a><span style="font-family: "georgia" , "serif";">
</span><o:p></o:p></div>
<br />
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<span style="font-family: "georgia" , "serif";"><o:p> </o:p></span></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">REFERENCES</span></b></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Andrews SM</span></b><span style="color: black; font-size: 10pt;">, Rosenfeld KA, Kraus AL, Wilner DJ. (2013)
The mass dependence between protoplanetary disks and their stellar hosts. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 771, 129.<o:p></o:p></span></span></div>
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Anglada-Escude G</span></b><span style="color: black; font-size: 10pt;">, Amado PJ, Barnes J,
Berdinas ZM, Butler RP, Coleman GAL, de la Cueva I, Dreizler S, Michael Endl M,
Giesers B, and 21 others. (2016) A terrestrial planet candidate in a temperate
orbit around Proxima Centauri. <i style="mso-bidi-font-style: normal;">Nature</i>
536, 437-440.<o:p></o:p></span></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Barnes R</span></b><span style="color: black; font-size: 10pt;">, Deitrick R, Luger
R, Driscoll PE, Quinn TR, Fleming DP, Guyer B, McDonald DV, Meadows VS, Arney
G, Crisp D, Domagal-Goldman SD, Lincowski A, Lustig-Yaeger J, Schwieterman E.
(2016) The habitability of Proxima Centauri b I: Evolutionary scenarios. In
press. Abstract: </span><span style="font-size: 10pt;"><a href="http://adsabs.harvard.edu/abs/2016arXiv160806919B"><span style="color: #000099;">2016arXiv160806919B</span></a><o:p></o:p></span></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Coleman GAL</span></b><span style="color: black; font-size: 10pt;">, Nelson RP,
Paardekooper SJ, Dreizler S, Giesers B, Anglada-Escude G. (2016) Exploring
plausible formation scenarios for the planet candidate orbiting Proxima
Centauri. <i style="mso-bidi-font-style: normal;">Monthly Notices of the Royal Astronomical
Society</i>, in press. Abstract:</span><span style="font-size: 10pt;"> <a href="http://adsabs.harvard.edu/abs/2016arXiv160806908C"><span style="color: #000099;">2016arXiv160806908C</span></a><o:p></o:p></span></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Davenport JRA</span></b><span style="color: black; font-size: 10pt;">, Kipping DM,
Sasselov D, Matthews JM, Cameron C. (2016) <i style="mso-bidi-font-style: normal;">MOST</i>
observations of our nearest neighbor: Flares on Proxima Centauri. In press. <o:p></o:p></span></span><br />
<span style="color: black; font-size: 10pt;"><span style="font-size: x-small;"><strong>Goldblatt C. </strong>(2016) Tutorial models of the climate and habitability of Proxima Centauri b: A thin atmosphere is sufficient to distribute heat given low stellar flux. In press.</span></span><br />
<span style="color: black; font-size: 10pt;"><span style="font-size: x-small;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Hansen B, Murray N.</span></b><span style="color: black; mso-themecolor: text1;"> (2013) Testing <i style="mso-bidi-font-style: normal;">in situ</i> assembly with the Kepler planet
candidate sample. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i>
775, 53. Abstract:</span><span> </span></span><a href="http://adsabs.harvard.edu/abs/2013ApJ...775...53H"><span><span style="color: #000099; font-size: x-small;">2013ApJ...775...53H</span></span></a><span style="font-size: x-small;"> </span></span><span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Kaib N</span></b><span style="color: black; font-size: 10pt;">, Raymond S, Duncan M. (2013) Planetary
system disruption by Galactic perturbations to wide binary stars. <i style="mso-bidi-font-style: normal;">Nature</i> 493, 381-384. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2013Natur.493..381K"><span style="font-size: 10pt;"><span style="color: #000099; font-size: x-small;">2013Natur.493..381K</span></span></a><span style="font-size: 10pt;"><span style="font-size: x-small;"> <span style="color: black;"><o:p></o:p></span></span></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Luger
R, Barnes R.</span></b><span style="color: black; font-size: 10pt;"> (2015)
Extreme water loss and abiotic O<sub>2</sub> buildup on planets throughout the
habitable zones of M dwarfs. <i style="mso-bidi-font-style: normal;">Astrobiology</i>
15, 119-143. Abstract: </span></span><a href="http://adsabs.harvard.edu/abs/2015AsBio..15..119L"><span style="font-size: 10pt;"><span style="color: #000099; font-size: x-small;">2015AsBio..15..119L</span></span></a><span style="color: black; font-size: 10pt;"><o:p></o:p></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">McKiernan K.</span></b><span style="color: black; font-size: 10pt;"> (2016) Earth’s twin
beckons: Scientists say planet proves it’s a “Star Trek” universe. <i style="mso-bidi-font-style: normal;">Boston Herald</i>, Thursday, August 25,
2016.<o:p></o:p></span></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="font-size: 10pt;">Meadows VS</span></b><span style="font-size: 10pt;">, Arney GN, <span style="color: black; mso-themecolor: text1;">Schwieterman E</span>, <span style="color: black; mso-themecolor: text1;">Lustig-Yaeger
J</span>, Lincowski AP, Robinson T, <span style="color: black; mso-themecolor: text1;">Domagal-Goldman SD</span>, Barnes RK, <span style="color: black; mso-themecolor: text1;">Fleming DP</span>, <span style="color: black; mso-themecolor: text1;">Deitrick R, Luger R, Driscoll PE, Quinn TR, </span>Crisp D. (2016) <span style="mso-bidi-font-weight: bold;">The habitability of Proxima Centauri b: II:
Environmental states and observational discriminants. In press.</span><span style="color: black; mso-themecolor: text1;"><o:p></o:p></span></span></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Owen JE, Mohanty S</span></b><span style="color: black; font-size: 10pt;">. (2016) Habitability
of terrestrial-mass planets in the HZ of M Dwarfs. I. H/He-dominated
atmospheres. <i style="mso-bidi-font-style: normal;">Monthly Notices of the Royal
Astronomical Society</i> 459, 4088-4108.<o:p></o:p></span></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Ribas I</span></b><span style="color: black; font-size: 10pt;">, Bolmont E, Selsis
F, Reiners A, Leconte J, Raymond SN, Engle SG, Guinan EF, Morin J, Turbet M,
Forget F, Anglada-Escude G. (2016) The habitability of Proxima Centauri b I.
Irradiation, rotation and volatile inventory from formation to the present. <i style="mso-bidi-font-style: normal;">Astronomy & Astrophysics</i>, in press. <o:p></o:p></span></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Turbet M</span></b><span style="color: black; font-size: 10pt;">, Leconte J, Selsis
F, Bolmont E, Forget F, Ribas I, Raymond SN, Anglada-Escude G. (2016) The
habitability of Proxima Centauri b II. Possible climates and observability. In
press. <o:p></o:p></span></span><br />
<span style="font-size: x-small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Williams JP, Cieza
LC.</span></b><span style="color: black; font-size: 10pt;">
(2011) Protoplanetary disks and their evolution. <i style="mso-bidi-font-style: normal;">Annual Review of Astronomy and Astrophysics</i> 49, 67-117. Abstract:</span><span style="font-size: 10pt;"> </span></span><a href="http://adsabs.harvard.edu/abs/2011ARA%26A..49...67W"><span style="font-size: 10pt;"><span style="color: #000099; font-size: x-small;">2011ARA&A..49...67W</span></span></a><span style="font-size: 10pt;"><o:p></o:p></span><br />
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<span style="color: black; font-size: 10pt;"><o:p> </o:p></span></div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-75625503702954993572016-07-30T15:39:00.001-07:002016-08-02T02:03:35.579-07:00Jupiter Re-Ascending<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://4.bp.blogspot.com/-3_FJH2l54_o/V50sD4yEJPI/AAAAAAAAA2c/l5joBCMgBoMGd20ZZTHdgQ6QvRdteUk-gCLcB/s1600/Les%2BGrands%2BZigzags.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="380" src="https://4.bp.blogspot.com/-3_FJH2l54_o/V50sD4yEJPI/AAAAAAAAA2c/l5joBCMgBoMGd20ZZTHdgQ6QvRdteUk-gCLcB/s640/Les%2BGrands%2BZigzags.gif" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Figure 1. <i style="mso-bidi-font-style: normal;">Les
Grands Zig-zags</i>.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"> The assembly and migration of Jupiter and
Saturn through the primordial Solar nebula, according to a scenario presented
by Sean Raymond & colleagues (2016).Numbers alongside the concentric
semicircles indicate radii in astronomical units (AU), where 1 AU equals the
radius of Earth’s present orbit and 5.2 AU equals the radius of Jupiter’s
present orbit. </span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">-------------------<o:p></o:p></span></div>
<br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">This one slipped past me during </span></span><a href="http://www.austinchronicle.com/daily/music/2016-05-02/levitation-live-shot-king-gizzard-and-the-lizard-wizard/"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">the excitement</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"> of the
spring music festivals: a new contribution by Sean Raymond & colleagues to
the ongoing debate over the </span></span><a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">history of the inner Solar System</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">. Here’s
the background:</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Our Solar System is </span></span><a href="http://backalleyastronomy.blogspot.com/2012/05/how-weird-is-our-solar-system.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">weird</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">. Among its most notable
oddities is the dearth of mass inside a radius of one astronomical unit (AU, where
1 AU equals the average separation between Earth and Sun). Outside our system,
the nearest G-type star known to harbor planets is 82 Eridani, at a distance of
20 light years. This metal-poor G8 star supports three planets with an
aggregate mass at least 10 times Earth (10 Mea) inside an area equivalent to
the orbit of Mercury. The next-nearest G-type host is 61 Virginis, located
about 28 light years away. This G5 star hosts three planets with an aggregate
mass in excess of 46 Mea inside the equivalent of the orbit of Venus. Both of
these architectures are typical of multiplanet systems discovered by radial
velocity and transit searches.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">By contrast, the three planets closest to our Sun –
Mercury, Venus, and Earth – have an aggregate mass smaller than 2 Mea.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Last year, </span></span><a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">two studies</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"> tried
to find an explanation for the missing mass. <b style="mso-bidi-font-weight: normal;">Batygin & Laughlin (2015)</b>, hereafter BL15, proposed a solution
that extends the popular scenario of the Grand Tack, which has been advocated
by </span></span><a href="http://adsabs.harvard.edu/abs/2011A%26A...533A.131P"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">Pierens & Raymond (2011)</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"> and is discussed
</span></span><a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">here</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"> and </span></span><a href="http://backalleyastronomy.blogspot.com/2015/12/evolutionary-twist.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">here</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">. BL15 hypothesized that
several Super Earth-type planets formed inside the present orbit of Mercury during
the first few million years after the birth of our Sun, when it was still surrounded by an extensive nebula. (In this context, Super
Earths should be understood as rocky or gassy planets of a few Earth masses.) No
sooner had these objects assembled than the growing core of Jupiter began migrating
from the outer Solar nebula to the vicinity of the present orbit of Mars. Young
Jupiter’s inward passage swept a huge swarm of planetesimals into the inner
system, causing perturbations that destabilized the orbits of the Super Earths.
As a result, both the planets and the planetesimals were engulfed by the Sun.
At this point, Jupiter executed a dramatic course change (the Grand Tack itself)
which carried it back into the outer system on the nebular tides, courtesy of an intimate connection
with its smaller and chillier sidekick, Saturn. Meanwhile, the inner system
catastrophe left behind a ring of debris that coalesced into the four
terrestrial planets we know today.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Volk
& Gladman (2015)</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">, hereafter VG15, also proposed that our
system originally harbored a cluster of planets inside the present orbit of
Mercury (“intra-Mercurians”). In all other ways, however, their model differs markedly
from BL15. To start with, their intra-Mercurian objects are similar in mass to
Earth and Venus, not to Super Earths like 82 Eridani b or 61 Virginis b. A more
striking difference is that VG15 implicitly reject both the </span></span><a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">Nice Model</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"> and the
Grand Tack, since their scenario does not involve the outer Solar System at all.
Instead, VG15 suggest that the four terrestrial planets assembled exactly where
we see them today, outside the orbits of the intra-Mercurian planets, with
which they peacefully coexisted for tens of millions of years. Then, one bright
millennium, an abrupt dynamical instability upset the apple cart. The
intra-Mercurians devolved into a donnybrook of orbit crossings and collisions
that rapidly ground them into dust. The dust itself was then engulfed by our
Sun, while the four terrestrial planets continued circling the scene of the
catastrophe like horrified onlookers. The only remaining evidence of those
disintegrated inner planets can be found in the cratered surfaces of Mercury,
the Moon, and Mars, which were pelted by fragments during the period of
annihilation. In sharp contrast, the Nice Model argues that these rocky worlds were
etched by a storm of asteroids that originated in a dynamic instability among
the four outer planets – Jupiter, Saturn, Uranus, and Neptune.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Inside-out planet formation<o:p></o:p></span></span></div>
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://2.bp.blogspot.com/-mDzBf8pyQRQ/V50sQbnVygI/AAAAAAAAA2g/LIM-IVqoS5wZf00q1O1GUdtfaNEH8WqIQCLcB/s1600/Inside-Out%2BPlanet%2BFormation%2B-%2BChatterje%2B%2526%2BTan%2B2014.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="632" src="https://2.bp.blogspot.com/-mDzBf8pyQRQ/V50sQbnVygI/AAAAAAAAA2g/LIM-IVqoS5wZf00q1O1GUdtfaNEH8WqIQCLcB/s640/Inside-Out%2BPlanet%2BFormation%2B-%2BChatterje%2B%2526%2BTan%2B2014.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">This image appears as Figure 1 of Chatterjee
& Tan 2014. (i) The magnetic field of the young star creates a cavity in
the center of the gaseous protoplanetary disk. Immediately outside the cavity
is a “dead zone” through which pebbles drift. (ii) Pebbles accumulate in a ring
around the edge of the cavity, where the gas pressure is at its maximum. (iii)
The pebble ring coalesces into an Earth-size planet. (iv) The dead zone
retreats from the star, creating a new pressure maximum at a larger radius
where new pebbles accumulate and potentially form a new planet. <i style="mso-bidi-font-style: normal;">Abbreviations</i>: MRI = magnetorotational
instability; P max = pressure maximum.</span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> </span></div>
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">-------------------<o:p></o:p></span></div>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">pebble pile-up
with outward migration</span></span></b><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span></b> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Raymond & colleagues (hereafter R16) take a
completely different approach to the problem, one that does not involve either
a primordial clutch of small planets or an inner system catastrophe. Although R16
note that their model is consistent with the Grand Tack, they emphasize its
theoretical independence. Their starting point is actually the scenario of
“Inside-Out Planet Formation,” as presented in an article of the same name by
Sourav Chatterjee and Jonathan Tan (2014). <b style="mso-bidi-font-weight: normal;">Figure
2</b> provides a high-level summary.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">R16 begin with the very earliest stages of accretion in
the Solar nebula. They hypothesize that a large rocky planet assembled out of
pebbles orbiting near the inner edge of the nebula, at an approximate semimajor axis of 0.1 AU. This object was
proto-Jupiter (Figure 2.iii). Once it attained a few Earth masses, it was
subject to torques exerted by the ambient gases, which caused it to migrate
outward to a semimajor axis of about 5 AU. Along the way, the migrating core
cleared all solid material from the nebula interior to 1 AU, shepherding some
of it onto exterior orbits. This shepherding process likely resulted in the
formation of a second core – proto-Saturn – which young Jupiter continued to
herd during its journey into cooler regions of the nebula. Both objects
accreted mass during this process, eventually initiating the runaway accretion
of extensive envelopes of hydrogen and helium (H/He).</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">As R16 argue, this scenario explains why the area inside
Mercury’s orbit is completely empty of mass. The rocky material that originally
accumulated in this space was accreted by young Jupiter, which continued accreting
and scattering planetesimals as it ascended beyond the radii that would later
mark the orbits of Earth and Mars. Once Jupiter attracted sufficient H/He to
become a gas giant planet, it opened a gap in the surrounding nebula,
initiating the process of Type II migration. Then, having arrived in the cool
zone, Jupiter immediately turned around and retraced its path.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">By this stage in the narrative we’ve reached the threshold
of the Grand Tack. Saturn was now following Jupiter instead of being herded
ahead of it, and the gap between the two planets kept narrowing until their
orbits entered a mean motion resonance. At that point, when Jupiter had reached
a radial distance of about 2 AU (Brasser & al. 2016), the direction of their
migration switched again and they sailed into the outer nebula for the last
time. </span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 3.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Jupiter Ascending with Ganymede<o:p></o:p></span></span></div>
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://4.bp.blogspot.com/-vaUWkSDNu_U/V50saG_szII/AAAAAAAAA2k/ClSp4G7ClwIBq9SjrHFr6Zwtxq3hXcI_wCLcB/s1600/Budweiser1906-cropped.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://4.bp.blogspot.com/-vaUWkSDNu_U/V50saG_szII/AAAAAAAAA2k/ClSp4G7ClwIBq9SjrHFr6Zwtxq3hXcI_wCLcB/s640/Budweiser1906-cropped.jpg" width="400" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">A boy and his eagle: Ganymede & Jupiter.
In Greek mythology, <i style="mso-bidi-font-style: normal;">Ganymede</i> was a
Trojan prince of uncommon beauty. Spying him from on high, <i style="mso-bidi-font-style: normal;">Zeus</i> (Latin <i style="mso-bidi-font-style: normal;">Jupiter</i>)
assumed the form of an eagle, swooped down to Earth, and carried the boy back
up to Mount Olympus, where Ganymede became the cupbearer of the gods. The
beverage he served in heaven was <i style="mso-bidi-font-style: normal;">nectar</i>,
a delicious liquid that confers immortality. In 1906, the <i style="mso-bidi-font-style: normal;">Anheuser-Busch Brewing Ass’n</i> appropriated the myth of Ganymede to
promote their own version of heavenly nectar: Budweiser beer, now one of the
most popular alcoholic beverages on Earth. Credit: Wikimedia. </span></div>
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">-------------------</span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">The full scenario involves a double zigzag that calls to
mind the</span><span style="font-family: "georgia" , "serif";"> <span class="MsoHyperlink"><a href="https://www.youtube.com/watch?v=nB8tiSMCwRE"><span style="color: blue;">mark
of Zorro</span></a></span> <span style="color: black; mso-themecolor: text1;">(<b style="mso-bidi-font-weight: normal;">Figure 1</b>). Jupiter’s
outward-inward-outward path also recalls a Greek myth in which Zeus (the Greek
equivalent of Roman Jupiter) swooped down into the terrestrial zone, abducted a
youth named Ganymede, and carried him aloft into the upper spheres of heaven (<b style="mso-bidi-font-weight: normal;">Figure 3</b>). Indeed, it seems likely that
Jupiter’s satellite system – of which Ganymede is the most prominent member –
formed just as the Solar nebula was dissipating (Alibert & al. 2005). In
the Grand Zigzag scenario proposed by R16, this process is constrained to occur
when Jupiter arrived in the vicinity of its present orbit, potentially bearing
a circumplanetary disk of solids (proto-Ganymede) enriched by the planet’s
wanderings through the nebula.</span></span></span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;"><span style="font-size: large;"></span></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;"></span></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Wisely, R16 ask whether this intricate sequence of events
is “a generic process” or a relatively rare occurrence “confined to only a
limited range of conditions.” While they lean toward the first option, they
concede that planetary systems containing both an inner system of Super Earths
and an outer system of gas giants would be challenging to explain in the
context of the Grand Zigzag. In this regard I hasten to note that Chatterjee
& Tan (2014) originally developed their model of inside-out planet
formation to explain the architecture of tightly packed systems of low-mass
planets such as </span></span><a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">Kepler-11</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"> and
Kepler-20, not mixed-mass systems resembling </span></span><a href="http://backalleyastronomy.blogspot.com/2016/04/hd-219134-take-three.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">HD 219134</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">, </span></span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">Kepler-167</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">, or our
Solar System. The adoption of this approach by R16 appears quite novel in the
context of the prehistory of the Solar System, where its application - in my
view – seems tortuous.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">critique
of competing models</span></span></b></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span></b> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">My favorite section of R16 is their discussion of
alternative explanations for the missing mass in the inner Solar System. The
authors make quick work of two studies to which they themselves contributed:
Morbidelli & al. 2016, which explains the void as a lingering effect of the
condensation front for silicate dust in the primordial Solar nebula, and
Izidoro & al. 2015, which argues that the formation of Jupiter blocked the
migration of solids from the outer system, starving the inner system of the
mass it needed to form short-period Super Earths. Neither approach holds up
under their analysis. I find this skeptical attitude particularly impressive, since
healthy self-criticism is essential in scholarly
work.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">R16
dispose of VG15 with similar ease, arguing that the furious impacts invoked to
explain the disappearance of several primeval planets “would not have fallen in
the ‘super-catastrophic’ regime” needed to achieve total annihilation. Instead,
the debris from any erosive impacts would simply be swept up by the remaining
planets in the inner Solar System. R16 thereby confirm my own doubts that a
whole subsystem of planets could vanish without a trace, but they do so with a better-informed argument than I could hope to make. As they
conclude, “we do not expect that a system of close-in terrestrial planets could
self-destruct.”</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">R16 devote considerably more space to their takedown of
BL15. First, they find that the “massive pulse of collisional debris” generated
by Jupiter’s inward migration in that scenario would merely accelerate mass
accretion by any planets forming in the inner system. Second, they argue that this
swarm of debris would not be physically capable of shepherding a clutch of
Super Earths into ever-shrinking orbits. Instead, they suggest that this catastrophic
outcome was just an artifact of the simulation code used by BL15. In reality,
they contend, a mass of planetesimals would “self-interact and grow” rather
than push a planet to the brink of its host star’s gravity well. Finally, R16
challenge the notion that planets can simply be pushed into their stars,
because all evolving protoplanetary disks develop a cavity interior to about
0.1 AU where gas dynamics cease. An object forced into this void, they say,
would be more likely to stabilize on a new orbit than to </span></span><a href="https://en.wikipedia.org/wiki/Set_the_Controls_for_the_Heart_of_the_Sun"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">set the controls for the heart of the Sun</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">In parting, R16 note a striking irony in the model presented
by BL15. Whereas Batygin & Laughlin “invoke the rapid inward drift of
solids to destroy super-Earths,” many other models have proposed the same
process to create them.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 4.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> </span></span><a href="http://www.imdb.com/title/tt1617661/"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">Jupiter
Ascending (2015)</span></span></a><span class="MsoHyperlink"><span style="font-family: "georgia" , "serif";"><o:p></o:p></span></span></div>
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://1.bp.blogspot.com/-DMJ67rD9hb8/V50sm_n83iI/AAAAAAAAA2o/yR9K7l5ZixkRJ3JcAuuiyCYzjqYffM_SQCLcB/s1600/Tatum-Redmayne-Jupiter-Ascending.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="184" src="https://1.bp.blogspot.com/-DMJ67rD9hb8/V50sm_n83iI/AAAAAAAAA2o/yR9K7l5ZixkRJ3JcAuuiyCYzjqYffM_SQCLcB/s640/Tatum-Redmayne-Jupiter-Ascending.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="font-size: small;">Twenty-first century superstars Channing
Tatum and Eddie Redmayne appeared in this star-crossed science fiction epic.
“Jupiter” herself was played by Mila Kunis. All three actors have seen much
more success in other movies.</span> <o:p></o:p></span></div>
<div style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">-------------------<o:p></o:p></span></div>
<br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Sometimes I wonder if I’m getting mental whiplash from
the barrage of new theories about the evolution of our own planetary
system and others. Nevertheless, when it comes to activities of the mind, I’d
prefer an embarrassment of riches to an empty cupboard! <o:p></o:p></span></span></div>
<span style="font-size: large;">
</span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p> </o:p></span></div>
<br />
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="font-size: large;"><span style="color: blue;">Click
here for a permanent index by topic of blog posts</span><span style="color: blue;">
on Back Alley Astronomy</span></span></span></i></a><span style="font-family: "georgia" , "serif";">
</span><o:p></o:p></div>
<br />
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<span style="font-family: "georgia" , "serif";"><o:p> </o:p></span></div>
<br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">REFERENCES</span></b></div>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Alibert Y</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">, Mousis O, Benz
W. (2005) Modeling the Jovian subnebula. I. Thermodynamic conditions and
migration of proto-satellites. <i style="mso-bidi-font-style: normal;">Astronomy
& Astrophysics</i> 439, 1205-1213.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Batygin K, Laughlin G.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">
(2015) Jupiter’s decisive role in the inner Solar System’s early evolution. <i style="mso-bidi-font-style: normal;">Proceedings of the National Academy of
Sciences</i> 112, 4214-4217. Abstract:</span><span style="color: black; font-family: "arial" , "sans-serif";"> </span><span style="font-family: "arial" , "sans-serif";"><a href="http://adsabs.harvard.edu/abs/2015PNAS..112.4214B"><span style="color: blue;">2015PNAS..112.4214B</span></a>
<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Brasser R</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">, Matsumura
S, Ida S, Mojzsis SJ, Werner SC. (2016) Analysis of terrestrial planet
formation by the Grand Tack model: System architecture and tack location. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 821, 75.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Chatterjee S, Tan JC.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">
(2014) Inside-out planet formation <i style="mso-bidi-font-style: normal;">Astrophysical
Journal</i> 780, 53.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Izidoro A</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">, Raymond S,
Morbidelli A, Hersant F, Pierens A. (2015) Gas giant planets as dynamical
barriers to inward-migrating Super-Earths. <i style="mso-bidi-font-style: normal;">Astrophysical
Journal Letters </i>800, L22. <o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Morbidelli A</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">, Bitsch B,
Crida A, Gounelle M, Guillot T, Jacobson S, Johansen A, Lambrechts M, Lega E.
(2016) Fossilized condensation lines in the Solar System protoplanetary disk. <i style="mso-bidi-font-style: normal;">Icarus</i> 267, 368-376.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Pierens A, Raymond SN</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">.
(2011) Two phase, inward-then-outward migration of Jupiter and Saturn in the
gaseous solar nebula. <i style="mso-bidi-font-style: normal;">Astronomy &
Astrophysics</i> 533, A131. Abstract:</span><span style="font-family: "arial" , "sans-serif";"> <a href="http://adsabs.harvard.edu/abs/2011A%26A...533A.131P"><span style="color: blue;">2011A&A...533A.131P</span></a><o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif";">Raymond SN</span></b><span style="color: black; font-family: "arial" , "sans-serif";">, Izidoro A, Bitsch B,
Jacobson SA. (2016) Did Jupiter’s core form in the innermost parts of the Sun’s
protoplanetary disk? <i style="mso-bidi-font-style: normal;">Monthly Notices of
the Royal Astronomical Society</i> 458, 2962-2972. Abstract: </span><span style="font-family: "arial" , "sans-serif";"><a href="http://adsabs.harvard.edu/abs/2016MNRAS.458.2962R"><span style="color: blue;">2016MNRAS.458.2962R</span></a><span style="color: black;"><o:p></o:p></span></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Volk K, Gladman B</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">. (2015) Consolidating and crushing exoplanets: Did it
happen here? <i style="mso-bidi-font-style: normal;">Astrophysical Journal
Letters</i>, 806: L26. Abstract: </span><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;"><a href="http://adsabs.harvard.edu/abs/2015ApJ...806L..26V"><span style="color: blue; font-size: small;">2015ApJ...806L..26V</span></a><span style="color: black;"><o:p></o:p></span></span><br />
<br />
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><o:p> </o:p></span></div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-11060455003336526972016-07-10T00:46:00.003-07:002016-07-10T00:49:02.109-07:00HIP 41378: A Compact Planet Sampler<br />
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<a href="https://1.bp.blogspot.com/-kRXwAs7CMK0/V4H8mS6528I/AAAAAAAAA10/3-V56dDCyjAhX0uV1XrE-YBo5YEfSmRdACLcB/s1600/HIP%2B41378%2Bsystem%2Barchitecture.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="602" src="https://1.bp.blogspot.com/-kRXwAs7CMK0/V4H8mS6528I/AAAAAAAAA10/3-V56dDCyjAhX0uV1XrE-YBo5YEfSmRdACLcB/s640/HIP%2B41378%2Bsystem%2Barchitecture.gif" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="font-family: "arial" , "sans-serif";">Figure 1.</span></b><span style="font-family: "arial" , "sans-serif";"> Candidate planets of
<b style="mso-bidi-font-weight: normal;">HIP 41378</b> represented at their
relative sizes (radii from Vanderburg et al. 2016). Planets <b style="mso-bidi-font-weight: normal;">b</b> and <b style="mso-bidi-font-weight: normal;">c</b> are the only objects with formal validation. The orbital periods
of the three outer candidates remain uncertain, and the discovery team provided
no data on semimajor axes. Nor could they establish whether the period of candidate
<b style="mso-bidi-font-weight: normal;">e</b> is longer or shorter than that of
candidate <b style="mso-bidi-font-weight: normal;">d</b>. </span></div>
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<span style="font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="font-size: small;">-------------------<o:p></o:p></span></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The ongoing </span><a href="http://backalleyastronomy.blogspot.com/2016/07/k2-and-tweens.html"><span style="color: blue; font-size: large;">K2
Mission</span></a><span style="font-size: large;"> can’t compare with the original </span><a href="https://en.wikipedia.org/wiki/Kepler_(spacecraft)"><span style="color: blue; font-size: large;">Kepler Mission</span></a><span style="font-size: large;"> in
terms of the sheer number of planet candidates identified. Nevertheless, some K2
discoveries rival the astrophysical interest of Kepler’s crown jewels. Last
year’s game-changing results from </span><a href="http://backalleyastronomy.blogspot.com/2015/08/the-blazing-wasp-47s.html"><span style="color: blue; font-size: large;">WASP-47</span></a><span style="font-size: large;">
have been </span><a href="http://backalleyastronomy.blogspot.com/2015/12/evolutionary-twist.html"><span style="color: blue; font-size: large;">discussed</span></a><span style="font-size: large;">
</span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="color: blue; font-size: large;">a
few times</span></a><span style="font-size: large;"> in this blog. A few weeks ago the world received advance notice
of another amazing K2 find: a rich, high-multiplicity, mixed-mass system of
five planets transiting <b style="mso-bidi-font-weight: normal;">HIP 41378</b>, described
in a recent preprint by Andrew Vanderburg & colleagues (<b style="mso-bidi-font-weight: normal;">Figure 1</b>).</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">As the discovery team notes at the outset, “K2 is not as
sensitive to planetary systems with complex architectures as the original <i style="mso-bidi-font-style: normal;">Kepler</i> mission” (Vanderburg et al.
2016). That’s because K2 can observe a given star for a maximum of
approximately 80 days, instead of the three-plus years of continuous staring
enabled by Kepler. But in the case of HIP 41378, a happy coincidence enabled
the detection of five different companions within the available period of data
collection (<b style="mso-bidi-font-weight: normal;">Figure 2</b>). Two of them have
repeating transits and the other three have a single transit each.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure
2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Light curves for five transiting planet candidates around HIP 41378<o:p></o:p></span></span><br />
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<a href="https://3.bp.blogspot.com/-BSYhlmvTNRg/V4H8uEzp2xI/AAAAAAAAA14/qWcQtpyefrojdQfNLYl4YA2elTnVvwjPgCLcB/s1600/HIP%2B41378%2BLight%2BCurves%2B-%2BVanderburg%2B2016.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="376" src="https://3.bp.blogspot.com/-BSYhlmvTNRg/V4H8uEzp2xI/AAAAAAAAA14/qWcQtpyefrojdQfNLYl4YA2elTnVvwjPgCLcB/s640/HIP%2B41378%2BLight%2BCurves%2B-%2BVanderburg%2B2016.gif" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Phase-folded light curves for each of the
candidate planets in the HIP 41378 system. Adapted from Figure 2 in Vanderburg
& al. 2016.</span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">A few definitions are in order. “High-multiplicity” is
now in general use as a descriptor for systems with three or more planets.
“Mixed-mass” is (I believe) my own coinage, referring to systems with at least
one low-mass planet (an object less massive than about 50 Earth masses [50 Mea]
with a bulk composition dominated by heavy elements) and one gas giant planet (an
object of 50 Mea or more with a bulk composition dominated by hydrogen/helium).
“Rich mixed-mass” is my own concept, referring to mixed-mass systems containing
a minimum of two low-mass planets. (According to my terminology, then, all
“rich mixed-mass” systems are automatically “high-multiplicity.”)</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Note that all three of these descriptors apply to our
Solar System. To date, however, fewer than 20 exoplanetary systems – among the
many thousands now known – can be described as “rich mixed-mass.” HIP 41378
adds to their number.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Among systems published to date, the one that most
closely resembles HIP 41378 is our friend </span><a href="http://backalleyastronomy.blogspot.com/2014/02/a-new-design-for-planetary-systems.html"><span style="color: blue; font-size: large;">Kepler-90</span></a><span style="font-size: large;">,
which supports an astonishing ensemble of seven planets ranging in radius from 1.2
Rea (Earth-like) to 11.3 Rea (Jupiter-like) on orbits shorter than a single
Earth year.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Before cueing the Hallelujah Chorus, however, we should
be aware of the limits of the K2 data. As the investigators concede, only two
of the candidate planets around HIP 41378 have been formally validated by
analysis of at least two transits. These two are HIP 41378 <b style="mso-bidi-font-weight: normal;">b</b> and <b style="mso-bidi-font-weight: normal;">c</b>, with respective
orbital periods of 15.6 days and 31.7 days. For the other three candidates,
Vanderburg & colleagues used the duration of each recorded transit to distinguish
among them. They applied the letters <b style="mso-bidi-font-weight: normal;">d</b>
through <b style="mso-bidi-font-weight: normal;">f</b> to designate objects with
increasingly longer transit times and (potentially) longer orbital periods.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The investigators then calculated the probability that
one or more of the candidates with a single transit were false positive
detections. Using the K2 dataset, they found that a system with five transiting
planets was 100 million times more likely than a system with two planets and
three false positives. But they also found that the five-planet model was only
200 times more likely than a model with four planets and one false positive. Thus,
the five-planet interpretation is probable but hardly certain.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">The SIMBAD database assigns a spectral type of G1 IV to
the host star, HIP 41378. The investigators prefer to describe it as a
“slightly evolved late F-type star.” Its mass is 1.15 times Solar (1.15
Msol),<span style="mso-spacerun: yes;"> </span>similar to Kepler-90 at 1.13
Msol. Its metallicity (endowment of elements heavier than hydrogen) is -0.11,
meaning that the star is less enriched in metals than our Sun. Its distance is
estimated at 116 parsecs, much nearer than most systems in the Kepler sample. Nevertheless,
HIP 41378 still lies </span><span class="MsoHyperlink"><a href="http://backalleyastronomy.blogspot.com/2016/03/the-nearest-20-parsecs.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue;">outside the region</span></span></a></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> where
radial velocity searches have typically been able to identify low-mass planets
in the Super Earth range.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Vanderburg & colleagues argue that candidates HIP 41378
<b style="mso-bidi-font-weight: normal;">d</b> through <b style="mso-bidi-font-weight: normal;">f</b> have likely orbital periods exceeding 100 days. Their best guess for
each period appears in <b style="mso-bidi-font-weight: normal;">Table 1</b>. For
simplicity, both the table and <b style="mso-bidi-font-weight: normal;">Figure 1</b>
depict the five planets in alphabetical order, since the confidence intervals
for the period of planet <b style="mso-bidi-font-weight: normal;">d</b> overlap
with those for planet <b style="mso-bidi-font-weight: normal;">e</b>. Luckily, the
estimates for planetary radii are more firmly grounded in transit data.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> HIP 41378 system parameters</span></span></div>
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<a href="https://2.bp.blogspot.com/-5mbw8qvhpS0/V4H83nHFyJI/AAAAAAAAA18/RAky9tXWJHQqvLGz0c2fnqdWa_08DG50wCLcB/s1600/HIP-41378-system-parameters.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="162" src="https://2.bp.blogspot.com/-5mbw8qvhpS0/V4H83nHFyJI/AAAAAAAAA18/RAky9tXWJHQqvLGz0c2fnqdWa_08DG50wCLcB/s640/HIP-41378-system-parameters.gif" width="640" /></a></div>
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<i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Tags:</span></i><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"> Radius = planet radius
in Earth units (Rea); Preferred Period = likely orbital period in days; Period
Range = likely range in days of orbital period.<o:p></o:p></span></div>
<div style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">-------------------</span></div>
<div style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><o:p></o:p></span> </div>
<div style="text-align: left;">
<span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;"><span style="font-size: large;">According to comparisons with transiting
planets that have well-constrained mass estimates, planets <b style="mso-bidi-font-weight: normal;">b</b> and <b style="mso-bidi-font-weight: normal;">c</b> have likely
masses in the range of 2 to 10 Mea; planet <b style="mso-bidi-font-weight: normal;">d</b>
is in the range of 5 to 26 Mea; planet <b style="mso-bidi-font-weight: normal;">e</b>
is a </span><a href="http://backalleyastronomy.blogspot.com/2016/07/k2-and-tweens.html"><span style="color: blue; font-size: large;">tween</span></a><span style="font-size: large;">
in the range of 15 to 40 Mea; and planet <b style="mso-bidi-font-weight: normal;">f</b>
is a gas giant in the range of 63 Mea (the mass of HAT-P-18b) to 318 Mea (the
mass of Jupiter). Given their ample radii, even the smallest of these planets
must have hydrogen/helium envelopes.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;"><span style="font-size: large;">Despite the absence of purely rocky
planets, HIP 41378 counts as a rich mixed-mass system. Its assortment of radii
likely corresponds to an assortment of planetary species and bulk compositions.
Like Kepler-90, it resembles a compact planet sampler (<b style="mso-bidi-font-weight: normal;">Figure 3</b>), offering a planet population almost as diverse as our
own Solar System within a space comparable to the orbital radius of the Earth.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: Arial; mso-themecolor: text1;"><span style="font-size: large;">Vanderburg & colleagues note that
the system’s host star is bright enough for high-precision radial velocity
measurements. Such data could confirm the reality of the transiting candidates
and constrain the mass and orbital period of HIP 41378 f, and potentially of
the smaller planets as well. The information could also help us understand the
formation process of systems like HIP 41378 and our own Solar System. <o:p></o:p></span></span></div>
<a href="https://2.bp.blogspot.com/-7yCytMhzScA/V4H9CZhSDaI/AAAAAAAAA2A/2n6yTkfvob4OStwm5kqWumHxdJG4DC9KgCLcB/s1600/saratoga-box-of-chocolates.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="442" src="https://2.bp.blogspot.com/-7yCytMhzScA/V4H9CZhSDaI/AAAAAAAAA2A/2n6yTkfvob4OStwm5kqWumHxdJG4DC9KgCLcB/s640/saratoga-box-of-chocolates.jpg" width="640" /></a><br />
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Figure 3.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><span style="font-size: small;"> A
chocolate candy sampler<o:p></o:p></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p> </o:p></span></div>
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here for a permanent index by topic<br />of blog posts</span><span style="color: blue;">
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<b><span style="color: black; mso-themecolor: text1;">REFERENCE<o:p></o:p></span></b></div>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; mso-themecolor: text1;">Andrew Vanderburg</span></b><span style="color: black; mso-themecolor: text1;">, Juliette C. Becker,
Martti H. Kristiansen, Allyson Bieryla, Dmitry A. Duev, Rebecca Jensen-Clem,
Timothy D. Morton, David W. Latham, Fred C. Adams, Christoph Baranec, Perry
Berlind, Michael L. Calkins, Gilbert A. Esquerdo, Shrinivas Kulkarni, Nicholas
M. Law, Reed Riddle, Maissa Salama, Allan R. Schmitt. Five Planets Transiting a
Ninth Magnitude Star. <i style="mso-bidi-font-style: normal;">Astrophysical
Journal Letters</i>, in press. Abstract: </span><a href="http://adsabs.harvard.edu/abs/2016arXiv160608441V"><span style="color: blue;">2016arXiv160608441V</span></a><u><span style="color: blue; font-size: 10pt;"><o:p></o:p></span></u><br />
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<u><span style="color: blue; font-size: 10pt;"><o:p><span style="text-decoration: none;"> </span></o:p></span></u></div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-53181544250699092352016-07-04T16:06:00.000-07:002016-07-23T13:50:21.574-07:00K2 and the Tweens<br />
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<a href="https://2.bp.blogspot.com/-5ZAv75JbLxM/V3rrJHai89I/AAAAAAAAA1Q/9z6OVtKutIQtkS8XvmyugxorzzYdcfiWgCLcB/s1600/K2-and-the-Tweens.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="264" src="https://2.bp.blogspot.com/-5ZAv75JbLxM/V3rrJHai89I/AAAAAAAAA1Q/9z6OVtKutIQtkS8XvmyugxorzzYdcfiWgCLcB/s640/K2-and-the-Tweens.jpg" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Selected low-mass
planets with measured masses that have radii between those of Uranus and
Saturn. </span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------<o:p></o:p></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The Kepler space telescope was launched in 2009 on an
ambitious mission to find Earth-like planets transiting stars in deep space.
Initial results started arriving in 2010, and by 2011 it was clear that Kepler
had revolutionized our understanding of exoplanets. In 2013, Kepler scientists
began publicizing their best candidates for </span></span><a href="http://backalleyastronomy.blogspot.com/2013/04/holy-grail-earthman.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">potentially Earth-like planets</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">. Then,
just one month after the first such announcement, the spacecraft’s </span></span><a href="http://backalleyastronomy.blogspot.com/2013/05/alas-kepler.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">pointing system malfunctioned</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">, ending
the original mission.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Unlike the Hubble Space Telescope, which is an artificial
satellite orbiting the Earth at a low altitude, the Kepler spacecraft follows
an independent orbit around the Sun. Thus no rescue operation was possible.
Nonetheless, the astronomical community responded to Kepler’s mishap by
applying the same ingenuity that they devoted to the initial problem of
identifying Earth-like planets around distant stars.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Kepler originally relied on four reaction wheels to aim
its grid of light sensors with the exquisite precision needed for continuous
observation of the many thousands of stars selected for study. Although one reaction
wheel broke down early in the mission, the spacecraft was designed to function
with three, so the search for transiting planets continued. It was the
breakdown of the second reaction wheel that terminated the telescope’s capacity
for precision pointing.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Astronomers determined that Kepler could still observe
stars whose sky-projected locations coincided with the </span><span style="font-family: "georgia" , "serif";"><a href="https://en.wikipedia.org/wiki/Ecliptic"><span style="color: blue;">ecliptic</span></a><span style="color: black; mso-themecolor: text1;"> – that is, the plane of the Earth’s
orbit around the Sun. Gravity combined with the pressure of Solar radiation
could keep the sensors pointed for observing runs lasting a maximum of about 80
days. At the end of each run, the spacecraft’s thrusters could be fired to reposition
the field of view (Howell & al. 2014).</span></span></span></div>
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<span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;"><span style="font-size: large;"></span></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;"></span></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">This plan was nicknamed “K2” or “Second Light” (Figure
2). It went into effect early in 2014, and the Kepler telescope has continued
to find new planets ever since. Given finite fuel supplies and the need to fire
thrusters periodically, K2 has a limited lifetime.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure
2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Overview of the K2 Mission</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-wlMr3eFVVy8/V3rrXxdrLsI/AAAAAAAAA1U/WyzFL70yX9IQqZ6YtKNTVEQCJZEAWOyDQCLcB/s1600/KeplerSecondLight-K2-Explained-NASA.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="576" src="https://3.bp.blogspot.com/-wlMr3eFVVy8/V3rrXxdrLsI/AAAAAAAAA1U/WyzFL70yX9IQqZ6YtKNTVEQCJZEAWOyDQCLcB/s640/KeplerSecondLight-K2-Explained-NASA.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">As of
early July, K2 has unveiled 44 new planets, including four new Hot Jupiters and
a few dozen low-mass planets. Some are members of multiplanet systems. K2 has
also contributed new data on a handful of previously discovered Hot Jupiters. Many
results have been spectacular, such as the discovery of <b style="mso-bidi-font-weight: normal;"><a href="http://backalleyastronomy.blogspot.com/2015/08/the-blazing-wasp-47s.html"><span style="color: blue;">WASP-47d<span style="font-weight: normal;"> and </span></span><span style="color: blue;">e</span></a></b>, two low-mass planets in a
previously known Hot Jupiter system; </span></span><a href="http://backalleyastronomy.blogspot.com/2016/01/a-data-driven-year.html"><b style="mso-bidi-font-weight: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">K2-25b</span></span></b></a><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">, the </span><span style="font-family: "georgia" , "serif";">first small planet<span style="color: black; mso-themecolor: text1;"> ever identified in a star cluster (the Hyades, our
next-door neighbor in the Orion Spur); </span></span></span><a href="http://adsabs.harvard.edu/abs/2016arXiv160606729D"><b style="mso-bidi-font-weight: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">K2-33b</span></span></b></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">, a puffy
newborn planet orbiting an M dwarf in the nearby Upper Scorpius star-forming
region; and </span></span><a href="http://adsabs.harvard.edu/abs/2016arXiv160608441V"><b style="mso-bidi-font-weight: normal;"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">HIP
41378</span></span></b></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">, a compact mixed-mass system orbiting a bright F star.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">One
bonus associated with K2 is that the stars it observes are generally closer
than those targeted by Kepler. Their proximity makes it easier to collect
follow-up data with other instruments. In the </span><a href="http://backalleyastronomy.blogspot.com/2016/05/1284-new-kepler-planets-none-like-earth.html"><span style="color: blue; font-size: large;">most
recent release</span></a><span style="font-size: large;"> of Kepler findings, 76% of Kepler host stars have distance
estimates, with a median of 772 parsecs. A similar percentage of K2 host stars (71%)
also have distance estimates, with a much smaller median of 183 parsecs. (All
population-level data on K2 planets were retrieved from the Table of Confirmed
K2 planets at </span></span><a href="http://kepler.nasa.gov/Mission/discoveries/"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">http://kepler.nasa.gov/Mission/discoveries/</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">.)</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Unfortunately,
K2 doesn’t seem very sensitive to rocky planets like Earth, defined as those
with radii of 1.5 Earth units (1.5 Rea) or less. Whereas 25% of planets in the
full Kepler sample have radii in this range, only one K2 planet does (K2-19d).
Similarly, the median radius among Kepler planets is 2.15 Rea, notably smaller
than the median of 2.59 Rea for the K2 sample. <em>(Check out the postscript at the end for a more recent perspective.)</em></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The silver lining in this cloud is the trove of K2 <b style="mso-bidi-font-weight: normal;">tweens</b>, which I define as planets with
radii between those of Uranus (4 Rea) and Saturn (9.4 Rea). Objects of this
size are unknown in our Solar System and relatively rare in the Kepler sample (7%
of the total). To date, however, K2 has returned 10 such exoplanets,
representing almost 25% of the total. Seven out of 10 have mass measurements,
obtained by analysis of transit timing variations (TTVs) or radial velocity (RV)
observations or both. These discoveries make a valuable contribution to the
field of planetology, since the blurry boundary between the two major planetary
species – low-mass planets and gas giants – falls somewhere between Uranus and
Saturn.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Although everyone agrees that Jupiter and Saturn are gas
giant planets, the exact definition varies somewhat from source to source. Here’s
the one I find most appropriate: A gas giant planet is an object whose bulk
composition is more than 50% hydrogen and helium. The range of masses among
extrasolar giants is approximately 60 to 4000 Mea, corresponding to about 0.20
to 13 Jupiter masses (Mjup). The median mass is about 1.8 Mjup. However, the
precise upper and lower mass boundaries of the mass distribution are uncertain.
Some authors have placed the lower boundary as low as 30 Mea (0.10 Mjup), while
others have placed the upper boundary as high as 25 Mjup.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 1</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> shows three
well-characterized K2 tweens alongside four better-characterized planets in our
Solar System and its immediate neighborhood. All the objects pictured have
hydrogen/helium atmospheres, but the proportion of heavy elements (rock, metal,
and ice) varies substantially.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Twenty
Transiting Tweens with Measured Masses<o:p></o:p></span></span></div>
<a href="https://3.bp.blogspot.com/-fsQW2X2rOyg/V3rrhelRHlI/AAAAAAAAA1Y/7IIK2UllBCQifGb5uiK-tzdCTaRZQ6LVQCLcB/s1600/Transiting%2BTweens%2Bwith%2BMeasured%2BMasses.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="524" src="https://3.bp.blogspot.com/-fsQW2X2rOyg/V3rrhelRHlI/AAAAAAAAA1Y/7IIK2UllBCQifGb5uiK-tzdCTaRZQ6LVQCLcB/s640/Transiting%2BTweens%2Bwith%2BMeasured%2BMasses.gif" width="640" /></a><br />
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">*Several conflicting masses have been
reported for K2-19b and c. Mass estimates for the K2 planets are generally less
secure than those available for the other exoplanets of this selection. <o:p></o:p></span></div>
<i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Tags</span></i><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">: Rea = planet radius in
Earth units; Mea = planet mass in Earth units; H/He = planet mass fraction
attributed to hydrogen/helium; Period = planet orbital period in days; a =
planet semimajor axis in Earth units; Msol = star mass in Solar units; Dist. =
system distance in parsecs (1 parsec = 3.26 light years)</span><br />
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------------<o:p></o:p></span></div>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> offers
a larger and more representative sample of tweens selected from various
populations: the Kepler catalog, which encompasses a prominent subset of
circumbinary tweens; ground-based transit searches, sometimes supplemented by
space-based observations; the CoRoT catalog; and the current K2 sample. As
available, the table includes an estimate of the mass fraction attributable to the
hydrogen/helium (H/He) envelope around each planet (Dodson-Robinson &
Bodenheimer 2009, Doyle & al. 2011, Cochran & al. 2011, Welsh & al.
2012, Lopez & Fortney 2014, Bakos & al. 2015, Petigura & al. 2016).</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The criterion for inclusion in this table is the
availability of a mass estimate. Nevertheless, some estimates are more robust
than others. For example, the five nearest tweens have more precise RV data
than any of the K2 tweens.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The K2-19 system in particular presents a unique conundrum.
Two of its three planets (<b style="mso-bidi-font-weight: normal;">b</b> and <b style="mso-bidi-font-weight: normal;">c</b>) orbit near a 3:2 mean motion
resonance, which produces significant TTVs, while the host star is bright
enough to permit ground-based RV studies. This happy coincidence might be
expected to yield highly precise results for the masses of both planets. However,
that’s not how things have turned out. Through an analysis of TTVs, Barros
& al. (2015) estimated masses of 44 Mea for K2-19b and 15.9 Mea for K2-19c.
In another TTV study, Narita & al. (2015) could not determine a mass for
K2-19b, but they found a value of about 20 Mea for K2-19c, formally consistent
with the results of Barros & al. A subsequent RV study by Dai & al.
(2016) found 28.5 Mea for K2-19b and 25.6 Mea for K2-19c. Finally, a study by
Nespral & al. (2016) obtained new RV data indicating a mass of 71.7 Mea for
K2-19b, but K2-19c was undetectable in their dataset. The authors followed up
with a TTV analysis that found a mass of 57.7 Mea for K2-19b and 9.4 Mea for
K2-19c.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">The conflicting results of these four studies give me
pause: it looks like less data is better than more! We can keep pursuing our
dreams of illumination by distant stars only if we’re ready to shrug off a lot
of troublesome details.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Having done so, let’s look again at the tweens in Table 1.
First, the bulk mass fraction of H/He appears to increase along with increasing
radius, consistent with the hypotheses of Lopez & Fortney (2014). Second, overall
mass also tends to increase with radius, although the trend is less consistent.
Third, the two tweens with the largest radii – Kepler-16b and Kepler-34b – come
close to fulfilling my definition of a gas giant, since they have H/He
fractions in the vicinity of 50%, and they fall in the expected mass range for
giant planets (both in excess of 60 Mea).</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">However, this neat picture is complicated by predictions
for the internal structure of the four most massive tweens in the table. Although
CoRoT-8b, HD 149026 b, Kepler-16b, and Kepler-34b have larger endowments of
H/He than most of the other tweens, their envelope fractions are still much
smaller than the norm for well-constrained gas giant planets, which range from 75%
to 95%. Given estimated heavy element masses of 30 to 60 Mea, each of these big
tweens could have contributed solid cores to as many as six planets like Jupiter
and Saturn.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">super-puff
tweens</span></span></b><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Note that my selection in Table 1 is biased in favor of RV
data. Many studies have found that mass estimates derived from RV measurements
are systematically larger than those based on TTV analyses. Furthermore, for
systems where both TTVs and RV observations are available, the two methods
frequently produce widely divergent results. The exceptions are those rare cases
with data of extremely high precision.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">When we turn to the sample of tweens for which TTVs
provide the only evidence of mass, we find unexpectedly lightweight objects.
Such tweens have been nicknamed “super-puffs.” The classic example is </span><a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html"><span style="color: blue; font-size: large;">Kepler-11</span></a><span style="font-size: large;">,
whose radius (4.18 Rea) is about 5% larger than Neptune’s, while its TTV mass
(8.4 Mea) is less than 50% of Neptune’s. Lopez & Fortney (2014) calculate
its mass fraction of H/He as 15%. The four well-constrained tweens in Table 1
with similar mass fractions (GJ 3470 b, GJ 436 b, HAT-P-11b, HATS-7b) are
notably more massive, with values in the range of 13 to 38 Mea.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Another multiplanet system, Kepler-223, rivals the
compact structure and extensive TTVs revealed in Kepler-11. The four planets of
Kepler-223 have orbital periods ranging from 7 to 20 days, arranged in an
interlocking pattern of resonances: planets <b style="mso-bidi-font-weight: normal;">b</b> and <b style="mso-bidi-font-weight: normal;">d</b> orbit very close
to a 2:1 mean motion resonance, as do planets <b style="mso-bidi-font-weight: normal;">c</b> and <b style="mso-bidi-font-weight: normal;">e</b> (Mills & al.
2016). TTV analyses yield a narrow selection of masses for these four, ranging
from 4.8 Mea for planet <b style="mso-bidi-font-weight: normal;">e</b> to 8 Mea
for planet <b style="mso-bidi-font-weight: normal;">d</b>. The two bookends of
this range also happen to fit my definition of tweens: Kepler-223d has a radius
of 5.24 Rea and Kepler-223e of 4.6 Rea. Although Mills & al. do not
calculate the H/He mass fraction for these objects, they must surpass our
estimates for Uranus and Neptune.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Despite their status as outliers, the four super-puffs orbiting
Kepler-223 are easier to explain than the four massive tweens discussed in the
previous section. Considerable theoretical work has established that planets as
lightweight as 2 Mea can accrete H/He envelopes from their native nebulae and retain
them against stripping by stellar flux, as long as their obits are cool enough.
Still larger core masses can withstand still higher levels of irradiation,
maintaining extended atmospheres over the main sequence lifetimes of their host
stars. The real problem is to account for musclebound tweens like CoRoT-8b and HD
149026 b.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">why
tweens but not giants?</span></span></b></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">As Erik Petigura & colleagues recently noted (2016),
the K2 tweens and their cousins in other extrasolar catalogs raise critical
questions about planet formation. How can these objects assemble such massive heavy
element cores and deep H/He envelopes without undergoing runaway accretion –
the pathway that leads to the birth of a <i style="mso-bidi-font-style: normal;">bona
fide</i> gas giant?</span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Before Kepler, we already knew that the mass range for planets
dominated by H/He extended as low as 60 Mea. HAT-P-18b, with a mass of only 63
Mea, is a twin of Jupiter in radius, requiring a highly inflated H/He envelope.
Similarly, HAT-P-12b has a radius of 10.8 Rea (about 4% smaller than Jupiter’s)
and a slightly larger mass of 67 Mea, requiring an envelope almost as deep.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">Petigura & colleagues suggest that puffy tweens (and
presumably puffy gas giants like these two HATs) formed far beyond their
systems’ ice line, where extensive envelopes can be accreted quickly and
easily, and then migrated inward to their present orbits. Ironically, this is similar to the scenario that Dodson-Robinson & Bodenheimer (2009) proposed some years
earlier to account for the opposite problem – the anomalously massive core of another
tween, HD 149026 b. <br />In both cases, long-range migration has been invoked: through a gas-rich nebula to form tweens with unusually deep H/He envelopes, and through a metal-rich nebula to form tweens with unusually large heavy-element cores.</span></span></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">I suspect that the true explanation for these dissimilar
outliers will be more complicated, likely involving the chemical composition of the nebula but also requiring evolving
structures that alternately concentrate mass at particular radial locations and then encourage migration, either inward or outward. Recent
advances in the imaging and analysis of planet-forming nebulae have inspired a
growing number of theoretical studies that are bound to shed light on these
questions.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><br />
<span style="font-size: large;"><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><strong>postscript on July 23, 2016</strong></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">
</span></span></span><div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">This past week,</span><span style="font-family: "georgia" , "serif";"> </span><a href="http://adsabs.harvard.edu/abs/2016arXiv160705263C"><span style="font-family: "georgia" , "serif";"><span style="color: blue;">Ian Crossfield & colleagues</span></span></a><span style="font-family: "georgia" , "serif";"> <span style="color: black; mso-themecolor: text1;">issued a preprint entitled “197 Candidates and 104 Validated Planets in K2’s First Five Fields” (hereafter C16). It upends some of the descriptive statistics on the K2 sample in this blog post, even though I used data retrieved only three weeks ago. C16 offer a table listing key parameters of the planet candidates they identified (their Table 8). It includes 93 confirmed planets, versus NASA’s list of 44 confirmed planets on July 4. Where the two samples overlap, the stellar and planetary parameters listed in C16 are often at odds with the ones offered by NASA, and it’s not clear to me which numbers are preferable.</span></span></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;"></span></span></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;">C16 find 16 planets smaller than 1.5 Rea (meaning that they’re likely to be terrestrial rather than gassy), whereas I identified only one in the recent NASA data. Among those 16 small planets from C16, all but 3 have orbital periods shorter than 10 days, and all but 2 orbit M dwarfs. The 2 proposed terrestrials orbiting Sun-like stars – K2-36b and K2-80c – have periods of 1.42 and 5.61 days, respectively, implying surface conditions that are literally hellish.</span></span></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;"></span></span></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;">I also observed a median orbital period of 8.5 days and a median radius of about 2.6 Rea. C16 find a similar median period (8.6 days), but for radii they present a smaller median of 2.3 Rea. Oddly, my own inspection of their Table 8, using the same approach that I applied to the older NASA data, suggests a median of 2.4 Rea. Regardless of which number is more accurate, the new value is closer to the median in the Kepler sample, which is just 2.15 Rea.</span></span></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;"></span></span></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;">Finally, C16 report 12 planets with radii between Uranus and Saturn, similar to the 10 I found in the NASA selection. Given their larger overall sample, the fraction of tweens amounts to 13% of the total, versus 23% for the NASA sample.</span></span></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;"></span></span></span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;">C16 conclude with a cautionary statement: “L<span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: SFRM1095; mso-fareast-font-family: Calibri; mso-themecolor: text1;">ists of </span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: SFTI1095; mso-fareast-font-family: Calibri; mso-themecolor: text1;">K2 </span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: SFRM1095; mso-fareast-font-family: Calibri; mso-themecolor: text1;">candidates and/or validated planets are not currently suitable for the studies of planetary demographics that </span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: SFTI1095; mso-fareast-font-family: Calibri; mso-themecolor: text1;">Kepler</span></i><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: SFTI1095; mso-fareast-font-family: Calibri; mso-themecolor: text1;"> </span><span style="color: black; font-family: "georgia" , "serif"; mso-bidi-font-family: SFRM1095; mso-fareast-font-family: Calibri; mso-themecolor: text1;">so successfully enabled.” I am duly chastened.</span> <o:p></o:p> <span style="mso-spacerun: yes;"></span></span></span></span></span></span></div>
<br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><div style="text-align: center;">
<a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"><span style="font-size: large;"><span style="color: blue;">Click here for a permanent index by topic of blog posts</span><span style="color: blue;"> on Back Alley Astronomy</span></span></span></i></a><span style="font-size: large;"><span style="font-family: "georgia" , "serif";"></span></span></div>
</span></div>
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<b><span style="color: black; font-size: 10pt;"></span></b><br />
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509-512.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Narita N</span></b><span style="color: black; font-size: 10pt;">, Hirano T, Fukui A, Hori Y, Sanchis-Ojeda R, Winn JN, et al. (2015)
Characterization of the K2-19 multiple-transiting planetary system via
high-dispersion spectroscopy, AO imaging, and transit timing variations. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 815, 47. Abstract:
</span><a href="http://adsabs.harvard.edu/abs/2015ApJ...815...47N"><span style="font-size: 10pt;"><span style="color: blue;">2015ApJ...815...47N</span></span></a><span style="font-size: 10pt; mso-fareast-font-family: Calibri;"><o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Nespral D</span></b><span style="color: black; font-size: 10pt;">, Gandolfi D, Deeg HJ, Borsato L, Fridlund MCV, Barragán O, et al. (2016)
<span style="mso-bidi-font-weight: bold;">Mass determination of K2-19b and K2-19c
from radial velocities and transit timing variations. In press.<o:p></o:p></span></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Petigura EA</span></b><span style="color: black; font-size: 10pt;">, Howard AW, Lopez ED, Deck KM, Fulton BJ, Crossfield IJM, et al. (2016)
Two transiting low density sub-Saturns from K2. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 818, 36. Abstract:</span><span style="font-size: 10pt; mso-fareast-font-family: Calibri;"> </span><a href="http://adsabs.harvard.edu/abs/2016ApJ...818...36P"><span style="font-size: 10pt;"><span style="color: blue;">2016ApJ...818...36P</span></span></a><span style="font-size: 10pt;"><o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Van Eylen V</span></b><span style="color: black; font-size: 10pt;">, Nowak G, Albrecht S, Palle E, Ribas I, Bruntt H, Perger M, Gandolfi D,
Hirano T, Sanchis-Ojeda R, Kiilerich A, Arranz JP, Badenas M, Dai F, Deeg HJ,
Guenther EW, Montanes-Rodriguez P, Narita N, Rogers LA, Bejar VJS, Shrotriya TS,
Winn JN, Sebastian D. (2016) The K2-ESPRINT Project II: Spectroscopic follow-up
of three exoplanet systems from Campaign 1 of K2. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 820, 56. Abstract: </span><a href="http://adsabs.harvard.edu/abs/2016ApJ...820...56V"><span style="font-size: 10pt;"><span style="color: blue;">2016ApJ...820...56V</span></span></a><span style="font-size: 10pt; mso-fareast-font-family: Calibri;"><o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Welsh WF</span></b><span style="color: black; font-size: 10pt;">, Orosz JA, Carter JA, Fabrycky DC, Ford EB,
Lissauer JJ, et al. (2012) Transiting circumbinary planets Kepler-34 b and
Kepler-35 b. <i style="mso-bidi-font-style: normal;">Nature</i> 481, 475-479. <o:p></o:p></span><br />
<br />
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<span style="font-size: 10pt;"><o:p> </o:p></span></div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-9476945755187296852016-05-25T22:14:00.003-07:002016-05-26T10:50:49.964-07:001,284 New Kepler Planets, None Like Earth<br />
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<a href="https://2.bp.blogspot.com/-k7Pk8BgdSfw/V0aEozFJXdI/AAAAAAAAA0o/Y4uFgcGFueQKi4JUaNE7MR2gXFh1cF_JgCLcB/s1600/Candidate%2B%2526%2Bconfirmed%2BKeplers%2B-%2BAfter%2BMorton%2B2016.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="434" src="https://2.bp.blogspot.com/-k7Pk8BgdSfw/V0aEozFJXdI/AAAAAAAAA0o/Y4uFgcGFueQKi4JUaNE7MR2gXFh1cF_JgCLcB/s640/Candidate%2B%2526%2Bconfirmed%2BKeplers%2B-%2BAfter%2BMorton%2B2016.gif" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"> Candidate
and confirmed planets in the final Kepler dataset. This image is based on
Figure 7 in Morton et al. (2016), with the addition of the red box outlining
the parameter space occupied by Earth-like planets (radii 0.7-1.4 Earth units
or Rea) orbiting in the habitable zones (periods 100-800 days) of Sun-like
stars (masses 0.7-1.2 Solar units or Msol). <o:p></o:p></span></span></div>
<div style="text-align: center;">
<span style="font-family: "arial" , "helvetica" , sans-serif;">
</span><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">------------------------------<o:p></o:p></span></span></div>
<br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Two weeks
ago, with considerable fanfare, the Kepler team announced the confirmation of
1,284 new transiting planets (Overbye 2016, Kluger 2016). This remarkable feat
was accomplished through an automated analysis of the complete Kepler dataset
(Morton et al. 2016). Following an </span></span><a href="http://backalleyastronomy.blogspot.com/2015/01/much-ado-about-earth-2.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-family: "georgia" , "times new roman" , serif; font-size: large;">established tradition</span></span></a><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">, the
announcement highlighted a selection of small planets that some investigators
might consider somewhat Earth-like. This time around, however, the significance
of the selection was downplayed, since the nine objects singled out for
presentation were described only as candidates that “</span><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif"; mso-fareast-font-family: Calibri;">may</span></i><span style="font-family: "georgia" , "serif"; mso-fareast-font-family: Calibri;"> </span><span style="font-family: "georgia" , "serif"; mso-bidi-font-family: AdvOTf9433e2d; mso-fareast-font-family: Calibri;">fall within
the optimistic habitable zones of their host stars</span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">” (italics
in original). As we’ll see below, that claim is a surprisingly watered-down response
to the Kepler Mission’s original goal, which was to characterize Earth-size
planets on habitable orbits around Sun-like stars.</span></span></span></div>
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<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span> </div>
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<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">All
1,284 new planets were immediately incorporated into the exoplanetary census
maintained by the </span></span><a href="http://exoplanet.eu/"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-family: "georgia" , "times new roman" , serif; font-size: large;">Extrasolar Planets Encyclopaedia</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> (EPE).
Thus, as of today, the count for all exoplanets detected by all search methods stood
at 3,411. In combination with 19 other transiting planets reported earlier this
year, the new data release has doubled the transit population in a span of only
four months. The grand total is now 2600 transiting planets, representing 76%
of all exoplanets detected by any method.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">lonely little planets</span></span></span></b></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Unlike
previous releases of Kepler data, this one is dominated by single-planet
systems. Systems of high multiplicity, defined as those with at least three
planets, are quite scarce. Among 1,284 new detections, 110 occur in 55 newly
announced systems with 2 planets each; 15 occur in 5 new systems with 3 planets
each; and 12 occur in 3 new systems with 4 planets each. In addition, one
planet (Kepler-436c) was found in a system where a single planet was already
known, raising the multiplicity of this system to 2; 14 were found in systems
where 2 planets were known, raising the multiplicity of these 14 systems to 3;
and 9 planets were found in 7 systems where 2 or 3 planets were known, raising
the multiplicity of these 7 systems to 4. No new systems with five or more
planets were identified.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Given such
a modest increase in the population of multiple systems, 87% of the new planets
orbit their stars alone, without any detectable companions. Among all Kepler
planets confirmed since the spacecraft launched (including objects designated EPIC,
KIC, KOI, and K2), singleton planets now comprise 53% of the total. Among all
Kepler planetary systems, 75% contain only one planet, 16% contain two planets,
and 9% contain three or more planets. The latter group represents the
high-multiplicity subsample; only 16 systems in that group contain more than
four detected planets.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">One caveat
is in order for these statistics: several Kepler multiplanet systems contain planets
detected by radial velocity measurements or transit timing variations but not
observed in transit. Therefore, counts and percentages will vary depending on
the sample queried. In addition, most Kepler systems of any multiplicity are
likely to contain additional planets that are undetectable by existing search
methods. Thus the terms “single-planet,” “two-planet,” and so on represent our
state of knowledge about a system rather than the true number of planets it
contains.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Caveats
aside, these new numbers signal a dramatic shift in our perspective on the
demographics of transiting systems. The Kepler sample available as recently as </span></span><a href="http://backalleyastronomy.blogspot.com/2016/03/the-nearest-20-parsecs.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-family: "georgia" , "times new roman" , serif; font-size: large;">March</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> was biased against
singletons, an inescapable product of extant validation methods that relied on
evidence of planetary multiplicity to rule out false positives. By correcting
that bias, the automated approach to validation taken by Morton and colleagues
(2016) has revealed that singletons are just as common in Kepler systems as
they are in the </span></span><a href="http://backalleyastronomy.blogspot.com/2016/03/the-nearest-20-parsecs.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-family: "georgia" , "times new roman" , serif; font-size: large;">Sun’s back yard</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">.</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span> </div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Characteristics
of three exoplanetary populations</span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><a href="https://1.bp.blogspot.com/-VY5jJ0lYbpo/V0aFV2SQsvI/AAAAAAAAA00/P4ApiVOGdn4HyBaT9Ev0xFC6eGzZnfJ-gCLcB/s1600/Three%2BExoplanetary%2BPopulations%2BCompared%2B-%2BMay%2B2016.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="308" src="https://1.bp.blogspot.com/-VY5jJ0lYbpo/V0aFV2SQsvI/AAAAAAAAA00/P4ApiVOGdn4HyBaT9Ev0xFC6eGzZnfJ-gCLcB/s640/Three%2BExoplanetary%2BPopulations%2BCompared%2B-%2BMay%2B2016.gif" width="640" /></a></span></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><br />
<div class="separator" style="clear: both; text-align: left;">
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></div>
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> updates the summary of exoplanetary
demographics originally posted in March. The frequency of selected planetary
and system characteristics is compared across three samples: 1) 129 planets in
73 exoplanetary systems located at a distance of 20 parsecs or less, 2) 707
planets in 606 systems detected by transit or radial velocity searches outside
20 parsecs, excluding Kepler discoveries, and 3) the full Kepler sample of 2342
confirmed planets in 1677 systems, as characterized in a recent query of EPE.</span></span></span><br />
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<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">One
demographic feature hasn’t changed: the full Kepler sample is still dominated
by small planets. The median radius is 2.15 Earth radii (Rea), and 95% are
smaller than 8 Rea, which is the practical cut-off between low-mass objects
like Earth and Uranus and gas giants like Jupiter and Saturn. Fully 25% of
Kepler planets are smaller than 1.5 Rea, another practical cut-off that
approximates the boundary between terrestrial planets and gas dwarfs (that is, planets
with hydrogen/helium envelopes accounting for less than half their mass).</span></span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span> </div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Transiting planets by
radius (N = 2570)<o:p></o:p></span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://1.bp.blogspot.com/-PfmfKsGL3lo/V0aE5LySnMI/AAAAAAAAA04/mCYXOtirGH41pT8FFvGU7lMl-ukpqzlzwCKgB/s1600/Radii%2Bof%2B2570%2BTransiting%2BPlanets%2B-%2BMay%2B13%252C%2B2016.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="344" src="https://1.bp.blogspot.com/-PfmfKsGL3lo/V0aE5LySnMI/AAAAAAAAA04/mCYXOtirGH41pT8FFvGU7lMl-ukpqzlzwCKgB/s640/Radii%2Bof%2B2570%2BTransiting%2BPlanets%2B-%2BMay%2B13%252C%2B2016.gif" width="640" /></a></div>
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
</span><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">Distribution of sizes among 2570 transiting
planets with radii up to 18 times Earth (18 Rea). The letters M, E, U, S, J
indicate the positions of Mars, Earth, Uranus, Saturn, and Jupiter on the same
scale. All data were retrieved from the </span></span><a href="http://exoplanet.eu/"><span style="font-family: "arial" , "sans-serif";"><span style="color: blue; font-family: "arial" , "helvetica" , sans-serif;">Extrasolar Planets
Encyclopaedia </span></span></a><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">on May 13, 2016. Although the recent
confirmation of 1,284 new planets in the Kepler dataset doubled the number of
planets observed in transit, the relative frequency of planetary radii remains
essentially unchanged since the previous Kepler dump in 2014; compare this
graph with the one posted </span></span><a href="http://backalleyastronomy.blogspot.com/2014/03/small-planets-then-all-rest.html"><span style="font-family: "arial" , "sans-serif";"><span style="color: blue; font-family: "arial" , "helvetica" , sans-serif;">two years ago</span></span></a><span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">. <o:p></o:p></span></span><br />
<span style="font-family: "arial" , "helvetica" , sans-serif;">
</span><br />
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; mso-themecolor: text1;"><span style="font-family: "arial" , "helvetica" , sans-serif;">------------------------------<o:p></o:p></span></span></div>
<span style="font-family: "arial" , "helvetica" , sans-serif;">
</span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">medium cool</span></span></span></b><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">That’s
right: one in four Kepler planets is potentially rocky, like Earth or Venus. Such
odds sound intoxicating until we tune into the orbital characteristics of these
small, dense objects (see <b style="mso-bidi-font-weight: normal;">Figures 1 and
2</b>).</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">The vast
majority of planets of 1.5 Rea or less orbit Sun-like stars. Yet their median
semimajor axis is only 0.06 AU, which implies equilibrium temperatures that
range from “infernal” to “Tartarean.” The sunny side of rocky worlds on such
tight orbits will likely be molten. More than 70% of Kepler planets of 1.5 Rea
or less have periods shorter than 10 days, qualifying them as potential
Hellworlds. Only six have periods longer than 100 days. Among the cool six,
just one – our old friend </span></span><a href="http://backalleyastronomy.blogspot.com/2013/04/holy-grail-earthman.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-family: "georgia" , "times new roman" , serif; font-size: large;">Kepler-62f</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"> – occupies
the habitable zone of a Sun-like star.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Given this
background, I’m puzzled by the decision of Morton & colleagues (hereafter
M16) to showcase nine mostly puffy planets in their Table 3. These objects are
described rather oddly as “newly validated planets in the optimistic habitable
zone.”</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Now,
it’s a truism of astrobiology that an orbit in the habitable zone is no
guarantee of habitability. That’s because a planet must have an appropriate </span></span><a href="http://backalleyastronomy.blogspot.com/2015/12/quantifying-earth-like.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-family: "georgia" , "times new roman" , serif; font-size: large;">mass and composition</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">, in
addition to an appropriate level of stellar flux, in order to maintain surface
bodies of water. Only one of the nine planets presented by M16 is smaller than
1.5 Rea: Kepler-1229b, which has an estimated radius of 1.12 Rea. This value is
consistent with a rocky planet of 1.5 Mea, but unfortunately, the planet’s host
star is an M dwarf of only 0.43 Msol.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Several characteristics of young M dwarfs (stellar masses
0.12-0.65 times Solar or Msol) are antagonistic to the formation of habitable
planets. Their luminosities are one to two orders of magnitude higher than
mature stars of the same mass; they emit high levels of extreme ultraviolet
radiation; and they are subject to frequent flaring events (Ramirez &
Kaltenegger 2014, Luger & Barnes 2015). These factors ensure that volatiles
will be stripped from any small, low-mass planets that happen to form in the mature
habitable zone of an M dwarf. The likely outcome of such a “boil-off” will be
desiccated rocks like Venus, unfriendly to the emergence of life. It’s no
coincidence that Kepler’s mission was to seek Earth-size planets around
Sun-like stars, not around M dwarfs.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Nevertheless, certain contingencies – possibly quite rare
– might still permit the formation of habitable planets around red stars.
Imagine a puffy rock/ice planet of a few Earth masses in the mature habitable
zone of a young M dwarf. This world happens to support just enough hydrogen and
water to boil down to the level of an Earth ocean during the first billion
years of system evolution. Thus unveiled, the cooling rocky core could outgas a
new atmosphere and rain down shallow seas where life could begin.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Absent such a history, Kepler-1229b does not meet the
criteria for a </span></span><a href="http://backalleyastronomy.blogspot.com/2015/12/quantifying-earth-like.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-family: "georgia" , "times new roman" , serif; font-size: large;">potentially habitable planet</span></span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">. The
same verdict applies to five other objects selected by M16, which also orbit
host stars with masses ranging from 0.30 Msol to 0.55 Msol. Unlike
Kepler-1229b, these five M dwarf planets are quite bulky, with radii ranging
from 1.56 Rea to 1.97 Rea. Such values imply either blasted monoliths of 6 to
10 Earth masses (Mea) or less massive worlds originally so rich in volatiles
that even the tantrums of a baby red star were insufficient to deplete them.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Only three of the nine planets in the current selection
orbit Sun-like stars. Unfortunately, these three have estimated radii in the
range of 1.70 Rea to 1.98 Rea. Theory indicates that such objects, if composed
entirely of heavy elements, will be too massive for plate tectonics, and thus
not habitable (Unterborn et al. 2016). Both theory and observation suggest that
objects of this size are likely to include a substantial volatile component –
either water or hydrogen or both – around rocky cores (Rogers 2015). Such a composition
would lower their mass, but it would also render them uninhabitable.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Yet M16 embellished their text with happy talk about
“optimistic habitable zones.” This approach evidently convinced the editors of <i style="mso-bidi-font-style: normal;">TIME Magazine</i> that astronomers are
closing in on Earth 2, even though not much has changed in years. Jeffrey
Kluger, the author responsible for <i style="mso-bidi-font-style: normal;">TIME</i>’s
coverage of the new data release, described the nine objects selected by
Morton’s team as “earth-like planets,” and his headline proclaimed that their
detection “Boosts Odds of Life in Space.” Sadly, they’re not, and it doesn’t.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">the
drought continues</span></span></span></b><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">Three years ago, Kepler-62f enjoyed a moment of celebrity
as the Holy Grail of exoplanetary astronomy. That distant world (368
parsecs/1200 light years away) remains the best candidate for an Earth-like
planet orbiting an alien sun. Yet Kepler-62f is clearly a borderline case,
since the planet’s estimated radius of 1.41 Rea falls near the outer limit for
a rocky composition, while the estimate itself depends on limited data.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">I’m still waiting for a thorough assessment that would
explain why Kepler has been unable to detect a significant population of small
rocky planets in the habitable zones of Sun-like stars. Maybe those planets are
really out there, but transit photometry isn’t well suited to detecting them.
Or maybe transit photometry is perfectly fine, but the Kepler Mission ended too
soon to disentangle the faint transit signals predicted for such objects from
other stellar activity. Maybe if the Kepler spacecraft had kept functioning for
three more years, it could have returned enough data to identify dozens of
Earth-like planets.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">But maybe such planets are so rare that the one we happen
to be standing on has very few siblings in the entire Milky Way Galaxy. Maybe
all the others are so far away that twenty-first century technologies can never
find them. I don’t like that possibility, but if it’s the best explanation for
Kepler’s disappointing results, I’d rather know than remain ignorant. <o:p></o:p></span></span></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">
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here for a permanent index by topic of blog posts<br />on Back Alley Astronomy</span></span></span></span></i></a><br />
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p> </o:p></span><b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">REFERENCES<o:p></o:p></span></b></div>
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Kluger J.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> </span><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Kepler Telescope’s New Planets Discovery Boosts Odds for
Life in Space. <i style="mso-bidi-font-style: normal;">TIME</i>, May 10, 2016.</span><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Luger R, Barnes R.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> (2015) Extreme water loss and abiotic O<sub>2</sub>
buildup on planets throughout the habitable zones of M dwarfs. <i style="mso-bidi-font-style: normal;">Astrobiology</i> 15, 119-143. <o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Morton TD</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">, Bryson ST,
Coughlin JL, Rowe JF, Ravichandran G, Petigura EA, Haas MR, Batalha NM. (2016)
False positive probabilities for all Kepler objects of interest: 1284 newly
validated planets and 428 likely false positives. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 822, 86.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Overbye D</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">. Kepler
Finds 1284 New Planets. <i style="mso-bidi-font-style: normal;">New York Times</i>,
May 10, 2016. <o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Rogers L</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">. (2015) Most 1.6 Earth-radius planets are not rocky. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 801, 41.<o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Ramirez RM, Kaltenegger L.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> (2014) The habitable zones of pre-main sequence stars. <i style="mso-bidi-font-style: normal;">Astrophysical Journal Letters</i> 797, L25. <o:p></o:p></span><br />
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Unterborn CT</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">, Dismukes EE, Panero WR. </span><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">(2016)</span><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Scaling the
Earth: A sensitivity analysis of terrestrial exoplanetary interior models. </span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Astrophysical Journal</span></i><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> 819, 32.<o:p></o:p></span><br />
<br />
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><o:p> </o:p></span></div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com1tag:blogger.com,1999:blog-7485863064099307059.post-3033451160356173572016-04-27T21:34:00.001-07:002016-04-29T19:31:27.236-07:00HD 219134: Take Three<br />
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<a href="https://3.bp.blogspot.com/-Q-LPebVwg74/VyGSIDygtNI/AAAAAAAAA0E/RXdLFvQlg_sAxhjrUX3wHfkaDilq_DvqACLcB/s1600/HD%2B219134%2Bsystem%2Barch%2B-%2BTake%2BThree.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="586" src="https://3.bp.blogspot.com/-Q-LPebVwg74/VyGSIDygtNI/AAAAAAAAA0E/RXdLFvQlg_sAxhjrUX3wHfkaDilq_DvqACLcB/s640/HD%2B219134%2Bsystem%2Barch%2B-%2BTake%2BThree.gif" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Analyses of
longitudinal radial velocity data by Johnson & colleagues have confirmed three
planets orbiting HD 219134, a small Sun-like star of spectral class K3 located
at a distance of only 6.53 parsecs (21 light years). Additional low-mass planets
were reported in this system by Motalebi & colleagues (2015) and Vogt &
colleagues (2015), but Johnson’s group had insufficient data to confirm or
reject those candidates. In the figure, the planets colored aquamarine (b, d) are
supported by Johnson et al. 2016, Motalebi et al. 2015, and Vogt et al. 2015;
the large blue planet (e (h)) is supported by Johnson et al. 2016 and Vogt et
al. 2015; and the orange planet (c) is supported by Motalebi et al. 2015 and
Vogt et al. 2015<o:p></o:p></span></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------<o:p></o:p></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">I love science! Replication of results, falsification of invalid
hypotheses! Teams of scientists working on singular questions, converging on
robust answers, shining the light of understanding on the farthest reaches of
the universe!</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Or at least that’s the ideal.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Since last summer, we’ve seen </span><a href="https://en.wikipedia.org/wiki/Science_in_Action"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">science in action</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"> around HD 219134, an appealing amber star of
0.78 Solar masses (0.78 Msol) located </span><a href="http://backalleyastronomy.blogspot.com/2016/03/the-nearest-20-parsecs.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">very nearby</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"> in the northern
constellation of Cassiopeia. That star and its planetary system have been
featured in three previous blog posts (</span><a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">here</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">, </span><a href="http://backalleyastronomy.blogspot.com/2015/10/a-different-picture-of-hd-219134.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">here</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">, and </span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">here</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">), which record unfolding developments
both in the number of the reported planets and in the most appropriate
characterization of the system architecture.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Two major studies last year presented conflicting analyses
based on two different datasets. Motalebi & colleagues (2015; hereafter M15)
proposed a </span><a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">four-planet architecture</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">
comprising three low-mass planets inside 0.25 AU and a gas giant of 0.19 Mjup (62
Mea) just outside 2 AU. In order of distance from the star, the planets were
designated <b style="mso-bidi-font-weight: normal;">b</b> through <b style="mso-bidi-font-weight: normal;">e</b>. Almost simultaneously, Vogt &
colleagues (2015; hereafter V15) proposed a </span><a href="http://backalleyastronomy.blogspot.com/2015/10/a-different-picture-of-hd-219134.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">six-planet architecture</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"> with
five low-mass planets inside 0.4 AU and a gas giant of 0.34 Mjup (108 Mea) just
outside 3 AU. In the manuscript circulated before publication, these six were
designated <b style="mso-bidi-font-weight: normal;">b</b> through <b style="mso-bidi-font-weight: normal;">g</b> in order of distance from the star,
but on publication they were renamed <b style="mso-bidi-font-weight: normal;">b</b>
through <b style="mso-bidi-font-weight: normal;">h</b> without reference to their
distance, omitting <b style="mso-bidi-font-weight: normal;">e</b> altogether
(perhaps reserving that letter for the planet reported by M15?). M15 also
proposed a rotation period of 42.3 days for the host star, while V15 proposed a
period of approximately 20 days.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Now a third team, one of whose members (Meschiari) also
participated in V15, has weighed in on this system (Johnson et al. 2016;
hereafter J16). Most of the authors are based at the University of Texas at
Austin and involved in the McDonald Observatory Planet Search. A key strength
of this work is its radial velocity coverage, which spans 27 years. Another is its
data on stellar activity, which span 17 years and were obtained by the
Keck/HIRES spectrograph. A third strength is the analytical acumen of the investigative
team, which includes Artie Hatzes and Paul Robertson. A notable limitation, as
the authors concede, is the low precision of the radial velocity data, which
did not permit definitive results regarding any of the proposed low-mass
planets apart from the innermost, <b style="mso-bidi-font-weight: normal;">planet
b</b> (see <b style="mso-bidi-font-weight: normal;">Figure 1</b>).</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">The most direct contribution of J16 is their finding that
HD 219134 has a stellar activity cycle with a period of 11.6 years. This is
very similar to the 11-year cycle of our Sun, which manifests in the ebb and
flow of sunspots on the Sun and auroras on Earth. Identifying this cycle in HD
219134 assisted the analysis of the radial velocity data, enabling J16 to rule
out systematic effects originating in stellar activity. However, they were unable
to determine the star’s rotation period, which is essential for a robust data analysis.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">As the authors note, a star’s rotation has a critical
relationship with its age. For HD 219134, a period of about 20 days would imply
an age of about 1.3 billion years. Notably, V15 reported a period of 22.8 days
for their <b style="mso-bidi-font-weight: normal;">planet f</b> – very close to
their estimate of the stellar rotation period. A period of 42.3 days, as
reported by M15, would imply an age of 4.1 billion years, making the star a bit
younger than our Sun. J16 favor a period similar to M15’s estimate, which they
say is most consistent with the stellar activity data.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Summary
of findings on the HD 21934 planetary system<o:p></o:p></span></span><br />
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://2.bp.blogspot.com/-Y39p40UlQWs/VyGSWsaYwkI/AAAAAAAAA0I/aUDwtoGwtWMo7evfDf2Msb92x7XckOIZACLcB/s1600/HD%2B219134%2B-%2BSummary%2Bof%2Bfindings.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="178" src="https://2.bp.blogspot.com/-Y39p40UlQWs/VyGSWsaYwkI/AAAAAAAAA0I/aUDwtoGwtWMo7evfDf2Msb92x7XckOIZACLcB/s640/HD%2B219134%2B-%2BSummary%2Bof%2Bfindings.gif" width="640" /></a></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Tags:</span></i><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> <b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;">P</i></b> = approximate periodicity in days;
<b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;">PL</i></b>
= planet designation; <b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;">d</i></b> = reported period in days; <b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;">Mea</i></b>
= reported mass in Earth units; <b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;">a</i></b> = semimajor axis in astronomical
units (AU; Earth’s semimajor axis = 1 AU); <b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;">e</i></b> = orbital eccentricity. <o:p></o:p></span></div>
<div style="text-align: center;">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">-----------------------------<o:p></o:p></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">In broad terms, J16 support M15’s analysis of the
low-mass planets in the inner system and V15’s analysis of the gas giant in the
outer system (<b style="mso-bidi-font-weight: normal;">Table 1</b>).</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">In the inner system, J16 had no difficulty recovering a
signal corresponding to <b style="mso-bidi-font-weight: normal;">planet b</b>,
which was characterized by M15 and V15 as a planet less massive than 5 Mea with
a period just over 3 days. J16 also recovered periodicities at 22.8 days and
79.1 days, but they concluded that both signals were caused by stellar rotation
(as an alias or harmonic of the period) rather than by planets. They were much
more confident of another signal at about 47 days, which corresponds to <b style="mso-bidi-font-weight: normal;">planet d</b> in the models of M15 and V15.
However, the two earlier studies provided conflicting estimates of this
object’s minimum mass, with M15 proposing 8.67 Mea and V15 proposing 21.3 Mea.
J16 did not comment on this candidate’s mass.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">J16 also failed to recover any periodicity corresponding
to <b style="mso-bidi-font-weight: normal;">planet c</b>. Nevertheless, both M15
and V15 characterized this object in very similar terms, while J16 noted that
their own radial velocity precision was significantly lower than that of the
two earlier studies. Thus it seems safe to regard planet c as validated.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">In the outer system, J16 confirmed the small gas giant
detected by M15 and V15, noting that the duration of M15’s dataset was too brief
to inform a robust estimate of the planet’s orbital period. Benefiting from
their extended radial velocity coverage, J16 calculated a period similar to
that reported by V15 (2121 days versus 2247 days from V15), but they favored a
mass closer to M15 (76 Mea, versus 62 Mea from M15 and 108 Mea from V15). Sadly,
they retained the confusing designation of <b style="mso-bidi-font-weight: normal;">planet
h</b> for this object.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">I say “sadly” because <b style="mso-bidi-font-weight: normal;">h</b> implies a system with seven exoplanets, whereas only one extrasolar
system with that many companions has been confirmed to date: </span><a href="http://backalleyastronomy.blogspot.com/2014/02/a-new-design-for-planetary-systems.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">Kepler-90</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">. The
planeticity of this remarkable system is rivaled only by HD 10180, which has
six exoplanets (<b style="mso-bidi-font-weight: normal;">c</b> through <b style="mso-bidi-font-weight: normal;">h</b>) confirmed by radial velocity measurements
and a seventh (<b style="mso-bidi-font-weight: normal;">b</b>) whose existence
has remained tentative ever since the system’s initial publication in 2011. Notably,
no study has yet proposed more than six planets for HD 219134, and J16 were
dubious (though not dismissive) of two of the six reported by V15. So I contend
that <b style="mso-bidi-font-weight: normal;">planet e</b> is a more appropriate
designation for the cool gas giant in this system, on the grounds of logical
consistency.</span><br />
<span style="font-size: large;"></span><br />
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid; text-align: center;">
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Selected mixed-mass
planetary systems</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://1.bp.blogspot.com/-fbO2IX75Sm0/VyGSW_vHDxI/AAAAAAAAA0M/E3xw0BNXJCohmEU8T7r7OBitA5RrXoBNgCKgB/s1600/Mixed-mass%2BRV%2B%2526%2BTransiting%2BSystems%2B-%2Bselected%252C%2B2016.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://1.bp.blogspot.com/-fbO2IX75Sm0/VyGSW_vHDxI/AAAAAAAAA0M/E3xw0BNXJCohmEU8T7r7OBitA5RrXoBNgCKgB/s640/Mixed-mass%2BRV%2B%2526%2BTransiting%2BSystems%2B-%2Bselected%252C%2B2016.gif" width="464" /></a></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt; text-align: left;">
<i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Tags</span></i><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">: ME = Earth masses; RE
= Earth radii; AU = astronomical units (Earth-Sun separation = 1 AU). <i style="mso-bidi-font-style: normal;">Selection criteria</i>: At least two
low-mass planets and at least one gas giant on an exterior orbit. In each
system with transiting planets, all transiting orbits are co-planar. Note that
gas giants Kepler-89d and WASP-47b are also observed in transit, while a single
transit is reported for the innermost planet of HD 219134.<o:p></o:p></span></div>
<div style="text-align: left;">
</div>
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------<o:p></o:p></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">As we learn more and more about its architecture, HD
219134 becomes a more and more interesting example of what I might call “a rich
mixed-mass system.” <b style="mso-bidi-font-weight: normal;">Figure 2</b> illustrates
the construct I have in mind. It updates Figure 3 from a recent posting (</span><a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">Almost Jupiter</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">) by
revising the orbit of the outer planet of HD 219134 and adding our Solar
System.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">These 11 systems look like variations on a single theme. All
center on a Sun-like star (o.70-1.25 Msol) and include at least two low-mass
planets with at least one gas giant on a wider orbit. In 9 out of 11 systems,
at least one low-mass planet orbits inside 0.1 AU, and in 8 out of 11, the gas
giant orbits outside 1 AU. In three systems (including our own), at least one
additional low-mass planet orbits outside the (outer) gas giant. One system
(WASP-47) hosts two gas giants inside the system ice line, such that one
dominates the inner cluster of planets and the other orbits in the system
habitable zone. Another (Kepler-90) might host two warm giants, since planets <b style="mso-bidi-font-weight: normal;">g</b> and <b style="mso-bidi-font-weight: normal;">h</b> both have radii larger than 8 Rea.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Although the Solar System is typically the oddball in any
line-up of planetary systems orbiting Sun-like stars, it bears a distinct family
resemblance to most systems in Figure 2. Specifically, it is one of nine
systems in this sample with a pronounced gap between the inner aggregation of
low-mass planets and the cool giant. In our system the gap appears in the void
between Mars and Jupiter. Such a gap is absent only from the two most compact
systems (Kepler-89, 289), where all known planets orbit inside 0.6 AU.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Even one of our system’s oddest features – the absence of
planets inside 0.3 AU – has an extrasolar counterpart, since Kepler-289 reveals
a similar dearth of planets inside 0.2 AU. Evidently gaps can appear almost
anywhere within 5 AU of a host star.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Nevertheless, the Solar System’s two cold gas giants make
it unique among the systems considered here. Ours is also the only one with
confirmed or even proposed objects of habitable mass (i.e., 0.3-3.0 Mea)
orbiting in the liquid water zone. Three other systems in this line-up harbor
planets in that favored region (WASP-47, Kepler-68, HD 10810). Unfortunately,
none are habitable: two are gas giants and the third, HD 10810 h, is more
massive than Neptune.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">What about the seven systems where the gap occupies the habitable
zone? In one of them (Kepler-90), an adjacent planet might prevent habitable
planets from forming or surviving. But in six other systems (HD 219134 and
Kepler-48, 87, 89, 167, 289) the habitable zone looks empty. For each of these,
I’d love to see dynamical simulations of the long-term stability of hypothetical
Earth-mass planets on habitable orbits. </span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">migration through a radially structured disk<o:p></o:p></span></span></b></div>
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">We have lots of theories about the evolution of our Solar
System. These days, the </span><a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">most popular</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"> hinge
on the unique relationship </span><a href="http://backalleyastronomy.blogspot.com/2012/05/how-weird-is-our-solar-system.html"><span style="font-family: "georgia" , "serif";"><span style="color: blue; font-size: large;">between Jupiter and Saturn</span></span></a><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">. As
noted above, however, none of the other systems in Figure 2 contain such a
power couple. Could some other aspect of system evolution, some factor
independent of primordial resonance between two gas giants, be responsible for
the structural commonalities visible in these systems?</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">G</span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">avin Coleman and Richard Nelson recently produced two studies
in quick succession on their developing model of system evolution (2016a, 2016b;
hereafter CN16A and CN16B). Each study reports a set of N-body simulations of
planet formation around a star of 1 Msol in a realistic protoplanetary disk that
includes gas, pebbles, boulders, planetesimals, and a cavity inside a radius of
0.05 AU. The simulations modeled key physical processes known to affect planet
formation and disk evolution, such as magnetohydrodynamic turbulence at the
inner edge, aerodynamic drag on planetesimals, gas accretion on planetary
cores, Type I migration, Type II migration, and photoevaporation. Both sets of
simulations produced planetary systems similar to examples in Figure 2.</span></span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">CN16A reported an ensemble of 72 simulations. Each one
began with a disk of radius 40 AU containing 52 protoplanets (also called
planetary embryos) of 10% Earth mass each (0.1 Mea), traveling on orbits
between 1 and 20 AU. The full ensemble used initial gas disks that were either 1,
1.5, or 2 times the mass of the minimum mass Solar nebula, with disk
metallicities of either 0.5, 1, or 2 times Solar.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">This set of simulations readily formed Hot Jupiters,
sometimes with low-mass companions on adjacent orbits; the latter systems recall
WASP-47. The same setup also produced compact systems of low-mass planets similar
to Kepler-11. However, it failed to produce any gas giants with periods longer
than 10 days.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">The second study, CN16B, reported an unspecified number
of simulations with disks initially containing 44 planetary embryos of 0.2 Mea each
on orbits between 1 and 20 AU. The full ensemble used initial disk masses that
were either 1 or 2 times the minimum mass Solar nebula, with metallicities of
either 0.5, 1, or 2 times Solar. Disk lifetimes ranged from 3.5 to 8.5 million
years.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">The most significant change in the new setup was the inclusion
of radial structures in the protoplanetary disk arising from discontinuities in
viscous stress and the local surface density of solid particles. Such structures
can also be described as planet traps or dead zones: well-defined regions with
finite lifetimes where migrating solids become stranded and mutual interactions
result in the accretion of planetesimals and protoplanets. Each simulation in
CN16B included four radial structures whose location and duration varied from
run to run. As one structure subsided, another formed at a different radius. All
were located outside the system ice line at semimajor axes wider than 5 AU (see
Figure 5 in CN16B).</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; page-break-after: avoid;">
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 3.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> The protoplanetary disk around TW Hydrae<o:p></o:p></span></span></div>
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://2.bp.blogspot.com/-A8wvim-b-m4/VyGSW4CG8zI/AAAAAAAAA0Q/DiRFYV90Apw8mHwhG7Cc4gmuP8Az3TR2gCKgB/s1600/TW-Hydrae-protoplanetary-straight-on-2016.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="478" src="https://2.bp.blogspot.com/-A8wvim-b-m4/VyGSW4CG8zI/AAAAAAAAA0Q/DiRFYV90Apw8mHwhG7Cc4gmuP8Az3TR2gCKgB/s640/TW-Hydrae-protoplanetary-straight-on-2016.jpg" width="640" /></a></div>
<div class="MsoNormal" style="margin: 0in 0in 0pt;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">This young star has a huge protoplanetary
disk whose fortuitous alignment enables a face-on view from the Solar System. The
disk is about 5% as massive as our Sun (0.05 Msol), and its radius is about 90
AU. Radial structures can be observed throughout (Nomura et al. 2016, Debes et
al. 2016). </span></div>
<div align="center" class="MsoNormal" style="margin: 0in 0in 0pt; text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">-----------------<o:p></o:p></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">For disks of sufficient mass and metallicity, the
improved setup produced both Hot Jupiters and long-period gas giants. Overall
results replicated the widely reported “period valley” between 0.1 and 0.5 AU
(the range of semimajor axes where gas giant planets are rare). Final architectures
included systems with a Hot Jupiter plus an outer giant companion; compact
inner systems of low mass planets bounded by a gap with a cool giant orbiting outside
it (siblings of HD 219134 and Kepler-167); and systems with two cool gas giants
analogous to Jupiter and Saturn. This setup also produced compact low-mass
systems, some with inner gaps resembling those around Kepler-289 and our Sun.</span><br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"></span><br />
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;">As Coleman & Nelson readily concede, they’re working
with a “toy model” that isn’t intended to exactly recapitulate planet formation
or predict the relative frequency of system architectures. Nevertheless, their
model incorporates the latest theoretical perspectives and observational
insights (e.g., high-resolution imaging of the protoplanetary disks around HL
Tauri and TW Hydrae), and its most recent iteration compares very well with
other available approaches. As far as I’m aware, it’s the only evolutionary
model proposed to date that can produce system architectures whose diversity rivals
reality. I’m eager to see the next study this team produces.<o:p></o:p></span></span><br />
<span style="font-size: large;"></span><br />
<div style="text-align: center;">
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><o:p> </o:p></span></div>
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<br />
<div class="MsoNormal" style="margin: 0in 0in 0pt; mso-layout-grid-align: none;">
<span style="font-size: 10pt;"><o:p> </o:p></span></div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com2tag:blogger.com,1999:blog-7485863064099307059.post-33802555022684853852016-04-14T10:26:00.002-07:002016-06-03T12:22:29.799-07:00Daydream Destinations, Part 2<!--[if gte mso 9]><xml>
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<a href="https://2.bp.blogspot.com/-ptzsmLGSZ4A/Vw_RqR1TNCI/AAAAAAAAAys/IZDt_0irnNovUgKOOqfKVgb_UXGC3e-VACLcB/s1600/Chessmen%2B-%2BGahan%2Bscaled%2Bthe%2Btower-small.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://2.bp.blogspot.com/-ptzsmLGSZ4A/Vw_RqR1TNCI/AAAAAAAAAys/IZDt_0irnNovUgKOOqfKVgb_UXGC3e-VACLcB/s640/Chessmen%2B-%2BGahan%2Bscaled%2Bthe%2Btower-small.jpg" width="504" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> In this
illustration for <i style="mso-bidi-font-style: normal;">The Chessmen of Mars</i>
by </span><a href="https://en.wikipedia.org/wiki/Edgar_Rice_Burroughs"><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;">Edgar Rice Burroughs</span></a><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> (1922), artist </span><a href="https://en.wikipedia.org/wiki/J._Allen_St._John"><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;">J. Allen St. John</span></a><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> sketched the two Martian moons as an atmospheric
backdrop for the main action, which features Gahan of Gathol scaling a tower in
the lost city of Manator. In Martian mythology, the two moons represent a cold
husband (Cluros, the outer moon) and his mad and wayward wife (Thuria, the
inner moon). </span></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">This posting continues the discussion of daydream
destinations in extrasolar space begun in </span><a href="http://backalleyastronomy.blogspot.com/2016/04/daydream-destinations-part-1.html"><span style="font-family: "georgia" , "serif";">Part 1</span></a><span style="color: black; font-family: "georgia" , "serif";">. Today’s reveries involve
exomoons, tidally locked planets, and monoform worlds.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">exomoons</span></b></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Six out of eight planets in our Solar System have moons, with
Jupiter and Saturn between them hosting a total of 11 spherical and approximately
100 non-spherical moons. Most of the latter are defined as “irregular”
satellites on the basis of their orbital elements. In addition, Uranus has five
spherical moons (plus more than 20 irregulars) and Neptune and Pluto have one
each – respectively Triton and Charon. The ubiquity and similarity of satellite
systems in the Solar System argue that extrasolar planets will host analogous
systems. On this assumption, the term “exomoon” has become the standard designation
for a hypothetical satellite of an exoplanet.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">An extra moon or three in the night sky might simply be
decorative, like the </span><a href="https://en.wikipedia.org/wiki/Moon_Pilot"><span style="font-family: "georgia" , "serif";">seven moons of Beta Lyrae</span></a><span style="color: black; font-family: "georgia" , "serif";"> or the
two Martian satellites in <b>Figure 1</b>. There
seems to be no limit on the number of natural satellites an Earth-like exoplanet
could host. Given so much freedom, it would be interesting to explore the
effects of competing tides on the hypothetical oceans of a Super Earth of two
Earth masses (2 Mea) with three or four moons.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">More beguiling speculations are possible if we shift our
focus from habitable exoplanets to habitable exomoons (Tinney et al. 2011,
Heller et al. 2014). Writers in earlier eras, from </span><a href="https://en.wikipedia.org/wiki/Lucian"><span style="font-family: "georgia" , "serif";">Loukianos
of Samosata</span></a><span style="color: black; font-family: "georgia" , "serif";"> in the second century A.D. to </span><a href="https://en.wikipedia.org/wiki/H._G._Wells"><span style="font-family: "georgia" , "serif";">H.G.
Wells</span></a><span style="color: black; font-family: "georgia" , "serif";"> in the twentieth, offered intriguing visions of life on
our own Moon. Unfortunately, we now know that such scenarios are purely
fantastic. To support life, a world must also maintain an atmosphere and stable
bodies of water. No such environment has ever been available on the Moon.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">In the current generation of astronomers, 0.3 Mea has
been adopted as the minimum mass for an </span><a href="http://backalleyastronomy.blogspot.com/2015/12/quantifying-earth-like.html"><span style="font-family: "georgia" , "serif";">Earth-like planet</span></a><span style="color: black; font-family: "georgia" , "serif";">
(Raymond et al. 2006), and by extension a habitable exomoon. This is more than five
times the mass of Mercury and almost triple the mass of Mars.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Is it possible for an exomoon to attain such a high mass?
That would depend on its formation pathway. Planets in our Solar System evidently
acquired their moons by three different mechanisms: 1) accretion in a
circumplanetary disk during the first few million years of system evolution; 2)
a glancing collision with another protoplanet followed by accretion in the
resulting circumplanetary ring of debris; and 3) gravitational capture of a
fully formed object after planet formation finished, often resulting in a
retrograde orbit for the satellite. The first mechanism brought us the
spherical moons of Jupiter and Saturn, as well as a few of the non-spherical
ones; the second brought us our own Moon, Pluto’s moon Charon, and probably the
spherical moons of Uranus (but see Boue & Laskar 2010); while the third
brought us Neptune’s moon Triton, and probably all the irregular moons of the
other planets (Jewitt & Haghighipour 2007).</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif";"> View
from </span><a href="https://en.wikipedia.org/wiki/Fictional_universe_of_Avatar"><span style="font-family: "georgia" , "serif";">Pandora</span></a><span style="color: black; font-family: "georgia" , "serif";">, imagined as the moon of
an extrasolar gas giant</span></span></div>
<span style="font-size: large;">
</span><br />
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<a href="https://3.bp.blogspot.com/-uZUQS7ize9Q/Vw_RyjjrgVI/AAAAAAAAAy0/ftwUyBsdXtoHqUi3xQiHr2DpCK6FTnb4wCLcB/s1600/Exomoon5-Pandora.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="360" src="https://3.bp.blogspot.com/-uZUQS7ize9Q/Vw_RyjjrgVI/AAAAAAAAAy0/ftwUyBsdXtoHqUi3xQiHr2DpCK6FTnb4wCLcB/s640/Exomoon5-Pandora.jpg" width="640" /></a></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Each satellite system that formed along the first pathway
has an aggregate mass in a ratio of about 5,000:1 to the mass of its host
planet (see Canup & Ward 2006). For example, the aggregate of the four
Galilean moons of Jupiter is 0.066 Mea (a bit more than Mercury’s mass). Given
Jupiter’s mass of 318 Mea, the mass ratio of primary to satellites is about 4800:1.
Similarly, the aggregate of the seven spherical moons of Saturn is 0.0236 Mea,
while Saturn itself is 95.2 Mea, yielding a ratio of about 4000:1. Although
Jupiter’s four largest moons have approximately similar masses, Saturn’s moons
have an extremely lopsided mass distribution, with Titan accounting for 95% of
the total.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Under the right conditions, then, all three of these
mechanisms might be able to produce an exomoon of 0.3 Mea. The first would
require a gas giant primary with a mass of 5 Mjup or more. More than 100
exoplanets are currently known in this range, but only about half orbit
Sun-like stars; the rest are companions of hotter primaries. The second mechanism
would require a very specific kind of collision between two very specific kinds
of objects, at least one of which would need to have a large iron/silicate
content and be at least as massive as Uranus. The third would likely require a
very special three-body encounter between a gas giant and a habitable-mass
terrestrial planet with a gravitationally bound companion (Agnor & Hamilton
2006). All three scenarios seem possible, with the first apparently the most
straightforward and potentially the most frequent (see <b>Figure 2</b>).</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">In addition, the final orbit of the exoplanet/exomoon
system produced by each of these mechanism must lie in the local habitable
zone. Unfortunately, among the 100+ Super Jupiters noted above, fewer than 10%
are found in the habitable zone of a Sun-like star, and half of those have
eccentricities higher than 0.3, indicating extreme climates for any potential
satellites. If these numbers are any reflection of the true extrasolar
population, the most likely formation pathway for habitable exomoons would
yield few of them. The success rate of the other two mechanisms is harder to
assess. Although the planetary primaries involved are smaller than 5 Mjup, and
thus far more numerous than Super Jupiters, the conditions necessary to acquire
habitable exomoons through collision or capture might ensure that such objects
are rare.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">My hunch is that Earth-size exomoons are much less
frequent in our Galaxy than terrestrial planets of 0.3 to 3 Mea. The latter
population is already well-represented in the extrasolar census. Even though
few to none of them (depending on your definition) have yet been observed in
the habitable zone of a Sun-like star, their presence in such orbits is all but
assured.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">twilight
zones</span></b></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">In the early twentieth century, astronomers believed that
Mercury and Venus were tidally locked, meaning that each planet always turned
the same hemisphere toward the Sun. This arrangement was thought to result from
their short-period orbits, which made them vulnerable to the Sun’s
gravitational influence. Now we know that both planets actually rotate slowly.
Instead of permanent daysides and nightsides, each experiences an extended
day/night cycle. A “day” on Mercury is 58.6 Earth days long, whereas a year is
88 days. A day on Venus is 243 Earth days, whereas a year is 225 days.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Most low-mass exoplanets detected to date have orbital
periods shorter than Mercury, and almost all have periods shorter than Venus.
Theoretical models predict tidal locking for the great majority. This outcome
is especially significant for systems around cool stars less massive than 0.60
Msol, corresponding to the range of spectral types from late K through M. For these
stellar masses, planets orbiting anywhere in the habitable zone are very likely
to be tidally locked (Selsis et al. 2007). Even around higher-mass stars in the
range of 0.60-0.89 Msol, the inner regions of the habitable zone are subject to
tidal locking.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure 3.</span></b><span style="color: black; font-family: "georgia" , "serif";">
Insolation on a tidally locked planet</span></span></div>
<span style="font-size: large;">
</span><br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://2.bp.blogspot.com/-JwqQ4Hh-Fg0/Vw_SAM17RII/AAAAAAAAAy8/fVFGuN7WrJ0dOjnl3LuzwIbt2WrsqVVsgCLcB/s1600/Tidally%2Blocked%2Bplanet%2B-%2Binsolation.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="488" src="https://2.bp.blogspot.com/-JwqQ4Hh-Fg0/Vw_SAM17RII/AAAAAAAAAy8/fVFGuN7WrJ0dOjnl3LuzwIbt2WrsqVVsgCLcB/s640/Tidally%2Blocked%2Bplanet%2B-%2Binsolation.gif" width="640" /></a></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">One of my treasured books as a child was Kenneth Heuer’s <i>Men of Other Planets</i>, a popular
astronomy text originally published in 1951. Reflecting the consensus of the
1940s and 1950s, Heuer presents Mercury as a tidally locked world: “The country
of the Mercurians is really divided into three parts; there is the land of
eternal day, one of eternal night, and a borderland of alternate sunshine and
shadow.” This “borderland” is also designated “the twilight zone.” As Heuer explains:
“There are in reality two lands of sunrise and sunset [. . .] One at the east
and the other at the west, they are elliptical regions connected at the poles,
and bordering on eternal day and eternal night.”</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Applying Heuer’s perspective to an extrasolar locale,
let’s imagine an iron/silicate planet like Earth with a synchronous orbit of 150
days. This period places it in the habitable zone of an amber star of type K4.
Although our planet is tidally locked, its orbit is slightly eccentric, causing
it to </span><a href="https://en.wikipedia.org/wiki/Libration"><span style="font-family: "georgia" , "serif";">librate</span></a><span style="color: black; font-family: "georgia" , "serif";">. With respect to the host
star, it rocks back and forth so that the surface area exposed to illumination varies
slightly on a regular cycle. <b>Figure 3</b>
shows the relative insolation (exposure to stellar flux) of various regions on
the planet’s surface. At the substellar point, insolation is strongest; prevailing
winds from the darkside blow toward this point from all directions. Insolation
declines as distance from the substellar point increases, with thermal contours
shaped by wind patterns. The terminator (dividing line between day and night)
shifts back and forth as the planet librates. Nightward of the terminator is
the twilight zone, where the sky is still illuminated while the host star hovers
just below the horizon.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure 4.</span></b><span style="color: black; font-family: "georgia" , "serif";">
Twilight over the Bahariya Oasis, Egypt </span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://1.bp.blogspot.com/-tvUeLR7cgTU/Vw_SHpyiGCI/AAAAAAAAAzE/Ufy0X3n6vt4wRAMBdadjb0VasxvZwJcOgCLcB/s1600/Twilight%2B-%2BSalt%2BLake%252C%2BBahariya%2BOasis%252C%2BEgypt.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="478" src="https://1.bp.blogspot.com/-tvUeLR7cgTU/Vw_SHpyiGCI/AAAAAAAAAzE/Ufy0X3n6vt4wRAMBdadjb0VasxvZwJcOgCLcB/s640/Twilight%2B-%2BSalt%2BLake%252C%2BBahariya%2BOasis%252C%2BEgypt.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Glowing clouds are reflected in the Salt Lake of
Bahariya. Photo by Aymen Ibrahem;<br />
</span><a href="http://epod.usra.edu/blog/2011/03/civil-twilight-over-salt-lake-bahariya-oasis.html"><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;">Earth Science Picture
of the Day</span></a><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Depending on the planet’s geothermal flux and relative distribution
of land and sea, many environments are possible. If a body of water occupies
the substellar point, with continents surrounding it, that point will be
occupied by a permanent cyclone. Adjacent regions will be tropical, with
temperatures generally falling as distance from the center increases. Snowy
conditions might prevail along the terminator, while some organisms are bound
to colonize the darkside.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">As Yang and colleagues have shown, a reservoir of surface
water will strongly constrain the climate of a tidally locked planet (Yang et
al. 2014). If the darkside is land-locked, water will be carried toward the
antistellar point and freeze out of the atmosphere, creating glaciers on that
hemisphere. However, as long as seas cover at least 10% of the planet’s
surface, its atmosphere will circulate heat to the darkside and prevent a
complete freeze-out of water. If the planet has an ocean cover of 90%, ocean
currents will circulate heat to all longitudes and maintain warm temperatures throughout
the darkside.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">If you’re thinking about setting fantastic adventures on
a tidally locked planet, you might consider some form of extended quest. One
option would be a circuit of the twilight zone, imagined as a necklace of lakes
set in windy mountains or a chain of islands in the circling sea. Another would
be an ocean voyage from balmy latitudes through monster-infested straits to the
cyclonic inferno at the substellar point. For a dose of horror, I might advise
a trek beneath the cold, glittering stars of the night lands, around grimly
smoldering volcanoes with veins of red lava, through phosphorescent fungal
forests haunted by hallucinogenic mists, for a rendezvous with terror at the
heart of darkness . . .</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">monoform worlds</span></b></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Perhaps the most popular extrasolar destination is what I
call a “monoform planet.” Such a planet has a uniform environment that
corresponds to a single geographic region on Earth. Thus we have desert planets
like </span><a href="https://en.wikipedia.org/wiki/Arrakis"><span style="font-family: "georgia" , "serif";">Arrakis</span></a><span style="color: black; font-family: "georgia" , "serif";"></span>, <a href="https://en.wikipedia.org/wiki/Tatooine"><span style="font-family: "georgia" , "serif";">Tatooine</span></a><span style="color: black; font-family: "georgia" , "serif";">, <span style="color: black;">and </span><a href="http://starwars.wikia.com/wiki/Jakku" target="_blank">Jakku</a>; jungle
worlds like </span><a href="https://en.wikipedia.org/wiki/Yavin#Yavin_4"><span style="font-family: "georgia" , "serif";">Yavin IV</span></a><span style="color: black; font-family: "georgia" , "serif";"> and </span><a href="https://en.wikipedia.org/wiki/Dagobah"><span style="font-family: "georgia" , "serif";">Dagobah</span></a><span style="color: black; font-family: "georgia" , "serif";">, and
ice planets like </span><a href="https://en.wikipedia.org/wiki/Gethen"><span style="font-family: "georgia" , "serif";">Gethen</span></a><span style="color: black; font-family: "georgia" , "serif";"> and </span><a href="https://en.wikipedia.org/wiki/Hoth"><span style="font-family: "georgia" , "serif";">Hoth</span></a><span style="color: black; font-family: "georgia" , "serif";">. Their
terrestrial originals are obvious. The producers of <i>Star Wars</i> even went so far as to film desert scenes in Tunisia,
rain forest scenes in Guatemala, and snow scenes in Iceland.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure
5.</span></b><span style="color: black; font-family: "georgia" , "serif";"> Artist’s impression of a Carboniferous landscape</span></span></div>
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<a href="https://1.bp.blogspot.com/-q5pCD68U7q8/Vw_SS24KvLI/AAAAAAAAAzQ/HCi9B9mo_p4lcRGr-2ABEKqY-B6sNeFPgCLcB/s1600/Carboniferous-forest-ludek-pesek.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="382" src="https://1.bp.blogspot.com/-q5pCD68U7q8/Vw_SS24KvLI/AAAAAAAAAzQ/HCi9B9mo_p4lcRGr-2ABEKqY-B6sNeFPgCLcB/s640/Carboniferous-forest-ludek-pesek.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Artist: Ludek Pesek</span></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Earth itself has evidently sustained a succession of
relatively uniform environments at different phases in its history. At early
times our planet experienced extreme glacial periods (e.g., the Huronian,
Marinoan, and Ordovician) during which it probably resembled an ice planet.
According to one popular hypothesis, the </span><a href="https://en.wikipedia.org/wiki/Neoproterozoic"><span style="font-family: "georgia" , "serif";">Neoproterozoic</span></a><span style="color: black; font-family: "georgia" , "serif";"> brought about </span><a href="https://en.wikipedia.org/wiki/Snowball_Earth"><span style="font-family: "georgia" , "serif";">Snowball Earth</span></a><span style="color: black; font-family: "georgia" , "serif";"> conditions, when even the global ocean was
covered by ice or slush. Earth has also been a jungle planet, notably during
the </span><a href="https://en.wikipedia.org/wiki/Carboniferous"><span style="font-family: "georgia" , "serif";">Carboniferous</span></a><span style="color: black; font-family: "georgia" , "serif";">, when
global temperatures were higher, the atmosphere was denser, and tropical
forests thrived over a large area, despite lingering glaciation (<b>Figure 5</b>). Although Earth hasn’t yet seen
desert conditions from pole to pole, its ultimate fate is complete desiccation
as the Sun evolves and heats up. According to </span><a href="http://news.nationalgeographic.com/news/2013/13/130729-runaway-greenhouse-global-warming-venus-ocean-climate-science/"><span style="font-family: "georgia" , "serif";">some prognosticators</span></a><span style="color: black; font-family: "georgia" , "serif";">, our
planet’s hydrosphere will evaporate within a billion years.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Fortunately, water still covers more than three-quarters
of Earth’s surface, inspiring perennial visions of ocean planets. Science fictional
incarnations include Perelandra and Solaris, each the titular presence in a
classic novel (by </span><a href="https://en.wikipedia.org/wiki/C._S._Lewis"><span style="font-family: "georgia" , "serif";">C.S. Lewis</span></a><span style="color: black; font-family: "georgia" , "serif";"> and </span><a href="https://en.wikipedia.org/wiki/Stanis%C5%82aw_Lem"><span style="font-family: "georgia" , "serif";">Stanislav Lem</span></a><span style="color: black; font-family: "georgia" , "serif";">, respectively). But the ocean planets imagined
by exoplanetologists are quite different from those in science fiction
narratives, which typically assume an environment like the Pacific hemisphere (<b>Figure 6</b>). Instead of an average ocean
depth of 4.28 km, however, the ocean planets proposed by Leger & colleagues
(2004) have watery envelopes 100 km deep over a layer of high-pressure ice.
This layer would prevent liquid water from mixing with heavy elements in the
mantle, a situation unfriendly to the emergence of life (Alibert 2014). If we want
an exoplanet with extensive oceans, volcanic islands, and marine organisms,
we’re really looking for Earth 2.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure
6.</span></b><span style="color: black; font-family: "georgia" , "serif";"> A well-known Ocean Planet</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://2.bp.blogspot.com/-i1_bXLqEM7Y/Vw_SmWCQ_MI/AAAAAAAAAzY/id-477Cb8I4Mlg5P3lwxTGbgbgb-VLUMgCLcB/s1600/Earth%2Bas%2Ban%2BOcean%2BPlanet.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="586" src="https://2.bp.blogspot.com/-i1_bXLqEM7Y/Vw_SmWCQ_MI/AAAAAAAAAzY/id-477Cb8I4Mlg5P3lwxTGbgbgb-VLUMgCLcB/s640/Earth%2Bas%2Ban%2BOcean%2BPlanet.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">North is on the right. Image credit: NASA/JPL-CalTech</span></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Our planet has presented many different faces over the
course of its long history. We should expect no less from Earth-like planets
orbiting other stars.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">other
exotic destinations</span></b></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The itinerary of extrasolar daydream worlds can be
extended to include terrestrial planets with rings, carbon planets made of
diamonds, planets suspended in the event horizons of black holes, rogue planets
hurtling starless through the void, and probably others that Google will gladly
bring to your screen.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">We might also want to consider Earth-like planets in </span><a href="https://en.wikipedia.org/wiki/Open_cluster"><span style="font-family: "georgia" , "serif";">open clusters</span></a><span style="color: black; font-family: "georgia" , "serif";">, where bright stars like our Sun are packed
more tightly than in local space and stellar populations are rich in binaries
and triple star systems. Clusters make excellent settings for federated planets,
interstellar empires, and most other tropes of space opera. Sadly, they have
two drawbacks: 1) the vast majority break up within a billion years, such that
only the most massive and heavily populated can survive as long as the current
age of the Earth, and 2) any cluster that endured for 4.6 billion years would have
witnessed a succession of stellar cataclysms as its brightest stars evolved off
the main sequence and potentially fried everything within a few light years.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Then there are the artificial worlds, like Freeman
Dyson’s </span><a href="https://en.wikipedia.org/wiki/Dyson_sphere"><span style="font-family: "georgia" , "serif";">Sphere</span></a><span style="color: black; font-family: "georgia" , "serif";">, Larry Niven’s </span><a href="https://en.wikipedia.org/wiki/Ringworld"><span style="font-family: "georgia" , "serif";">Ringworld</span></a><span style="color: black; font-family: "georgia" , "serif";">, and
Iain Banks’ </span><a href="https://en.wikipedia.org/wiki/Orbital_%28The_Culture%29"><span style="font-family: "georgia" , "serif";">Orbitals</span></a><span style="color: black; font-family: "georgia" , "serif";"> and </span><a href="https://en.wikipedia.org/wiki/The_Culture"><span style="font-family: "georgia" , "serif";">Shellworlds</span></a><span style="color: black; font-family: "georgia" , "serif";">. All
marvels indeed, but beyond my bandwidth today! </span></span><br />
</div>
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<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10pt;">Leger A</span></b><span style="font-size: 10pt;">,
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<span style="color: black;"><b style="mso-bidi-font-weight: normal;"><span style="font-size: 10pt;">Raymond SN,</span></b><span style="font-size: 10pt;">
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<span style="color: black;"><b style="mso-bidi-font-weight: normal;"><span style="font-size: 10pt;">Selsis F</span></b><span style="font-size: 10pt;">,
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<span style="color: black;"><b><span style="font-size: 10pt;">Tinney CG</span></b><span style="font-size: 10pt;">, Wittenmyer RA, Butler RP, Jones HR, O’Toole SJ,
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<span style="color: black;"><b style="mso-bidi-font-weight: normal;"><span style="font-size: 10pt;">Yang
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<br /></div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com1tag:blogger.com,1999:blog-7485863064099307059.post-52148646977865045492016-04-09T16:10:00.000-07:002016-05-30T13:53:24.787-07:00Daydream Destinations, Part 1<!--[if gte mso 9]><xml>
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<a href="https://4.bp.blogspot.com/-HPdl05O40U4/VwmKt8BRyiI/AAAAAAAAAx0/69BmUgSJCPU9qtEFk2NE_A3u5pMXhPVDQ/s1600/Venus%2Bby%2BAir.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://4.bp.blogspot.com/-HPdl05O40U4/VwmKt8BRyiI/AAAAAAAAAx0/69BmUgSJCPU9qtEFk2NE_A3u5pMXhPVDQ/s640/Venus%2Bby%2BAir.jpg" width="370" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Retro
Venus, the ultimate daydream vacation. This artistic take on Venus as a lush jungle
planet reflects the pulp fiction of C.L. Moore in the early 1930s (“Black
Thirst”) and Leigh Brackett in the late 1940s (“The Moon That Vanished”). Image
by Steve Thomas. Prints are available for purchase </span><a href="http://www.stevethomasart.com/"><span style="font-family: "arial" , "sans-serif"; font-size: 10pt;">here</span></a><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Science fiction was the gateway drug for many of us who now
share a passion for exoplanetary astronomy. I suspect we’re hoping –
consciously or unconsciously – that science will eventually find worlds like
the ones we grew to love through speculative novels, comics, pulps, movies, and
television.</span></span><br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";"></span></span><br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">T</span></span><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";"><span style="color: black;">his two-part posting indulges those collective hopes with
some lucid daydreaming about popular extrasolar destinations. Part 1 looks at
systems with multiple stars, including circumbinary and conjoined systems. </span><a href="http://backalleyastronomy.blogspot.com/2016/04/daydream-destinations-part-2.html" target="_blank">Part 2</a> looks at exomoons, tidally locked planets, and monoform worlds.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">circumbinary
systems</span></b></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">You can be sure you’re on an alien planet if you can see
two suns in the sky. If you’re lucky you might even see three! (<b>Figure 2</b>) Ever since <i>Star Wars</i>, a multiplicity of host stars
has defined mass cultural expectations for extrasolar planets. Thanks to </span><a href="https://en.wikipedia.org/wiki/George_Lucas"><span style="font-family: "georgia" , "serif";">George Lucas</span></a><span style="color: black; font-family: "georgia" , "serif";">, objects in orbit around a pair of stars are
now frequently known as <a href="http://www.starwars.com/databank/tatooine"><span style="color: black;">Tatooine</span></a> planets, not just
among publicists but even in astronomical circles. The more technical modifier
for such a planet is “circumbinary,” meaning that it orbits both members of the
binary rather than just one. </span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure
2.</span></b><span style="color: black; font-family: "georgia" , "serif";"> Sunset over Trisol</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://1.bp.blogspot.com/-RcpCAIpM6dQ/VwmK41CXdeI/AAAAAAAAAx4/g1dqh6FZ5jQg4t19ScCCDdQryfnIIFYCA/s1600/MyThreeSuns%2B-%2BTriple%2BSunset.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="484" src="https://1.bp.blogspot.com/-RcpCAIpM6dQ/VwmK41CXdeI/AAAAAAAAAx4/g1dqh6FZ5jQg4t19ScCCDdQryfnIIFYCA/s640/MyThreeSuns%2B-%2BTriple%2BSunset.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">The planet Trisol orbits a triple star system
(types G, K, and M) located in the Forbidden Zone of the Galaxy of Terror. (<i style="mso-bidi-font-style: normal;">Futurama,</i> Season 1: episode “My Three
Suns,” originally aired May 4, 1999)</span></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">In the full stellar population, the likelihood of stellar
multiplicity increases with mass. As the least massive stars, M dwarfs tend to
be single, while virtually all of the most massive stars – types O and B – occur
in multiple systems. Binaries are the most common type of multiple star, with
larger assemblies becoming progressively less frequent. Among star systems of
all spectral types inside 10 parsecs, the RECONS survey reported 185 single
stars (71%), 55 binary systems (21%), 15 triples (6%), 3 quadruples (1%), and 1
quintuple (less than 1%). </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Because our Sun is well above the median stellar mass, 44%
of the Sun-like stars (spectral types F6-K3) located inside 25 parsecs occur in
multiple star systems (Raghavan et al. 2010). Within that volume, 33% of
Sun-like stars occur in binaries, 8% in triple systems, and 3% in quadruple systems
or higher. Raghavan & colleagues noted that higher-order multiples tend to
be younger than binaries, suggesting that systems with three or more stars in
their youth typically lose stars over time.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Even before the Kepler mission, many exoplanetary host
stars had known binary companions. Among non-Kepler binaries with planets, the
separation between the two stars ranges from about 20 AU (similar to the
distance of Uranus from our Sun) to 10,000 AU (Raghavan et al. 2010). Until
very recently, only one star in each planetic binary was known to host planets.
Now we know of at least two </span><a href="http://backalleyastronomy.blogspot.com/2014/12/hot-jupiter-twins-wasp-94.html"><span style="font-family: "georgia" , "serif";">conjoined systems</span></a><span style="color: black; font-family: "georgia" , "serif";"> in
which each star hosts its own planetary system (XO-2 and WASP-94).</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">These days, however, discussions of binary host stars often
center on circumbinary planets. Although theory predicted that very close
binaries could jointly host a single planetary system, Kepler was the first
telescope actually to detect such a configuration. These systems are rare: only
nine have been identified to date (<b>Table
1</b>). Eight out of nine host single planets, which include puffy gas dwarfs,
baby gas giants, and one Jupiter-sized giant (KOI-2939b). Except for the latter
planet, the radii of these singletons range from 4 to 8.6 Rea, staking out a
poorly populated area of the overall Kepler distribution. The remaining system
(Kepler-47) hosts a family of three small planets, all of low density. Two-thirds
of the parent binaries in these nine systems comprise a Sun-like star plus an M
dwarf; one-third comprise a pair of Sun-like stars of similar mass. Habitable
Earth-size planets might be possible in systems like these, but they are clearly
more difficult to observe than larger planets, and they may also be less
frequent. Notably, 40% of the known binary planets occupy their system
habitable zones (Kostov 2015). One of them is KOI-2939b, which is also an
excellent candidate for hosting large exomoons.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif";">
Circumbinary systems as of April 2016</span></span></div>
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<a href="https://4.bp.blogspot.com/-4mzzxfK0s4M/VwmLD8A2QvI/AAAAAAAAAx8/yih1-fe6d5AAUHuv2iOXjgs_zSRahO7wg/s1600/Circumbinary%2BPlanets%2BApril%2B2016.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="238" src="https://4.bp.blogspot.com/-4mzzxfK0s4M/VwmLD8A2QvI/AAAAAAAAAx8/yih1-fe6d5AAUHuv2iOXjgs_zSRahO7wg/s640/Circumbinary%2BPlanets%2BApril%2B2016.gif" width="640" /></a></div>
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<i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Tags</span></i><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">: Msol = star mass in
Solar units; period = period in days; a = semimajor axis in Earth units; e =
orbital eccentricity (0 = circular); Rea = planet radius in Earth units; Mea =
planet mass in Earth units. Distance = distance in parsecs</span></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Theory and observation alike show that stable multiple
star systems are built from binary units arranged in a strict hierarchy (<b>Figure 3</b>). Given the progressively
increasing scarcity of a) triple or higher-order multiples of any spectral
type, b) Earth-like planets, and c) circumbinary planetary systems, the
following prediction seems safe: Habitable planets or moons orbiting all
members of a multiple star system are much more likely to have binary hosts than
hosts that are triple (single + double) or quadruple (double + double).</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure 3.</span></b><span style="color: black; font-family: "georgia" , "serif";"> Orbital
configurations of selected quadruple and quintuple star systems</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://2.bp.blogspot.com/-iGBAPt4oRE4/VwmLLq7z60I/AAAAAAAAAyA/VyXVeFXX67c36pb9qCJErAHhVqjZhLNEA/s1600/Example%2Bquadruple%2B%2526%2Bquintuple%2Bstar%2Bsystems%2B-%2BRaghavan%2B2010.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="156" src="https://2.bp.blogspot.com/-iGBAPt4oRE4/VwmLLq7z60I/AAAAAAAAAyA/VyXVeFXX67c36pb9qCJErAHhVqjZhLNEA/s640/Example%2Bquadruple%2B%2526%2Bquintuple%2Bstar%2Bsystems%2B-%2BRaghavan%2B2010.gif" width="640" /></a></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br /></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">These three systems are located within 25
parsecs. They illustrate the hierarchy of binary orbits that constitute
higher-order multiple star systems. Based on Figures 23 and 24 of Raghavan et
al. 2010.</span></div>
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<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">wide
binaries</span></b></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Although I’m sensitive to their rarity and appeal, I’m
not as fascinated by circumbinary planets as the mass media expect me to be. Planets
in relatively wide binaries are not only vastly more common, but vastly more interesting
– not least because of their science fictional possibilities.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">At least two potential scenarios are available. The first
involves climatic fluctuations induced by an eccentric binary orbit.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Imagine an inhabited planet whose parent star is an early
K dwarf with a brighter, hotter binary companion of spectral type F7. The
binary orbit is wide and extremely eccentric, and its period is longer than a
thousand Earth years. When the two stars are at periastron (i.e., their closest
approach, which would last a few decades) the planet heats up. For several
years during periastron passage, the day/night cycle alternates between two
seasons: One features hot, bright days with both stars visible in the sky,
followed by dark nights with neither star. The other season sees endless light,
with the K star shining by day and the F star turning the nights white. Then
the two stars would separate, the white star would shrink and grow dimmer, and
temperatures would cool again. Assuming that the planet has polar caps and
oceans, at periastron the ice would shrink, sea levels would rise, and weather
would become unusually volatile. Moving toward apastron (the two stars’ widest
separation) the reverse would happen. The effects of this cycle on geology,
climate, organisms, psychologies, and cultures would be momentous.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure
4.</span></b><span style="color: black; font-family: "georgia" , "serif";"> The Helliconia Trilogy</span></span></div>
<span style="font-size: large;">
</span><br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-hAMgku7mOng/VwmLUvT2NXI/AAAAAAAAAyE/2iIancFyUqoOGF6gBoKckU4T-B4mPTrZA/s1600/Brian%2BAldiss_Helliconia_cover%2Bset_TRIAD%2BBOOKS.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="328" src="https://3.bp.blogspot.com/-hAMgku7mOng/VwmLUvT2NXI/AAAAAAAAAyE/2iIancFyUqoOGF6gBoKckU4T-B4mPTrZA/s640/Brian%2BAldiss_Helliconia_cover%2Bset_TRIAD%2BBOOKS.jpg" width="640" /></a></div>
<span style="font-size: large;"></span><br />
<div class="MsoNormal">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The overall impact would depend on the planet’s baseline
climate near apastron, where it would spend most of its orbit. If the baseline is
temperate, periastron might occasion fiery disasters, while if the baseline is
glacial, periastron might herald a brief, benign epoch of abundance. Brian
Aldiss has explored such an eccentricity-induced climate in his </span><a href="https://en.wikipedia.org/wiki/Helliconia"><i><span style="font-family: "georgia" , "serif";">Helliconia Trilogy</span></i></a><span style="color: black; font-family: "georgia" , "serif";"> (1982-1985;
<b>Figure 4</b>), and George Martin seems
to be playing with similar ideas in his ongoing </span><a href="https://en.wikipedia.org/wiki/A_Song_of_Ice_and_Fire"><i><span style="font-family: "georgia" , "serif";">Song
of Ice and Fire</span></i></a><span style="color: black; font-family: "georgia" , "serif";"> series (1996-?). However, as Martin has warned
us, the irregular climate of Westeros has more to do with magic and dragons
than with astrophysics (see also Kostov et al. 2013).</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">conjoined
systems</span></b></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The second binary scenario that floats my boat involves conjoined
planetary systems. The separations between the two stars in such a
configuration will be measured in astronomical units (AU) instead of parsecs.
Their proximity will enable interstellar travel within a human lifetime through
the use of propulsion technologies currently in development. Imagine
a binary system comprising one G2 star like our Sun and a cooler G8 star
separated by 150 AU. Each system has a different architecture, but both feature
at least one inhabited planet. The inhabitants might be indigenous to each
system, or indigenous to one and pioneers in the other, or colonists on both
from a far-future version of Earth. Conjoined systems plus space travel will
likely give rise to a federation of planets, rather like those envisioned in
the space operas of the 1930s and 1940s and later updated by both iterations of
</span><a href="https://en.wikipedia.org/wiki/Battlestar_Galactica_%281978_TV_series%29"><i><span style="font-family: "georgia" , "serif";">Battlestar</span></i></a><i><span style="color: black; font-family: "georgia" , "serif";"> </span></i><a href="https://en.wikipedia.org/wiki/Battlestar_Galactica_%282004_TV_series%29"><i><span style="font-family: "georgia" , "serif";">Galactica</span></i></a><span style="color: black; font-family: "georgia" , "serif";">.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure 5.</span></b><span style="color: black; font-family: "georgia" , "serif";"> Detail
of the Cyrannus quadruple system, home of the Twelve Colonies</span></span></div>
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<a href="https://4.bp.blogspot.com/-kgEUP3S2_rQ/VwmLfe3pP9I/AAAAAAAAAyM/YRWxfeBCQtgRS0GcoSppXwIaSBl9J6A7w/s1600/Cyrannus%2Bquadruple%2Bsystem%2B-%2Bdetail.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="246" src="https://4.bp.blogspot.com/-kgEUP3S2_rQ/VwmLfe3pP9I/AAAAAAAAAyM/YRWxfeBCQtgRS0GcoSppXwIaSBl9J6A7w/s640/Cyrannus%2Bquadruple%2Bsystem%2B-%2Bdetail.jpg" width="640" /></a></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Here I’m thinking of C.L. Moore’s tales of </span><a href="https://en.wikipedia.org/wiki/Northwest_Smith"><span style="font-family: "georgia" , "serif";">Northwest Smith</span></a><span style="color: black; font-family: "georgia" , "serif";">, an outlaw rocketeer whose wanderings take
him to Venus, Mars, and an unnamed moon of Jupiter, all of which have
indigenous life. Moore’s vision of a densely inhabited Solar System, as
presented in her fiction of the early 1930s, was resurrected a decade later in dozens
of stories by her friend Leigh Brackett. The latter created a similar outlaw
hero, </span><a href="https://en.wikipedia.org/wiki/Eric_John_Stark"><span style="font-family: "georgia" , "serif";">Eric John Stark</span></a><span style="color: black; font-family: "georgia" , "serif";">, an
Earthling who was raised from infancy – much like Tarzan – by a tribe of hairy
Mercurians. His adventures regularly bring him to Mars and Venus, which as in
Moore’s stories are envisioned respectively as a decadent desert planet and a savage
jungle planet.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The creators of <i>Battlestar
Galactica</i> modernized this many-worlds setting by leaving our Solar System
altogether and situating the human race in an extrasolar locale called the
Twelve Colonies. These are twelve inhabited worlds in the conjoined planetary systems
orbiting a quadruple star system. Google searches returned several maps of this
remarkable construct; <b>Figure 5</b> shows
a detail of one. A similar approach was adopted by the people behind the short-lived
but much-loved </span><a href="https://en.wikipedia.org/wiki/Firefly_%28TV_series%29"><i><span style="font-family: "georgia" , "serif";">Firefly</span></i></a><span style="color: black; font-family: "georgia" , "serif";"> series when
they created their wild extrasolar frontier. Known as the Verse, this locale
appears to be a quintuple star system that seriously violates the laws of
binary hierarchy (a characteristic shared with <a href="https://en.wikipedia.org/wiki/Serenity_%28Firefly_vessel%29">Serenity’s
crew</a>). <b>Figure 6</b> shows the
official map, which highlights inhabited exomoons and exoplanets.</span></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure 6</span></b><span style="color: black; font-family: "georgia" , "serif";">. The Verse,
scene of the action in <i><a href="http://www.imdb.com/title/tt0303461/">Firefly</a></i> and <i><a href="http://www.imdb.com/title/tt0379786/">Serenity</a></i></span></span></div>
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</span><br />
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<a href="https://2.bp.blogspot.com/-2K6OCxEjzyE/VwmLp7ItmtI/AAAAAAAAAyU/hok_A5yREVgb4QS-M9qdlfG8TTTke-JDg/s1600/The%2BVerse.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="422" src="https://2.bp.blogspot.com/-2K6OCxEjzyE/VwmLp7ItmtI/AAAAAAAAAyU/hok_A5yREVgb4QS-M9qdlfG8TTTke-JDg/s640/The%2BVerse.jpg" width="640" /></a></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Next to these impossible wonders, </span><span class="MsoHyperlink"><a href="http://backalleyastronomy.blogspot.com/2014/12/hot-jupiter-twins-wasp-94.html"><span style="font-family: "georgia" , "serif";">WASP-94</span></a></span><span style="color: black; font-family: "georgia" , "serif";"> and </span><span class="MsoHyperlink"><a href="http://backalleyastronomy.blogspot.com/2014/12/hot-jupiter-twins-wasp-94.html"><span style="font-family: "georgia" , "serif";">XO-2</span></a></span><span style="color: black; font-family: "georgia" , "serif";"> offer
thin gruel indeed. But at least they’re real. And the odds are that somewhere
in our Galaxy, there’s at least one pair of conjoined systems with a habitable
planet or moon between them.</span></span></div>
<span style="font-size: large;">
</span><br />
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "georgia" , "serif";"><span style="font-size: large;"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i>Click here for a permanent index by topic of
blog posts<br />
on Back Alley Astronomy</i></a></span> </span></div>
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<b><span style="color: black; font-size: 10pt;">REFERENCES</span></b></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Kostov V</span></b><span style="color: black; font-size: 10pt;">, Allan D, Hartman N, Guzewich S, Rogers J. (2013) Winter is Coming.
Submitted for publication in <i style="mso-bidi-font-style: normal;">Oldtown
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Kostov V</span></b><span style="color: black; font-size: 10pt;">, Orosz JA, </span><span style="color: black; font-size: 10pt;">Welsh WF, </span><span style="color: black; font-size: 10pt;">Doyle LR, Fabrycky DC</span><span style="color: black; font-size: 10pt;">, Haghighipour N, et
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Welsh WF</span></b><span style="color: black; font-size: 10pt;">, Orosz JA, Short DR, Cochran WD,
Endl M, Brugamyer E, et al. (2015) Kepler 453 b – The 10th Kepler transiting
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Welsh WF</span></b><span style="color: black; font-size: 10pt;">, Orosz J, Quarles B,
Haghighipour N. (2015) Kepler-47: A Three-Planet Circumbinary System. American
Astronomical Society, ESS Meeting #3, id.402.01. Abstract: </span><span style="font-size: 10pt;"><a href="http://adsabs.harvard.edu/abs/2015ESS.....340201W">2015ESS.....340201W</a>
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-91691846314221920302016-03-27T21:36:00.003-07:002016-05-01T11:44:23.688-07:00The Nearest 20 Parsecs<!--[if gte mso 9]><xml>
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<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://1.bp.blogspot.com/-ErOLDEhStjM/Vvi0BlPdDaI/AAAAAAAAAxI/QMvfdBArrwcWzI3JFZQcSkjszd6Kg4CbA/s1600/20%2BParsecs%2B-%2BFeb%2B2016.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="586" src="https://1.bp.blogspot.com/-ErOLDEhStjM/Vvi0BlPdDaI/AAAAAAAAAxI/QMvfdBArrwcWzI3JFZQcSkjszd6Kg4CbA/s640/20%2BParsecs%2B-%2BFeb%2B2016.gif" width="640" /></a></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Familiar
bright stars and selected exoplanetary host stars within 20 parsecs (65 light
years) of our Sun. This stylized diagram illustrates the perspective of an
observer located in the north celestial hemisphere, in the direction of the
constellation Draco. Approximate stellar spectral types are coded. Names of
host stars appear in turquoise; names of stars without known planets appear in violet.
</span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------</span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">With a stellar population in excess of 100 billion and a
diameter of at least 30,000 parsecs (100,000 light years), our </span><span style="font-size: large;"><a href="http://apod.nasa.gov/apod/ap000130.html"><span style="color: #0070c0; font-family: "georgia" , "serif";">Milky Way</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> is a good-sized </span><a href="https://www.eso.org/public/images/ngc2280-potw/"><span style="color: #0070c0; font-family: "georgia" , "serif";">spiral galaxy</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">. Astrophotography
has unveiled some of its most spectacular features, from the claw-like </span><a href="http://apod.nasa.gov/apod/ap150118.html"><span style="color: #0070c0; font-family: "georgia" , "serif";">filaments</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> just north of </span><a href="https://en.wikipedia.org/wiki/Sagittarius_A*"><span style="color: #0070c0; font-family: "georgia" , "serif";">Sagittarius A*</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> (our resident
black hole, about 7900 parsecs away) to the billowing clouds and blazing young
stars of the </span><a href="http://www.eso.org/public/images/eso1208a/"><span style="color: #0070c0; font-family: "georgia" , "serif";">Carina Nebula</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> (2300
parsecs away) and the delicate filigree of supernovae remnants such as </span><a href="http://apod.nasa.gov/apod/ap121009.html"><span style="color: #0070c0; font-family: "georgia" , "serif";">Simeis 147</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> and </span><a href="http://apod.nasa.gov/apod/ap100910.html"><span style="color: #0070c0; font-family: "georgia" , "serif";">Vela</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> (920 and 250 parsecs away, respectively). Yet
even as these panoramic views of our local universe swim into focus, other
details remain fuzzy, especially when we turn to the question of potential planets
orbiting all those billions of stars.</span></span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Three decades of searching, using several different techniques,
have returned data on 2098 confirmed extrasolar planets in more than 1300
different star systems. The most complete and broadly comparable information
continues to flow from radial velocity (RV) and transit searches. Apart from two
outliers (SWEEPS-04 and -11, both in the Galactic Bulge), the most remote star
with planets detected by either method lies at a distance of 3200 parsecs. That’s
more than 10% of the diameter of the Galactic Disk.</span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">But if we take a closer look at the combined sample of RV
and transiting planets available in early January (n = 1867), stellar distances
shrink fast. Searches of the </span><span style="font-size: large;"><a href="http://exoplanet.eu/"><span style="color: #0070c0; font-family: "georgia" , "serif";">Extrasolar Planets
Encyclopaedia</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> (EPE), the </span><a href="http://kepler.nasa.gov/Mission/discoveries/"><span style="color: #0070c0; font-family: "georgia" , "serif";">Kepler Discoveries Table</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">, and
various discovery papers found me distance estimates for just 713 stars in that
sample. Although they range from 3 to 3200 parsecs, 50% are closer than 90
parsecs, and 70% are no farther than 200 parsecs. The size of those two fractions
exposes a critical deficiency in our picture of extrasolar planets: the subset
of robustly characterized systems skews sharply in favor of the nearest stars. Even
though more than 1000 transiting planets have been confirmed by the Kepler
Mission, fewer than 10% of their stellar hosts have distance estimates. The
ones with estimates typically reside between 200 and 2700 parsecs.</span></span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Evidently our knowledge of transiting as well as RV
planets has gaps of near-cosmic proportions.</span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">If we want to grasp the true diversity of extrasolar
planets and system architectures, we could do worse than examine a clearly
delimited sample of well-constrained systems. I suggest that the best place to
find them is the volume of space within 20 parsecs, where diversity is balanced
by precision of observation.</span></div>
<div class="MsoNormal">
<span style="font-size: large;"></span> </div>
<div class="MsoNormal">
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">the
immediate solar neighborhood</span></b></span></div>
<div class="MsoNormal">
<span style="font-size: large;"></span> </div>
<div class="MsoNormal">
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 1</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> represents
a sphere centered on our Sun with a diameter of 40 parsecs. Although this
region is devoid of O- and B-type stars – the rarest and most massive species
in our Galaxy – all of the more common varieties of main sequence and evolved
stars are represented. The brightest nearby objects are young blue-white stars
of type A, such as Vega and Sirius, and older red giants, such as Pollux and
Aldebaran. Many bright stars in our immediate neighborhood are binary or
multiple. For example, both Sirius and Procyon have tiny white dwarf companions
that were born as B stars and then rapidly bloomed and withered into their
present state. Capella, the third brightest star in northern skies, is actually
a quadruple system, consisting of a pair of tightly bound red giants accompanied
by a pair of red dwarfs at a separation of 10,000 astronomical units (AU). More
than a dozen exoplanetary host stars in our immediate neighborhood are also members
of binary or higher-order multiples.</span></span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Google couldn’t find me an authoritative estimate of the
full stellar population within 20 parsecs, but I did locate some approximate
counts of G-type stars in this space (about 130). A rough extrapolation from
available sources (Turnbull 2013, RECONS 2012) would suggest a population of 1500-2000
stars inside our 20-parsec radius, with about 75% classed as M dwarfs. (If
anyone reading this knows of an authoritative census, please let me know!) Within
the full population are 73 host stars accompanied by a total of 129 planets, suggesting
that only 3%-5% of stars in this volume can be described as “planetic.” Yet
recent studies argue that virtually every star of spectral types M through G (and
probably types F through A) harbors at least one planet (Winn & Fabrycky
2015). Evidently a vast population of planetary systems remains to be
discovered right on our Galactic doorstep.</span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 2</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> sorts
the known exoplanetary systems within 20 parsecs according to their distance
from our Sun, with each successive ring in the blue background representing an
increment of 5 parsecs in the radius of the expanding sphere.</span></span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure
2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> All host stars at 20 parsecs or less, arranged by distance from the Sun</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-XyweNCjH_ps/Vvi0Jgb1f6I/AAAAAAAAAxM/OO-4MrmzhkMbRlBQvJ2ehiCBqHgWoZjow/s1600/Exoplanets%2Bwithin%2B20%2Bpc-2016-dimmer-names.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="640" src="https://3.bp.blogspot.com/-XyweNCjH_ps/Vvi0Jgb1f6I/AAAAAAAAAxM/OO-4MrmzhkMbRlBQvJ2ehiCBqHgWoZjow/s640/Exoplanets%2Bwithin%2B20%2Bpc-2016-dimmer-names.gif" width="640" /></a></div>
<div class="MsoNormal">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br /></div>
<div class="MsoNormal">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">73 exoplanetary host stars sorted by distance
from our Sun. Spectral types are indicated by the color key at lower right. The
inner circle has a radius of 5 parsecs, and each successive ring represents an
increment of 5 parsecs. Stellar icons are arranged in 2 dimensions by right
ascension, which is marked at the edge of the outer circle. (Declination is
necessarily ignored.) This diagram shows the relative distance of planetary
systems from our Sun, but not from each other. </span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">-------------</span></div>
<div class="MsoNormal">
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Inspection of the diagram reveals that the frequency of
confirmed host stars per cubic parsec falls rapidly with increasing distance,
even within this limited space. So does the ratio of dim stars (spectral type
M) to brighter stars. G-type stars, on the other hand, appear to be over-represented.
They constitute less than 10% of field stars within 20 parsecs, but almost
one-third of the sample of host stars in this space.</span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">About 57% of the planets in Figure 2 are low-mass objects
in the same classification as Uranus and Earth (<b style="mso-bidi-font-weight: normal;">Table 1</b>). These planets have masses smaller than about 0.15 Jupiter
masses (0.15 Mjup), equivalent to 48 Earth masses (48 Mea). The remainder are
gas giants like Jupiter and Saturn, although very few of them have orbital
periods as long as our giants. A similar majority of nearby host stars (55%) harbors
a single detected planet. Nonetheless, multiplanet systems are abundant enough
that only one-third of all planets in this space occur in singleton systems.</span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Thus the “average” planet in our neighborhood is a low-mass
object with at least one other companion orbiting the same star. Notably, this
description applies to six out of eight planets in our Solar System.</span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure
3.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Transiting low-mass planets within 20 parsecs</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://2.bp.blogspot.com/-K1RMo8qGUHo/Vvi0ZI6vgkI/AAAAAAAAAxQ/MX8liv-n8UYzH10qMEVeDlK21wB9N2kPQ/s1600/Transiting%2Blow-mass%2Bplanets%2Bwithin%2B20%2Bpc.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="270" src="https://2.bp.blogspot.com/-K1RMo8qGUHo/Vvi0ZI6vgkI/AAAAAAAAAxQ/MX8liv-n8UYzH10qMEVeDlK21wB9N2kPQ/s640/Transiting%2Blow-mass%2Bplanets%2Bwithin%2B20%2Bpc.jpg" width="640" /></a></div>
<div class="MsoNormal">
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><br /></div>
<div class="MsoNormal">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">All these planets have been observed in
transit from Earth’s orbit. A single transit has also been reported for <a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html">HD
219134 b</a>. The extrasolar planets in this image have orbital periods shorter
than 3 days; the period of 55 Cancri e is shorter than 18 hours.</span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">----------------------</span></div>
<div class="MsoNormal">
<br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Our
neighborhood also contains several distinctive architectural designs. The most
common (11 systems; 15%) features at least <b style="mso-bidi-font-weight: normal;">two gas giants</b>.
More than half of this sample harbors three or more planets in total, and in 9
out of 11 systems, at least one planet orbits outside 2 AU. Again, this
description fits our Solar System, but it also applies to quite different
systems, including Upsilon Andromedae, 47 Ursae Majoris, and HD 128311.</span></span><br />
<span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="font-family: "georgia" , "serif";"></span><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">The
next most common architecture (12%) is the <b style="mso-bidi-font-weight: normal;">compact low-mass</b>
configuration, in which at least three low-mass planets orbit inside 1 AU and
no gas giants are detected. This architecture is mutually exclusive with the
previous type, and it is extremely common in the Kepler sample. Well-studied
examples within 20 parsecs include GJ 581 and HD 69830.</span></span><br />
<span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="font-family: "georgia" , "serif";"></span><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">The
third most common architecture overlaps with the two-giant design. This is the <b style="mso-bidi-font-weight: normal;">mixed-mass</b> type
(10%), in which a minimum of three planets are present: at least one gas giant
and at least one low-mass object. 55 Cancri, GJ 876, and Mu Arae are notable
nearby examples containing at least two gas giants each.</span></span><br />
<span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="font-family: "georgia" , "serif";"></span><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">In
the full exoplanetary sample, the most abundant and easily detectable
architecture is the Hot Jupiter configuration, defined as a star accompanied by
a gas giant planet with a period of 10 days or less. However, this is only the
fourth most common architecture in our immediate neighborhood. Just five (7%)
of the systems detected to date in this space contain a <b style="mso-bidi-font-weight: normal;">Hot Jupiter</b>.
Three of them (HD 189733, Tau Boötis, and 51 Pegasi) are singletons. Each of
the others(Upsilon Andromedae, HD 217107) harbors at least one additional gas
giant on a wider orbit, so these two systems can also be assigned to the
two-giant architecture.</span></span><br />
<span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="font-family: "georgia" , "serif";"></span><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">Among
<b style="mso-bidi-font-weight: normal;">singleton</b>
systems within 20 parsecs, giants have a slight edge, accounting for 58% of
this subsample. Hot Jupiters are in a minority, however: the median semimajor
axis for single giants in this volume of space is 1.3 AU, compared to 0.1 AU
for low-mass singletons. Single giants also tend to orbit more massive stars:
inside 20 parsecs, the median host star mass in such systems is 0.9 Msol
(maximum 1.65 Msol), whereas the median for hosts of low-mass singletons is
0.49 Msol (maximum 0.97 Msol). These numbers suggest a certain antipathy
between hot stars and low-mass planets, whether real or the result of
observational bias. Low-mass planets might truly be scarce around hot stars, or
they might simply be harder to detect in such environments.</span></span><br />
<span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="font-family: "georgia" , "serif";"></span><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;">One
of the least common architectures revealed to date is the Solar System analog.
I define this configuration as a Sun-like star (0.7-1.2 Msol) hosting a gas
giant on a circular orbit wider than 3 AU, without any gas giants on interior
orbits either occupying or perturbing the system habitable zone. This configuration
is a subset of the mixed-mass type. Inside a radius of 20 parsecs, only one
system besides our own features such an architecture: HD 154345. More than a
dozen others have been reported outside this volume, but the most distant is
only 57 parsecs away (see </span><a href="http://backalleyastronomy.blogspot.com/2012/05/how-weird-is-our-solar-system.html"><span style="color: blue; font-family: "georgia" , "times new roman" , serif; font-size: large;">How
Weird is Our Solar System?</span></a><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"> </span></span><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif";">and</span><span style="font-family: "georgia" , "serif";"> <a href="http://backalleyastronomy.blogspot.com/2016/03/almost-jupiter.html"><span style="color: blue;">Almost
Jupiter</span></a></span><span style="color: black; font-family: "georgia" , "serif";">).
None are known within the Kepler sample.</span></span></span><br />
<span style="color: black; font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"></span></span><br />
<span style="color: black; font-family: "georgia" , "serif";"></span><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif";">Their absence is not surprising, since Kepler collected data for
less than four years and observed only planetary transits. The cool gas giant
in a Solar System analog will have an orbital period longer than five years
(and probably at least twice as long) and it might not be co-planar with the
inner planets. In such a configuration, an observer with a fixed viewing angle could
never observe both the inner and outer planets in transit.</span></span></span></span><br />
<br />
<span style="font-size: small;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: medium;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">
Characteristics of three exoplanetary populations</span></span></span></span></span></span></div>
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<a href="https://1.bp.blogspot.com/-LsqnW4PSso0/Vvi0iHw1RGI/AAAAAAAAAxU/a0GNR6cwoL4jdDY8NGde978jb52dIRlyA/s1600/Three%2BExoplantary%2BPopulations%2BCompared.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="300" src="https://1.bp.blogspot.com/-LsqnW4PSso0/Vvi0iHw1RGI/AAAAAAAAAxU/a0GNR6cwoL4jdDY8NGde978jb52dIRlyA/s640/Three%2BExoplantary%2BPopulations%2BCompared.gif" width="640" /></a></div>
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<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Among objects listed in </span><span style="font-size: large;"><a href="http://exoplanet.eu/"><span style="font-family: "georgia" , "serif";">EPE</span></a><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> in
early 2016, two substantially larger, non-overlapping samples are available to
compare with the discrete population in our immediate neighborhood: 1) <b style="mso-bidi-font-weight: normal;">472</b> confirmed Kepler systems hosting
1038 planets, and 2) <b style="mso-bidi-font-weight: normal;">606</b> confirmed
transiting and RV systems outside 20 parsecs hosting 707 planets, excluding
Kepler discoveries. <b style="mso-bidi-font-weight: normal;">Table 1</b> compares
both samples to the population of <b style="mso-bidi-font-weight: normal;">73</b>
host stars and 129 planets within 20 parsecs.</span></span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">The descriptive statistics summarized in the table suggest
at least six salient points:</span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">1.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">
Although all three groups are strongly biased against M dwarfs (defined as
stars < 0.7 Msol) and in favor of hot stars (those ≥ 1.2 Msol), our
neighborhood population appears to have the most representative selection of
stellar hosts. For example, if we want a “typical” M-type sample, the 26
systems orbiting red dwarfs within 20 parsecs are a good place to start.</span></span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Only the
Kepler sample has a minority of single-planet systems, alerting us that even the
host stars in our local sample must have many additional planets. These objects
have eluded detection so far because they are either very low in mass (and thus
undetectable by ground-based RV monitoring) or traveling on long-period orbits
(which require a substantial investment of resources to ensure regular
observations over a period of decades).</span></span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">3.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> The
abundance of multiplanet systems in the Kepler sample (78%) is evidence of the opposite bias. It is much easier to confirm the detection of a
transiting planet with one or more candidate companions than it is to rule out
all sources of false positives for a single planet with similar transit data. Hence
Kepler systems with a single transiting planet (“tranet”) are less likely to be
confirmed than those with multiple tranets. Nevertheless, a system with a confirmed
tranet might still harbor invisible companions if their orbits are widely
spaced (Xie et al. 2014) or non-coplanar (Johansen et al. 2012).</span></span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">4.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> The
relative fractions of low-mass planets in these three samples indicate a more
straightforward bias: the RV technique is simply less sensitive to small
planets than transit monitoring is, and sensitivity declines rapidly outside 10
parsecs. That limitation probably indicates that hundreds of terrestrial
planets are awaiting discovery within our immediate neighborhood.</span></span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">5.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> The
diversity of system architectures is greatest within 20 parsecs, as
demonstrated by the low frequencies of two-giant, compact low-mass, and mixed-mass systems in the other two populations. This feature makes our neighborhood
sample especially valuable for comparison studies.</span></span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">6.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Hot
Jupiters are heavily represented in all three populations, with the most
obvious excess in the RV sample outside 20 parsecs. Nevertheless, the Kepler
fraction is quite similar to the fraction in the local population. If only
about 1% of Sun-like stars harbor a Hot Jupiter, as recent studies conclude (Bayliss
& Sackett 2011, Wright & al. 2012, Wang & al. 2015), both of these
samples are unbalanced.</span></span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">local
bubble</span></b></span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">If we expand our perspective by an order of magnitude to
a radius of 200 parsecs, we see a more varied starscape (<b style="mso-bidi-font-weight: normal;">Figure 3</b>). This larger volume of space encompasses most of the region
known as the Local Bubble, an irregular, gas-free cavity surrounded by
molecular hydrogen clouds that trace the Orion Spur of our local spiral arm. Whereas
stars in the immediate vicinity of our Sun appear to be a random sample of the
Galactic population, our enlarged perspective reveals localized structures.</span><br />
<span style="font-family: "georgia"; font-size: large;"></span> </div>
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<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure
3.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Selected star clusters and exoplanetary systems within 200 parsecs</span></span></div>
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<a href="https://3.bp.blogspot.com/-lPPtFYbYxwg/Vvi0tOTbYiI/AAAAAAAAAxc/NS0BjN60gMAG_hKXKWJq4LJMOyxdDJ73A/s1600/Exoplanets%2Bwithin%2B200%2BParsecs%252C%2BNorthern%2BStyle-2016.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="640" src="https://3.bp.blogspot.com/-lPPtFYbYxwg/Vvi0tOTbYiI/AAAAAAAAAxc/NS0BjN60gMAG_hKXKWJq4LJMOyxdDJ73A/s640/Exoplanets%2Bwithin%2B200%2BParsecs%252C%2BNorthern%2BStyle-2016.gif" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Selected exoplanetary systems and star
clusters within 200 parsecs (652 light years) of our Sun. Each successive ring
in the colored circle represents an additional 50 parsecs. In this flattened,
two-dimensional schema, stellar icons are distributed by right ascension
according to their distance from our Sun, not by their distance from each
other. Right ascension is marked by the numbers ascending counterclockwise
around the outer ring.<span style="mso-spacerun: yes;"> </span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">-------------</span><span style="color: black; mso-themecolor: text1;"></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Visible are 10 star clusters, some young and bright,
others older and dimmer. A star cluster is a compact association in which the
space density of stars is much higher than the surrounding environment, and
member stars share a similar motion through space. Each cluster population is
far from random, since cluster members were born together in the same molecular
cloud, and thus have very similar ages and chemical compositions. All stars of
the same mass in the same cluster are at the same evolutionary stage.</span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Star clusters can be described as either embedded or
open, depending on the presence (embedded) or absence (open) of remnant hydrogen
clouds. Several nearby clusters are still embedded: Rho Ophiuchi,
Taurus-Auriga, Chamaeleon I, and Coronet. Their estimated ages range from 1 to
10 million years. The open clusters are substantially older, with the Pleiades about
125 million years old, Praesepe 600 million, and the Hyades 625 million. Notably,
planets have been discovered in both Praesepe and the Hyades.</span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Apart from these astronomically interesting and
aesthetically captivating structures, we can also see several field stars that
harbor unusual planetary systems. </span><span style="font-family: "georgia" , "serif"; font-size: large;"><a href="https://en.wikipedia.org/wiki/Kepler-16"><span style="color: #0070c0;">Kepler-16</span></a><span style="color: black; mso-themecolor: text1;">, located only 65 parsecs (212 light
years) away, was the first </span><span class="MsoHyperlink"><b style="mso-bidi-font-weight: normal;"><span style="color: #0070c0;"><a href="http://backalleyastronomy.blogspot.com/2012/08/kepler-47-is-amazing.html"><span style="color: #0070c0;">circumbinary system</span></a></span></b></span><span style="color: black; mso-themecolor: text1;"> ever detected, and the nearest by a
margin of several hundred parsecs. This system includes a K-dwarf and an
M-dwarf sharing a binary orbit of 41 days, with an undersized gas giant (Kepler-16b)
orbiting their common center of mass in a period of 229 days. Remarkably, even
though gas giant environments are inimical to life as we know it, this giant
happens to reside in the system’s habitable zone. While we can be pretty sure
that circumbinary planets are rare, they might not be so rare that Kepler-16 is
alone in our Local Bubble.</span></span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Venturing into deeper space, we find not one but two
examples of another rare species: </span><span class="MsoHyperlink" style="font-size: large;"><span style="color: #0070c0;"><a href="http://backalleyastronomy.blogspot.com/2014/12/hot-jupiter-twins-wasp-94.html"><b style="mso-bidi-font-weight: normal;"><span style="color: #0070c0; font-family: "georgia" , "serif";">conjoined planetary systems</span></b></a></span></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">,
defined as stellar binaries in which each star hosts its own system of planets.
XO-2NS comprises two <span style="font-family: "georgia" , "serif";">G</span>-type stars with a binary semimajor axis of 4600 AU. Star
N hosts a Hot Jupiter; star S hosts two warm gas giants. WASP-94AB is a pair of <span style="font-family: "georgia" , "serif";">F</span>-type stars with a binary semimajor
axis of 2700 AU. Both stars host Hot Jupiters, with no other planets in
evidence.</span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">Then we find the rarest of the rare, the only one of its
kind: </span><span class="MsoHyperlink" style="font-size: large;"><span style="color: #0070c0;"><a href="http://backalleyastronomy.blogspot.com/2015/08/the-blazing-wasp-47s.html"><span style="color: #0070c0; font-family: "georgia" , "serif";">WASP-47</span></a></span></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">, which includes
a Hot Jupiter flanked by two low-mass planets inside 0.1 AU. No other Hot
Jupiter has any nearby companions. WASP-47<span style="font-family: "georgia" , "serif";">b </span>also belongs to another small club:
Hot Jupiters with outer giant companions, of which we identify two inside 20
parsecs, but few others in deeper space.</span><br />
<br />
<span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">An unmistakable feature of Figure 3 is the Kepler
treasure trail extending outward along right ascension 19 into infinity. Joining
Kepler-16 in this procession of wonders is </span><span class="MsoHyperlink" style="font-size: large;"><span style="color: #0070c0;"><a href="http://backalleyastronomy.blogspot.com/2016/01/a-data-driven-year.html"><span style="color: #0070c0; font-family: "georgia" , "serif";">Kepler-444</span></a></span></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;"> and its
Hot Martian quintuplets (five planets less massive than Earth orbiting a K-type
star in periods shorter than 10 days) as well as two M dwarf systems (</span><span class="MsoHyperlink" style="font-size: large;"><span style="color: #0070c0;"><a href="http://backalleyastronomy.blogspot.com/2015/01/much-ado-about-earth-2.html"><span style="color: #0070c0; font-family: "georgia" , "serif";">Kepler-186, 438</span></a></span></span><span style="color: black; font-family: "georgia" , "serif"; font-size: large; mso-themecolor: text1;">)
hosting small planets that have been widely promoted as potentially habitable
Super Earths. Also represented is one of the few mixed-mass systems in the
Kepler sample: Kepler-68, which hosts two small transiting planets inside 0.1
AU as well as a non-transiting gas giant detected by radial velocity
measurements orbiting just outside 1 AU. All these Kepler systems enhance the
diversity of the region between 50 and 200 parsecs, which otherwise presents a
bland continuum of relatively bright stars hosting lonely giants.</span></div>
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</span><span style="color: #0070c0;">on Back Alley Astronomy</span></a></span></i></span><span class="MsoHyperlink"><i style="mso-bidi-font-style: normal;"><span style="color: #0070c0;"> </span></i></span><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";"></span></i></div>
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com3tag:blogger.com,1999:blog-7485863064099307059.post-26851495649568168102016-03-20T14:25:00.002-07:002017-01-02T19:18:54.768-08:00Almost Jupiter<!--[if gte mso 9]><xml>
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{mso-style-name:"Table Normal";
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mso-tstyle-colband-size:0;
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mso-tstyle-diagonal-down:none;
mso-tstyle-diagonal-up:none;
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<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://1.bp.blogspot.com/-QUcMvGJFluE/Vu8UMkhpm6I/AAAAAAAAAvk/LiJFQ-lpLP4yPRagMsGiv-fwmRM3lbFmA/s1600/Jupiter%2B-%2BRed%2Bspot.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="382" src="https://1.bp.blogspot.com/-QUcMvGJFluE/Vu8UMkhpm6I/AAAAAAAAAvk/LiJFQ-lpLP4yPRagMsGiv-fwmRM3lbFmA/s640/Jupiter%2B-%2BRed%2Bspot.jpg" width="640" /></a></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Saturn has
the rings, but Jupiter has the Great Red Spot: a cyclone large enough to
swallow Earth. Telescopic observations attest that this storm system has been
raging for centuries, although it has shrunk by half in the past hundred years.
Similar vortices circulate elsewhere in the planet’s deep atmosphere. Objects
like Jupiter – i.e., gas giants following circular orbits with periods of
several Earth years – are apparently rare in our region of the Galaxy. Image credit:
NASA/Voyager 1</span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------</span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">David Kipping and colleagues recently announced the
discovery of a transiting gas giant with an orbital period of 2.9 years, the
longest ever confirmed for a transiting planet. Their analysis opens new
horizons in our understanding of planetology and system architecture. Kipping’s
group found the object by searching archival Kepler data, which reveal a
mixed-mass system of four planets orbiting Kepler-167 (alias KIC-3239945). The star
was already known as the host of two short-period Super Earths, to which
Kipping’s group has added a third. All three have semimajor axes smaller than 0.15
astronomical units (0.15 AU) and radii smaller than twice Earth’s (2 Rea).</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The new giant is the outermost of the four planets,
orbiting at a semimajor axis of almost 2 AU (which would fall between Mars and
the Asteroid Belt in our Solar System). According to the standard naming
protocol, it is designated <b>Kepler-167e</b>.
Only two transits could be detected during the four-year span of Kepler data
collection, but that’s just enough to validate the object’s reality.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Table 1.</span></b><span style="color: black; font-family: "georgia" , "serif";">
Characteristics of the Kepler-167 <span style="font-family: "georgia" , "serif";">p</span>lanetary <span style="font-family: "georgia" , "serif";">s</span>ystem</span></span></div>
<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span><a href="https://1.bp.blogspot.com/-aqwHbHqkCqc/VvS2sbsDwCI/AAAAAAAAAwk/pUq4FxibscIgxcT-vSkKRrTtmW9iWuuaw/s1600/Kepler-167-parameters2.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="178" src="https://1.bp.blogspot.com/-aqwHbHqkCqc/VvS2sbsDwCI/AAAAAAAAAwk/pUq4FxibscIgxcT-vSkKRrTtmW9iWuuaw/s640/Kepler-167-parameters2.gif" width="640" /></a><br />
<div class="separator" style="clear: both; text-align: left;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Column 1 shows the planet name; column 2, the
radius in Earth units (Rea); column 3, the semimajor axis (a) in astronomical
units; column 4, the eccentricity (e); and column 5, the orbital period in
days. </span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------</span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The host star has a likely spectral type of K3 or K4,
given its estimated mass of 0.77 Solar masses (Msol) and effective temperature
of 4890 K. Surface gravity measurements confirm its evolutionary status on the
main sequence. The discovery team estimated the star’s age as 3.3 billion years
and its distance as 330 parsecs (1075 light years).</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Table 1</span></b><span style="color: black; font-family: "georgia" , "serif";"> and <b>Figure 2</b> describe a virtual twin of HD
219134 (for two different perspectives on that system, see <a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html">Motalebi
& al. 2015</a> and <a href="http://backalleyastronomy.blogspot.com/2015/10/a-different-picture-of-hd-219134.html">Wright
& al. 2015</a>). Both Kepler-167 and HD 219134 are early K dwarfs with
identical masses, and both host a cluster of small planets inside 0.25 AU as
well as a presumed gas giant at an approximate semimajor axis of 2 to 3 AU.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure
2.</span></b><span style="color: black; font-family: "georgia" , "serif";"> Kepler-167 <span style="font-family: "georgia" , "serif";">s</span>ystem <span style="font-family: "georgia" , "serif";">a</span>rchitecture</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://2.bp.blogspot.com/-92GckctgYMs/Vu8U1be1MuI/AAAAAAAAAvw/kRBJa4QNKU0cCu36EY9BQ6X9aXgznvPFw/s1600/Kepler-167%2Bsystem%2Barchitecture.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="556" src="https://2.bp.blogspot.com/-92GckctgYMs/Vu8U1be1MuI/AAAAAAAAAvw/kRBJa4QNKU0cCu36EY9BQ6X9aXgznvPFw/s640/Kepler-167%2Bsystem%2Barchitecture.gif" width="640" /></a></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">The four planets of Kepler-167 are
represented at their relative sizes. All orbits must be co-planar, since all
planets are observed in transit. As an early K dwarf, the parent star is dimmer
and cooler than our Sun. Thus planet <b style="mso-bidi-font-weight: normal;">e</b>
orbits outside the system ice line. </span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------</span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">This architecture has conspicuous structural similarities
with the other multiplanet systems summarized in <b>Figure 3</b>. All 10 systems include a Sun-like star (0.7 to 1.2 Msol)
accompanied by at least two low-mass planets on short-period orbits and at least one gas
giant on a wider orbit. In 9 out of 10 systems, at least one low-mass planet
orbits inside 0.1 AU, and in 7 out of 10, the gas giant orbits outside 1 AU. In
two systems, an additional low-mass planet orbits outside the gas giant. One
system (WASP-47) hosts two gas giants, with the second one dominating the inner
cluster of planets. Another (Kepler-90) might also host two gas giants, since
planets <b>g</b> and <b>h</b> both have radii larger than 8 Rea. In Kepler-90, however, both
these planets are part of the outer cluster of planets.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">A striking feature shared by most systems is the pronounced
gap between the inner aggregation of low-mass planets and the outer giant. This
gap is absent from the two most compact systems (Kepler-89 and -289), where all
known planets orbit inside 0.6 AU, and it has a different configuration in
Kepler-90 and HD 10180, which host seven planets each. Altogether, these 10
systems look like a suite of variations on a single theme.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<div style="text-align: center;">
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure 3.</span></b><span style="color: black; font-family: "georgia" , "serif";"> Selected <span style="font-family: "georgia" , "serif";">m</span>ixed-<span style="font-family: "georgia" , "serif";">m</span>ass <span style="font-family: "georgia" , "serif";">p</span>lanetary <span style="font-family: "georgia" , "serif";">s</span>ystems</span></span></div>
</div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://3.bp.blogspot.com/-c10vXZCptic/Vu8U8N87COI/AAAAAAAAAv0/h5IDH9cZ3kA65BWK0pOgceyNELVZg5K1w/s1600/Mixed-mass%2BRV%2B%2526%2BTransiting%2BSystems%2B-%2Bselected%252C%2B2016.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="640" src="https://3.bp.blogspot.com/-c10vXZCptic/Vu8U8N87COI/AAAAAAAAAv0/h5IDH9cZ3kA65BWK0pOgceyNELVZg5K1w/s640/Mixed-mass%2BRV%2B%2526%2BTransiting%2BSystems%2B-%2Bselected%252C%2B2016.gif" width="484" /></a></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<br /></div>
<div class="MsoNormal">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"><i>Tags</i>: ME = Earth masses; RE = Earth radii; AU = as<span style="font-family: "arial" , "sans-serif";">tronomical unit<span style="font-family: "arial" , "sans-serif";">s (Earth-Sun sepa<span style="font-family: "arial" , "sans-serif";">ration = 1). </span></span></span><i>Selection criteria</i>: at least one gas giant and
at least two low-mass planets on interior orbits. In each system with
transiting planets, all transiting orbits are co-planar. Note that gas giants
Kepler-89d and WASP-47b are also observed in transit, while a single transit is
reported for the innermost planet of HD 219134.</span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------</span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Despite the proximity of the gas giant to the low-mass
planets in each one, many of these systems have an extremely “flat”
configuration, with all planets traveling in the same orbital plane (see </span><a href="http://backalleyastronomy.blogspot.com/2012/10/mass-matters.html"><span style="font-family: "georgia" , "serif";">Mass Matters</span></a><span style="color: black; font-family: "georgia" , "serif";">).</span><span style="color: black; font-family: "georgia" , "serif";"> In half
of them (Kepler-87, 89, 90, 167, and 289), all planets are observed in transit,
implying that all orbits are approximately co-planar. In WASP-47, all three
inner planets are seen in transit, with the same implication. Sensitive,
long-term monitoring will be needed to determine whether the outer giant also
transits.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">However, in two other systems (Kepler-48 and -68), no
transits of the outer giant were detected, suggesting that the inner and outer
systems are misaligned. Furthermore, we have no way to estimate the orbital
alignment of the last two systems. None of the planets around HD 10180 have
been observed in transit, and in HD 219134, a single transit has been reported
for the innermost planet only. Even in the case of a perfectly co-planar
system, our viewing angle might limit transit detection to the planet with the
shortest period, with all the rest orbiting just out of sight.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Co-planarity is commonly interpreted as evidence of a
calm dynamical history. Notably, the eight planets in the Solar System are approximately
co-planar (depending on viewing geometry), and in more than 100 Kepler systems
containing only low-mass planets, at least three planets per system are co-planar.
Data showing that at least half of the compact mixed-mass exoplanetary systems currently
known are also co-planar provide a useful constraint on their formation
history.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">planetology
of Kepler-167</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">We have little information on the composition of the
planets orbiting Kepler-167, since we know only their radii. No transit timing
variations are available to constrain their masses. Although the discovery team
offered hope that future radial velocity programs could characterize the outer
planet, that prospect remains hypothetical in view of the system’s distance.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Kepler-167e has a radius of 10.15 Earth units (Rea), or about
90% of Jupiter’s, which is consistent with either a gas giant planet or a brown
dwarf star. Thus Kipping’s group describes this object as a “degenerate world,”
given the uncertain implications of its radius. The minimum goal of a radial
velocity study would be to break this degeneracy by establishing whether the object’s
mass exceeds 13 Jupiter masses (Mjup), the nominal threshold for brown dwarfs.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Even without mass estimates, we can make informed guesses
about the composition of the three inner planets of Kepler-167. The radius of planet
<b>d</b> corresponds to a rock/metal world
of about 2 Mea, with a structure similar to Earth’s. If only its orbit were
wider – 0.35 AU instead of 0.14 AU – it would be a top candidate for habitability.
The other two planets might also be rocky, if they’re relatively massive (5-9
Mea), but the discovery team favors a mix of rock/metal and volatiles for each
one. Regarding the volatile contribution, radii smaller than 2 Rea suggest water
instead of hydrogen/helium envelopes. For Kepler-167b and -167c, a substantial
water fraction might take the form of a steam atmosphere surrounding a
high-pressure ice layer. Unfortunately, available models of planet structure
suffer from ice aversion, so I haven’t found much theoretical guidance for this
range of radii.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">a
Jupiter analog?</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Now we come to the most remarkable claim in the discovery
paper, evident in its title. The authors describe Kepler-167e as a <b>Jupiter analog</b>. This rare species has
attracted growing interest in the past decade, along with a few competing
definitions (see <a href="http://backalleyastronomy.blogspot.com/2012/05/how-weird-is-our-solar-system.html">How
Weird Is Our Solar System?</a>). According to Kipping & colleagues,
Kepler-167e fulfills three essential requirements: its radius is consistent
with a gas giant planet, its semimajor axis places it outside the system ice line,
and its orbital eccentricity is low.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">However, these criteria are not universally regarded as
sufficient. In two successive studies, Robert Wittenmyer & colleagues (2011,
2016; hereafter W11 and W16) proposed that Jupiter’s most important
characteristic is its dynamical role in the Solar System, both historically and
at the present epoch. Their first study (W11) defined a Jupiter analog as a gas
giant with an orbital period of at least 8 years and an eccentricity smaller
than 0.2. In their view, these parameters implied <i>in situ</i> formation and a relatively calm dynamical history. Their
more recent publication (W16) revised this formulation to a minimum mass of 0.3
Mjup, a semimajor axis outside the system ice line and wider than 3 AU, and an orbital
eccentricity no more than 0.3. W16 note that planets as lightweight as 0.15
Mjup could also fulfill Jupiter’s role, but they set their cut-off at twice
that mass because so few gas giants under 0.3 Mjup are known. To calculate the
frequency of these “Jupiter analogs” (see below) they also limited their
analytic sample to stars with at least 30 observations spanning 8 years.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">All these criteria make sense, but they omit a critical
aspect of Jupiter’s dynamic role: our system has no gas giants inside Jupiter’s
orbit. However that architecture came into being, the underlying mechanism is
very likely to involve Jupiter’s orbital dynamics. Thus, when Wittenmyer’s
group recently applied their criteria to the sample of planets detected by the
Anglo Australian Search Program (W16), the results were odd. They identified 8
systems altogether, among which only 4 host what I would regard as a plausible
Jupiter analog (though only one member of this quartet has a semimajor axis of
5 AU or more). In the other 4, the so-called Jupiter analog is accompanied by a
gas giant on an interior orbit with a semimajor axis near 1 AU. In one of those
systems (Mu Arae), a third gas giant and a Uranus-mass planet also occupy the
inner system. As these system architectures imply dynamical histories very
different from the Solar System, it seems unlikely that such candidate Jupiters
could have played a role truly analogous to the original Jupiter. As <a href="http://backalleyastronomy.blogspot.com/2012/05/how-weird-is-our-solar-system.html">in
the past</a>, therefore, I argue that a <i>bona
fide</i> Jupiter analog must have no interior giant companions.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">Figure
4.</span></b><span style="color: black; font-family: "georgia" , "serif";"> Jupiter and Io</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="https://4.bp.blogspot.com/-a-24R7f9Hcc/Vu8VJgj55LI/AAAAAAAAAv8/b8zItDIozGwJ7FOHdib0GV_a0u7qSswQA/s1600/Jupiter%252BIo%2Bcrescent%2B-%2BNASA-APOD-1100%2Bpx.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="510" src="https://4.bp.blogspot.com/-a-24R7f9Hcc/Vu8VJgj55LI/AAAAAAAAAv8/b8zItDIozGwJ7FOHdib0GV_a0u7qSswQA/s640/Jupiter%252BIo%2Bcrescent%2B-%2BNASA-APOD-1100%2Bpx.jpg" width="640" /></a></div>
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<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Extrasolar analogs of Jupiter are likely to
be accompanied by extensive retinues of satellites, since satellite formation
appears to be the final stage in the evolution of a gas giant planet. This
photograph shows Jupiter with Io, the innermost of the giant’s four <a href="https://en.wikipedia.org/wiki/Galilean_moons">Galilean moons</a>. Apart
from our own Moon, Io is the only spherical satellite in the Solar System with
a purely rocky composition. It supports extensive volcanism, as the image
reveals: the red glow and bluish plume signal active eruptions. Image credit:
NASA/JPL-Caltech.</span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------</span></div>
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<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Another recent study by Dominick Rowan & colleagues
(2016; hereafter R16) adopts a definition based on W11, and thus very similar
to W16: a minimum mass of 0.3 Mjup, a semimajor axis 3 AU or more around a
G-type star (scaling as a period of at least 5 years around stars of other
spectral types), and an orbital eccentricity under 0.3. Their study reports the
discovery of <b>HD 32963 b</b>, which they
describe as a Jupiter analog. To demonstrate the application of their criteria,
they offer a list that includes the new candidate along with 20 others already
known. This list is longer than the one in W16 because R16 drew from all
published planets, whereas W16 limited their selection to planets discovered by
their own group. (For whatever reason, R16 omit four of the eight planets
presented by W16, even though all four were published before R16 submitted
their manuscript for peer review.)</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Both lists are similar in one important way: R16, like
W11 and W16, extend the designation of Jupiter analog even to planets with gas
giant companions on interior orbits. This architecture characterizes 6 of the
21 systems presented by R16, with 3 of the 6 shared with W16. Notably, R16 also
include HD 219134 (omitted by W16), but only as analyzed by Vogt & al. 2015.
Vogt’s group described a six-planet system in which the outermost object is a
gas giant named <a href="http://backalleyastronomy.blogspot.com/2015/10/a-different-picture-of-hd-219134.html">HD
219134 g</a> with a minimum mass of 0.34 Mjup, a semimajor axis of 3.11 AU, and
a period of 6.16 years. A competing analysis by Motalebi & al. 2015 found a
distinctly different line-up, in which a gas giant named <a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html">HD
219134 e</a>, the outermost of four planets, has a minimum mass of 0.19 Mjup, a
semimajor axis of 2.14 AU, and a period of 3.74 years. The former description
meets all the criteria adopted by R16; the latter meets none of them.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Given the close resemblance between the architecture of
HD 219134, as presented by Motalebi and
colleagues, and that of Kepler-167 system, as presented by Kipping and
colleagues, the criteria of W16 and R16 also exclude Kepler-167e.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Another perspective on this issue is available from Sean
Raymond, who in 2006 published an article titled “The search for other Earths: limits
on the giant planet orbits that allow habitable terrestrial planets to form.” It’s
significant that the term “Jupiter analog” occurs neither in the title nor the
text. Instead, Raymond’s concern was to define the orbital architectures that would
permit the accretion of a terrestrial planet with a minimum mass 0.3 times
Earth (0.3 Mea) in the habitable zone of a Sun-like star. Anticipating W11,
W16, and R16, Raymond noted the critical importance of orbital circularity. He
found that semimajor axes smaller than 2.5 AU were inconsistent with habitable
terrestrial planets, while wider separations became increasingly friendly, such
that a sufficiently large semimajor axis could mitigate eccentricities of 0.2
to 0.4. Even though his analysis is 10 years old, its emphasis on the possibility
of terrestrial planet formation in the habitable zone is still relevant to
current investigations.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Yet despite the differences among these approaches, none
of them would characterize Kepler-167e as a Jupiter analog. Whether their
exclusion implies that systems like Kepler-167 are unlikely to host habitable
planets is another question.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">birth
versus survival</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Let’s look again at the 10 systems summarized in <b>Figure 3</b>. In 9 out of 10, the (outer)
gas giant orbits inside 2.5 AU, disqualifying all 9 of them as Jupiter analogs according
to the criteria of W16, R16, and Raymond 2006. However, the giant in the tenth
system (HD 10180 h) has a semimajor axis of 3.4 AU and an eccentricity of 0.08,
well within their limits. Why did R16 omit this system from their list? I’d
guess the deal-breaker was the planet’s low minimum mass: 0.203 Mjup (64 Mea).
Nevertheless, as W16 noted, the actual mass implied by this value is sufficient
to play a toned-down version of Jupiter’s dynamic role, since it exceeds 0.15
Mjup.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The HD 10180 system had not yet been announced when
Raymond presented his analysis, so he did not discuss the compatibility of its
architecture with rocky planet formation. Nor did he discuss the possibility
that the inner companions of a Jupiter-like planet might include both rocky
planets and gas dwarfs analogous to Neptune. Finally, none of his simulations produced
“hybrid” architectures like those featured in <b>Figure 3</b>. By hybrid I mean that these architectures resemble the
well-known class of <a href="http://backalleyastronomy.blogspot.com/2014/06/dwarfs-versus-giants-round-two.html">compact
low-mass systems</a>, except that they add a cool gas giant to the mix.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Recent observations have established that clusters of
small planets can include rock/metal spheres with masses similar to Earth alongside
planets with masses several times larger and radii puffed up by hydrogen/helium
atmospheres (e.g., Kepler-20, 62, 90, 169). This diversity of composition among
small planets foregrounds the importance of understanding the evolution and
orbital dynamics of HD 10180 (and similar hybrids) before we can effectively
assess the likelihood that these systems support Earth-like planets.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Three studies have already discussed the potential for
habitable planets around HD 10180 (Lovis & al. 2011, Tuomi 2012, Kane &
Gelino 2014). Unfortunately, none of them addressed the system’s formation
history. Since the host star is very similar to our Sun, its habitable zone falls
in the outer reaches of the gap between planet <b>f</b> (23 Mea at 0.49 AU) and planet <b>g</b> (24 Mea at 1.42 AU). This region is equivalent to the space
between Mercury and Mars in the Solar System, and thus approximately
co-extensive with our own habitable zone. Both Lovis’ group and Tuomi were
optimistic about the possibility of an additional planet surviving there. They
argued that, amid the complex web of dynamic interactions woven by the system’s
packed orbits, the empty region around 1 AU was an island of stability that
might harbor an Earth-mass planet. In a darker view, however, Kane & Gelino
have recently argued that planet <b>g</b>
has a substantially more eccentric orbit than previously reported. In their
analysis, planet <b>g</b> would either
prevent the formation of any habitable planets or eject such planets if they managed
to form. Accordingly, the orbital gap represents a forbidden zone instead of a life
zone.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">mind the
gap</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Most of the systems presented in <b>Figure 3</b> exhibit a similar gap between the inner and outer planets.
Indeed, our own Solar System has an analogous feature: the gap between Mars and
Jupiter, which extends from about 1.5 AU to 5 AU and separates our inner system
of terrestrial planets from the outer realm of the giants. The only occupants of
this gap are the battered objects in the Asteroid Belt, whose mass is largely
confined to the region between 2 and 3.3 AU.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">In Kepler-87 and 90, the gap occurs well inside the inner
edge of the habitable zone, which begins beyond the outermost planet in both
systems. However, in six other systems (including HD 10180, HD 219134, and
Kepler-167), the gap encompasses the habitable zone. Are all these gaps truly
empty, or might they hold one or more planets that have so far escaped
detection? Such objects would be missed if they were slightly misaligned with
the known planets (in the case of the transiting systems) or too lightweight to
reach the threshold of detectability (in the RV systems). I’d love to see more
research addressing this question, especially in the form of dynamical analyses
of the systems already known.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">It might turn out that the interesting hybrid
architectures highlighted in Figure 3 are, by nature, unfriendly to habitable
planets, as Kane and Gelino argued for HD 10180. In many cases, the principal
antagonist would most likely be the outer giant, which would sculpt interior
orbits the way Jupiter has sculpted our own system’s gap. But if they aren’t
intrinsically hostile, then the configurations of HD 219134 and Kepler-167
might offer us a new architectural signpost of potential habitability. This one
would supplement our existing biomarker of <i>bona
fide</i> Jupiter analogs.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";"><b>Fi</b><span style="font-family: "georgia" , "serif";"><b>gure 5.</b> Fi<span style="font-family: "georgia" , "serif";">fteen potenti<span style="font-family: "georgia" , "serif";">al Solar System analogs<a href="https://1.bp.blogspot.com/-bUl9PcmMYI8/VvS3y711xBI/AAAAAAAAAww/rqB9F6EM1No4BWeADcbH_74sAcpla_u3A/s1600/Jupiter%2BAnalogs%2BJanuary%2B2016.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="376" src="https://1.bp.blogspot.com/-bUl9PcmMYI8/VvS3y711xBI/AAAAAAAAAww/rqB9F6EM1No4BWeADcbH_74sAcpla_u3A/s640/Jupiter%2BAnalogs%2BJanuary%2B2016.gif" width="640" /></a><b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> </span></i></b></span></span></span></span></span><br />
<br />
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</span></span></span></span></span></span></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">*</span></i></b><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Not listed in Rowan et
al. 2016.</span></i></span></span></span></span></span></span></div>
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<br /></div>
<div style="text-align: left;">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">
</span></span></span></span></span></span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Tags:</span></i><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Msol = star mass in
Solar units; Type = spectral type; Dist. = distance in parsecs; Mjup = planet
mass in Jupiter units; a = semimajor axis in Earth units; e = orbital
eccentricity; Period = orbital period in years. <i style="mso-bidi-font-style: normal;">Selection criteria:</i> star mass 0.7-1.2 Msol; a > 3 AU; e <
0.3, no interior giants occupying or perturbing the system habitable zone. </span></div>
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<![endif]--><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"><span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">------------------------</span></span></span></span></span></span></span></span></span></span></span></span><br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">
</span></span></span></div>
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">how rare
is Jupiter?</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span></div>
<div class="MsoNormal">
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Compact low-mass systems are relatively abundant, both in
the Kepler catalog and within a few dozen parsecs of our Sun. But their
mixed-mass cousins are not. To date, however, no estimates are available for
their relatively frequency in the underlying population of exoplanetary
systems.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Happily, the landscape of Jupiter analogs is now emerging
from the haze, emboldening both W16 and R16 to estimate the true occurrence rate
of Jupiter-like planets in our Galactic neighborhood. W16 calculate a frequency
of about 6%, while R16 find 1%-4%. Since both groups used a more generous
definition of their target than I allow, I believe they would find a
substantially lower frequency if they refocused their sights on cool gas giants
that tolerate habitable planets. These might occur in only 1%-2% of planetary
systems.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Such a population share is much lower than I imagined <a href="http://backalleyastronomy.blogspot.com/2012/05/how-weird-is-our-solar-system.html">a
few years ago</a>. If my new guess is accurate, then systems like ours are rare
– possibly as rare as Hot Jupiters, for which recent studies have calculated a
prevalence of about 1% or less around Sun-like stars (Bayliss & Sackett
2011, Wright & al. 2012, Wang & al. 2015).</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">So there’s another reason to appreciate the Earth (not to
mention the Jupiter) we already know about.</span><i><span style="font-family: "georgia" , "serif";"> </span></i></span><br />
<br />
<div style="text-align: center;">
<span style="font-size: large;"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i><span style="font-family: "georgia" , "serif";">Click
here for a permanent index by topic of blog posts<br />
on Back Alley Astronomy</span></i></a></span><span style="font-family: "georgia" , "serif";">
</span></div>
</div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<b><span style="color: black; font-size: 10pt;">REFERENCES</span></b></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Bayliss DD, Sackett PD.</span></b><span style="color: black; font-size: 10pt;"> (2011) The frequency of Hot Jupiters in the Galaxy: Results from the
SuperLupus survey. </span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-size: 10pt;">Astrophysical Journal</span></i><span style="color: black; font-size: 10pt;"> 743, 103.</span><span style="color: black; font-size: 10pt;"></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Kipping DM</span></b><span style="color: black; font-size: 10pt;">, Torres G, Henze C, Teachey A, Isaacson H, Petigura E, Marcy GW,
Buchhave LA, Chen J, Bryson ST, Sandford E. (2016) A transiting Jupiter analog.
In press. Abstract:</span><span style="font-size: 10pt;"> </span><a href="http://adsabs.harvard.edu/abs/2016arXiv160300042K"><span style="font-size: 10pt;">2016arXiv160300042K</span></a><span style="font-size: 10pt;"></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Kane SR, Gelino DM.</span></b><span style="color: black; font-size: 10pt;"> (2014) </span><span style="color: black; font-size: 10pt;">On the
inclination and habitability of the HD 10180 system. </span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-size: 10pt;">Astrophysical Journal</span></i><span style="color: black; font-size: 10pt;"> 792,
111.</span><span style="color: black; font-size: 10pt;"></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Lovis C</span></b><span style="color: black; font-size: 10pt;">, Ségransan D, Mayor M, Udry S, Benz W,
Bertaux J-L, & al. (2011) The HARPS search for southern extra-solar
planets. XXVIII. Up to seven planets orbiting HD 10180: probing the
architecture of low-mass planetary systems. <i style="mso-bidi-font-style: normal;">Astronomy
& Astrophysics</i> 528, A112. Abstract:</span><span style="color: black; font-size: 10pt;"> </span><span style="font-size: 10pt;"><a href="http://adsabs.harvard.edu/abs/2011A%26A...528A.112L">2011A&A...528A.112L</a><span style="color: black;"></span></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Motalebi
F,</span></b><span style="color: black; font-size: 10pt;"> Udry S, Gillon M,
Lovis C, Ségransan D, Buchhave LA, Demory BO, Malavolta L, Dressing CD,
Sasselov D, et al. (2015) The HARPS-N Rocky Planet Search I. HD 219134 b: A
transiting rocky planet in a multi-planet system at 6.5 pc from the Sun. <i style="mso-bidi-font-style: normal;">Astronomy & Astrophysics</i> 584, A72.
Abstract: </span><a href="http://adsabs.harvard.edu/abs/2015A%26A...584A..72M"><span style="font-size: 10pt;">2015A&A...584A..72M</span></a><span class="MsoHyperlink"><span style="font-size: 10pt;"></span></span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Raymond SN</span></b><span style="color: black; font-size: 10pt;">. (2006) The search for other Earths:
Limits on the giant planet orbits that allow habitable terrestrial planets to
form. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 643,
L131–L134. Abstract: </span><a href="http://adsabs.harvard.edu/abs/2006ApJ...643L.131R"><span style="font-size: 10pt;">2006ApJ...643L.131R</span></a><span style="color: black; font-size: 10pt;"><span style="mso-spacerun: yes;"> </span></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Rowan D</span></b><span style="color: black; font-size: 10pt;">, Meschiari S, Laughlin G, Vogt SS, Butler RP, Burt J, Wang S, Holden B,
Hanson R, Arriagada P, Keiser S, Teske J, Diaz M. (2016) The Lick-Carnegie
exoplanet survey: HD 32963, A new Jupiter analog orbiting a Sun-like star. </span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-size: 10pt;">Astrophysical Journal</span></i><span style="color: black; font-size: 10pt;">, </span><span style="color: black; font-size: 10pt;">817:104.</span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Tuomi M</span></b><span style="color: black; font-size: 10pt;">. (2012) Evidence for 9 planets in the HD 10180 system. </span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-size: 10pt;">Astronomy & Astrophysics</span></i><span style="color: black; font-size: 10pt;"> 543, 52.</span><span style="color: black; font-size: 10pt;"></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b><span style="color: black; font-size: 10pt;">Vogt SS</span></b><span style="color: black; font-size: 10pt;">, Burt J, Meschiari S, Butler RP, Henry GW, Wang S, Holden B, Gapp C,
Hanson R, Arriagada P, Keiser S, Teske J, Laughlin G. (2015) A six-planet
system orbiting HD 219134. <i style="mso-bidi-font-style: normal;">Astrophysical
Journal </i>814, 12. Abstract: </span><a href="http://adsabs.harvard.edu/abs/2015ApJ...814...12V"><span style="font-size: 10pt;">2015ApJ...814...12V</span></a><span style="font-size: 10pt;"></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Wang J</span></b><span style="color: black; font-size: 10pt;"> , Fischer DA, Horch EP, Huang X. (2015) On
the occurrence rate of Hot Jupiters in different stellar environments.</span><span style="color: black; font-size: 10pt;"> </span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-size: 10pt;">Astrophysical Journal</span></i><span style="color: black; font-size: 10pt;"> </span><span style="color: black; font-size: 10pt;">799, 229.</span><span style="color: black; font-size: 10pt;"></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Wittenmyer R</span></b><span style="color: black; font-size: 10pt;">, Tinney CG, O’Toole SJ, Jones HRA, Butler
RP, Carter BD, Bailey J. (2011) On the frequency of Jupiter analogs. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 727, 102.
Abstract:</span><span style="font-size: 10pt;"> </span><a href="http://adsabs.harvard.edu/abs/2011ApJ...727..102W"><span style="font-size: 10pt;">http://adsabs.harvard.edu/abs/2011ApJ...727..102W</span></a><span style="font-size: 10pt;"> </span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Wittenmyer R</span></b><span style="color: black; font-size: 10pt;">, Butler RP, Tinney CG, Horner J, Carter BD, Wright
DJ, Jones HRA, Bailey J, O’Toole SJ. (2016) The Anglo-Australian planet search
XXIV: The frequency of Jupiter analogs. <i style="mso-bidi-font-style: normal;">Astrophysical
Journal</i> 819, 28.</span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Wright JT</span></b><span style="color: black; font-size: 10pt;">, Marcy GW, Howard AW, Johnson JA, Morton TD,
Fischer DA. (2012) The frequency of Hot Jupiters orbiting nearby Solar-type
stars. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 753, 160.</span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<br /></div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-70236363653539675812016-01-01T23:30:00.000-08:002016-05-24T20:17:04.113-07:00A Data-Driven Year<!--[if gte mso 9]><xml>
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<a href="http://2.bp.blogspot.com/-2lJe71A7hYM/Vod7CWdfECI/AAAAAAAAAs0/ZS994WwuSOY/s1600/Plutonian%2BSunrise.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="374" src="https://2.bp.blogspot.com/-2lJe71A7hYM/Vod7CWdfECI/AAAAAAAAAs0/ZS994WwuSOY/s640/Plutonian%2BSunrise.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif; font-size: small;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Sunrise on
Pluto with <i style="mso-bidi-font-style: normal;">New Horizons</i>: a
once-in-a-lifetime view of the dwarf planet’s icy mountains and plains. </span></span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">------------------------------</span></span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;">During the year just ended, our own Solar System made the biggest headlines in astronomy and space exploration. At the top of the list were the amazing images and detailed physical data returned by the <i>New Horizons</i> mission to the double-dwarf system of <a href="http://backalleyastronomy.blogspot.com/2015/07/four-fabulous-dwarfs.html">Pluto and Charon</a> (Stern et al. 2015). Close behind were the data sent back from</span> <a href="http://backalleyastronomy.blogspot.com/2015/07/four-fabulous-dwarfs.html">Ceres</a><span style="color: black; mso-themecolor: text1;"> by the <i style="mso-bidi-font-style: normal;">Dawn</i> spacecraft. Then came evidence from various sources elsewhere
in our system for liquid water, whether </span><a href="http://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars">flowing</a><span style="color: black; mso-themecolor: text1;"> on the surface of </span><a href="http://backalleyastronomy.blogspot.com/2015/03/mighty-throxeus.html">Mars</a>
<span style="color: black; mso-themecolor: text1;">or sloshing in a global ocean beneath the ice shell of Enceladus (Thomas et al. 2015). Extrasolar astronomy offered nothing comparable. </span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="color: black; mso-themecolor: text1;">Yet the growth</span> in our evidence for exoplanets – near and
far, individually and in systems, at the present epoch and across galactic
history – continued at a rapid pace in 2015. In November, the exoplanetary
census maintained by the </span><a href="http://exoplanet.eu/"><span style="font-family: "georgia" , "serif";">Extrasolar Planets Encyclopaedia</span></a><span style="font-family: "georgia" , "serif";"> passed
2000. Today it stands at 2041: double the count registered just </span><a href="http://backalleyastronomy.blogspot.com/2013/10/1010-extrasolar-planets.html"><span style="font-family: "georgia" , "serif";">two years ago</span></a><span style="font-family: "georgia" , "times new roman" , serif;">, and an
order of magnitude larger than the census in 2006. Most exoplanets (62%) are
known by the shadows they cast while crossing the face of their parent stars,
through the so-called <b>transit</b> method.
Those in the next largest sample (31%) were revealed by their gravitational
effects on the spectral lines of their parent stars, as shown by <b>radial velocity</b> observations. The remainder
were found variously by direct imaging, microlensing, and pulsation timing.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">As 2016 begins, the astronomical community is still
digesting the massive haul of data collected by the primary Kepler mission from
2009 to 2013, even as new discoveries by K2 (Kepler’s successor) and other
programs continue to accumulate. This posting will highlight some of the most
remarkable extrasolar detections of the past year, focusing on contributions to
our understanding of planet structure, planet formation, and system
architectures. Then we’ll glance at a few other important studies of a more
theoretical nature.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 2.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Aldebaran </span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://2.bp.blogspot.com/-wKLjoraxO-w/Vod7MxoaYNI/AAAAAAAAAs8/rNfjNxikNVI/s1600/Aldebaran_e_le_Iadi.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="266" src="https://2.bp.blogspot.com/-wKLjoraxO-w/Vod7MxoaYNI/AAAAAAAAAs8/rNfjNxikNVI/s640/Aldebaran_e_le_Iadi.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">The red
giant Aldebaran appears left of center as a foreground object against members
of the more distant Hyades Cluster. Also known as Alpha Tauri, Aldebaran is
only about 20 parsecs away, while the distance to the Hyades is about 46
parsecs. Twenty-five years of radial velocity observations support the presence
of a massive gas giant planet orbiting Aldebaran in a period of 629 days. Image
credit: Wikipedia</span></span></div>
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<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><b><span style="color: red; font-family: "arial" , "sans-serif";">Aldebaran</span></b><span style="color: black; font-family: "georgia" , "serif";"> is one
of the brightest stars in northern skies, visible even in light-polluted
environments (Figure 2). Its brightness is explained partly by its evolutionary
status as a red giant of spectral type K5 III and partly by its proximity to
Earth: it’s located just 66 light years away (20.4 parsecs). Back in 1993, when
extrasolar planets were still science fiction, Artie Hatzes and William
Cochrane reported six years of data demonstrating regular variations in the
radial velocities of Aldebaran and two other nearby red giants, Arcturus and
Pollux. As Hatzes & Cochrane noted at the time, “The exciting possibility
that the variations are due to the presence of planetary companions should be
treated with caution.” A planet around Pollux was eventually confirmed more than
a decade later, in 2006. It’s a gas giant of 2.3 Jupiter masses (Mjup) orbiting
at a semimajor axis of 1.6 AU (a little wider than the orbit of Mars). Although
Arcturus is still awaiting confirmation, good tidings came from Aldebaran this
past summer in the form of a Super Jupiter with an orbit similar to that of
Pollux b (Hatzes et al. 2015). Details on the new system appear in Table 1.</span></span><br />
<br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">In an era when splashy claims are sometimes rushed into press,
only to be refuted within a few years, Hatzes & colleagues have upheld the
most exacting scientific standards. They collected and analyzed data on
Aldebaran b for more than 25 years – equivalent to about 15 planetary orbits –
before ultimately confirming its reality. Their publications on this project
over the past two decades offer a window on our progress in understanding nearby
stars. When they reported their early findings, Pollux and Aldebaran were
credited with respective masses of 2.8 and 2.5 Msol, and these values informed
estimates of planetary masses. Since then, the numbers have been revised dramatically
downward to 1.7 and 1.1 Msol, respectively. Evidently Pollux is a “retired A
star” (Johnson et al. 2011), but Aldebaran seems to have begun its existence as
a Sun-like star. Among planet-hosting red giants, which now number at least 60,
that origin puts it in the minority.</span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 11pt;"> </span></b></span></div>
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<span style="font-family: "georgia" , "times new roman" , serif; font-size: medium;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 11pt;">Table 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 11pt;"> Parameters of the Aldebaran system</span></span></span></span></div>
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<a href="http://1.bp.blogspot.com/-ai3l_dMPj2I/Vod7Uk24KdI/AAAAAAAAAtE/p1CANmkNv2Y/s1600/Aldebaran%2Bsystem%2Bparameters.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="80" src="https://1.bp.blogspot.com/-ai3l_dMPj2I/Vod7Uk24KdI/AAAAAAAAAtE/p1CANmkNv2Y/s640/Aldebaran%2Bsystem%2Bparameters.gif" width="640" /></a><span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">For Aldebaran (the host star), column 1 =
mass in Solar units; column 2 = radius in Solar units; column 3 = metallicity
as the ratio of iron to hydrogen; column 4 = effective temperature in Kelvin,
column 5 = age in billions of years; column 6 = spectral type, column 7 =
distance in parsecs. For Aldebaran b (the planet), column 1 = mass in Jupiter
units; column 2 = orbital semimajor axis in astronomical units; column 3 =
orbital eccentricity; column 4 = orbital period in days.</span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">-----------------------------------</span></div>
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<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: red; font-family: "arial" , "sans-serif";">EPIC
210490365</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> <span style="font-family: "georgia" , "times new roman" , serif;">Aldebaran is the right eye of the bull in the zodiacal
constellation of Taurus. It’s surrounded by bluer, dimmer stars that trace the
rest of the bull’s face (Figure 2). All the other stars comprise a completely
unrelated structure known as the Hyades Cluster, which at 44 parsecs is more
distant than Aldebaran, though still quite nearby in Galactic terms. The Hyades
are also much younger than Aldebaran, representing stars of all colors born in a
single hydrogen cloud about 650 million years ago. Data collected by the K2
Mission reveal that one member of the cluster – a red dwarf of type M4.5 with a
fiendishly unmemorable catalog number – is accompanied by a </span></span><span style="font-family: "georgia" , "times new roman" , serif;"><a href="http://backalleyastronomy.blogspot.com/2012/04/between-earth-and-uranus-part-ii.html">gas
dwarf</a><span style="color: black; mso-themecolor: text1;"> of 3.43 Earth radii (Rea)
transiting every 3.5 days (Mann et al. 2015). Given its membership in the
Hyades Cluster, the host star’s characteristics are well constrained (0.29
Msol, [Fe/H] +0.15), but no data are available on the new planet’s mass. </span></span></span></div>
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<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><br /></span><span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Despite the cluster’s proximity and size (at least 200
star systems), EPIC 210490365b is only the third exoplanet ever detected in the
Hyades. The first was a Super Jupiter of 7.5 Mjup orbiting the red giant
Epsilon Tauri (the bull’s left eye) in a period of 595 days (Sato et al. 2007).
The second was a Hot Jupiter of 0.92 Mjup orbiting HD 285507 (spectral type K4.5,
0.73 Msol) in a period of 6 days (Quinn et al. 2013). Two more giant planets –
both Hot Jupiters – have been reported around Sun-like stars in another nearby
cluster, Praesepe, which shares a common origin with the Hyades (Quinn et al.
2012). Finally, Kepler Mission data revealed two low-mass planets (Kepler-66b
and -67b; Meibom et al. 2013) transiting G-type stars in NGC 6811, a star
cluster aged about 1 billion years located 1107 parsecs away (3600 light years).
With respective radii of 2.8 and 2.9 Rea, Kepler-66b and Kepler-67b appear
typical of the large population of puffy Kepler planets with hydrogen/helium
atmospheres and masses between 2 and 12 Mea. The new Hyades gas dwarf reported
by Mann & colleagues has an even puffier profile, although it isn’t
necessarily more massive. It’s the third low-mass planet detected in any star
cluster and the first to be identified in the Hyades. </span></span></div>
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<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></b></span><br />
<span style="font-family: "georgia" , "times new roman" , serif; font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 3.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Bright stars within 15 parsecs of the center
of the Hyades Cluster</span></span></div>
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<div class="separator" style="clear: both; text-align: center;">
<a href="http://4.bp.blogspot.com/-UZ6uOgAVowo/Vod7eiYEtsI/AAAAAAAAAtM/Cd33jmuW7RY/s1600/Hyades%2BStructure%2Bwith%2BEPIC%2B21049.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="518" src="https://4.bp.blogspot.com/-UZ6uOgAVowo/Vod7eiYEtsI/AAAAAAAAAtM/Cd33jmuW7RY/s640/Hyades%2BStructure%2Bwith%2BEPIC%2B21049.gif" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">The x-axis shows right ascension; the y-axis
shows declination. The position of <b style="mso-bidi-font-weight: normal;">EPIC
210490365</b> is marked by a red six-pointed star at RA 04:13:06, Dec +15:14:52
Except for this M dwarf exoplanet host, only stars of type K or earlier appear
in the diagram, color-coded by spectral classification. A halo around a star
indicates multiplicity. The inner dot-dashed circle traces the cluster’s core
radius (2.7 parsecs); the outer circle shows the tidal radius (10 parsecs). To
date, three exoplanetary systems have been identified in the Hyades Cluster. </span></span></div>
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<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: red; font-family: "arial" , "sans-serif";">HD 219134</span></b></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> <span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">is still
closer than Aldebaran and the Hyades, located just 21 light years (6.53
parsecs) away. With an enhanced metallicity of +0.11, this orange dwarf has
long been considered a potential planetary host. Its spectral type of K3 (not
too dim) and mass of 0.78 Msol (not too small) add to its appeal. This past
year, two different groups (Motalebi et al., Vogt et al.) reported a
fascinating system around this star, including at least three low-mass planets
on short-period orbits and one cool gas giant orbiting outside 2 AU. Motalebi’s
group even reported the detection of a single transit of the innermost planet
(HD 219134 b), a likely Super Earth. If this transit is confirmed, the planet
in question will join an exclusive club of small exoplanets with measured
masses and radii. This system is discussed in more detail </span></span></span><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html">here</a><span style="color: black; mso-themecolor: text1;"> and </span><a href="http://backalleyastronomy.blogspot.com/2015/10/a-different-picture-of-hd-219134.html">here</a><span style="color: black; mso-themecolor: text1;">. </span></span></span></div>
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<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: red; font-family: "arial" , "sans-serif";">Kepler-138</span></b></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> <span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;">The club
of small planets with measured masses inducted several new members over the
past year, including three from the same system. Analyses of transit data on a
faint M dwarf known as Kepler-138, located at an unspecified distance, enabled
the characterization of three undeniably terrestrial planets with orbital
periods between 10 and 23 days. The host star has an estimated mass of 0.52
Msol and metallicity of -0.28. Analyses of transit timing variations enabled Jontof-Hutter
& colleagues to calculate masses for all three planets, revealing a
startling mismatch in densities.</span></span></span><br />
<br />
<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: medium;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 11pt;">Table 2.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 11pt;"> Parameters
of the Kepler-138 system</span></span></span></span></span></div>
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<a href="http://3.bp.blogspot.com/-96PFKKNlL1k/Vod7pvBHWPI/AAAAAAAAAtU/DSXfqZNaJgU/s1600/Kepler-138%2Bsystem%2Bparameters.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="108" src="https://3.bp.blogspot.com/-96PFKKNlL1k/Vod7pvBHWPI/AAAAAAAAAtU/DSXfqZNaJgU/s640/Kepler-138%2Bsystem%2Bparameters.gif" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Masses & radii are shown in Earth units;
semimajor axis (a) in astronomical units (AU); period in days; and equilibrium temperature
(Teq) in Kelvin. </span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Kepler-138b is a rare example of a </span><a href="http://backalleyastronomy.blogspot.com/2013/03/subterrestrials.html">subterrestrial</a><span style="color: black; mso-themecolor: text1;"> exoplanet, almost identical to Mars
in radius but even less massive. Despite its much larger radius, Kepler-138d is
another subterrestrial; it is less massive than Venus. Both of these objects
must include a volatile component (presumably water) to explain their
unexpectedly large radii. Only Kepler-138c is a <i>bona fide</i> Super Earth, with an
estimated mass that implies an Earth-like composition. Of course, all three
planets are much hotter than Earth.</span></span></span></div>
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<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: red; font-family: "arial" , "sans-serif";">Kepler-444</span></b></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> <span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Yet
another remarkable system of small planets was announced at the beginning of
2015 (Campante et al.). Instead of its unprecedented architecture, though, the
headlines emphasized the system’s advanced age: at an estimated 11.2 billion
years, the host star is just a little younger than the Milky Way itself. As
expected of such an ancient star, Kepler-444 is poor in metals, with [Fe/H] at
-0.55. Nevertheless, it harbors five tightly packed, slow-roasted planets with
a likely aggregate mass of 1.5 Mea – rather less than the total mass of the
Solar System’s four terrestrial planets. All these objects are “Martians,” with
radii larger than Mercury but smaller than Venus, and all are expected to be
purely rocky and blazing hot, lacking volatiles. Even such dense objects are
still too lightweight to be detected by radial velocity surveys, despite the
brightness and proximity of the parent star. In fact, Kepler-444 is one of the
closest stars in the entire Kepler sample, located just 35.7 parsecs away (116
light years). Its respectable mass of o.76 Msol and spectral type of K0 have
enabled unusually precise estimates of system parameters, as summarized in
Table 3. </span></span></span><br />
</div>
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<span style="font-size: medium;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 11pt;">Table 3.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 11pt;"> Parameters
of the Kepler-444 system</span></span></span></span></span></div>
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<a href="http://2.bp.blogspot.com/-Ed2CGyuBGIc/Vod7wO5djJI/AAAAAAAAAtc/XMoNGNjfO0E/s1600/Kepler-444%2Bsystem%2Bparameters.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="172" src="https://2.bp.blogspot.com/-Ed2CGyuBGIc/Vod7wO5djJI/AAAAAAAAAtc/XMoNGNjfO0E/s640/Kepler-444%2Bsystem%2Bparameters.gif" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Radii are shown in Earth units; semimajor
axis (a) in astronomical units (AU); period in days; and equilibrium
temperature (Teq) in Kelvin. </span></span></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Campante & colleagues noted that all adjacent
planet pairs in this system orbit just outside first-order mean motion
resonances, suggesting that they migrated into their present configuration in
the presence of a protoplanetary nebula. The orbits are also almost perfectly
co-planar, consistent with dynamic stability over the lifetime of the Galaxy.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Adding to the intricacy of this celestial clockwork is
the fact that Kepler-444 itself is the primary or “A” component of a
hierarchical triple star system. Its companions, Kepler-444B and C, are M
dwarfs with respective masses of 0.29 and 0.25 Msol. They are locked in a tight
binary orbit with a semimajor axis of 0.3 AU or less. Currently, the binary is
separated from Kepler-444A by a projected distance of about 66 AU. A new study
by Dupuy & colleagues reports astrometric and radial velocity data on all
three stars, arguing that the binary is engaged in a highly eccentric orbit
around the system’s center of mass, sharing a semimajor axis of 36.7 AU and an
eccentricity of 0.864 with Kepler-444A. These values imply that the BC binary
and the A star have a minimum separation of 5 AU at each periastron passage
during their shared orbital period of about 200 years. Such an orbit would seem
to compromise the long-term survival of the planetary system, but Dupuy &
colleagues argue that both the planetary and stellar orbits are co-planar,
promoting dynamical stability over the age of the Milky Way.</span><b style="mso-bidi-font-weight: normal;"><span style="color: red; font-family: "arial" , "sans-serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b style="mso-bidi-font-weight: normal;"><span style="color: red; font-family: "arial" , "sans-serif";">WASP-47</span></b></span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> <span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">has
been recognized as the host of an “entirely typical” Hot Jupiter since 2012.
Located at a distance of about 200 parsecs, it’s a G-type star of 1.04 Msol
with substantial enrichment in metals, given its [M/H] of +0.36. New data from
two different sources – the spaceborne K2 Mission and the ground-based CORALIE
program – now portray a</span></span></span><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black;"> <a href="http://backalleyastronomy.blogspot.com/2015/08/the-blazing-wasp-47s.html">unique
mixed-mass system</a> (Becker et al. 2015) </span><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">that is already the focus
of theoretical efforts to explain its singular architecture. Two short-period,
low-mass planets were announced this summer, one inside and the other outside
the orbit of the known Hot Jupiter (WASP-47b). Shortly afterward, a second gas
giant (WASP-47c) was reported at a semimajor axis of 1.36 AU (Neveu-VanMalle et
al. 2015).</span><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> </span></b></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 4.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> The three inner planets of WASP-47</span></span></span></div>
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<a href="http://3.bp.blogspot.com/-qtTXnqr3T1A/Vod8FQWkURI/AAAAAAAAAtk/EgkDmeCT-Wg/s1600/WASP-47-system-architecture.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="590" src="https://3.bp.blogspot.com/-qtTXnqr3T1A/Vod8FQWkURI/AAAAAAAAAtk/EgkDmeCT-Wg/s640/WASP-47-system-architecture.gif" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">The three transiting planets of WASP-47 are
shown at their relative sizes. The number inside each sphere refers to the
planet’s estimated mass in Earth units (Mea). The value for planet e is a guesstimate
within narrow limits; the other two are supported by radial velocity and
transit timing data. A second gas giant planet (WASP-47c) is also present on a wider orbit outside the scale of this diagram. </span></span></div>
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<span style="color: black; font-family: "georgia" , "serif"; font-size: large;">Still
more recently, two theoretical studies that appeared as preprints offer
findings relevant to the system’s evolution. The first, by Batygin &
colleagues, proposes that the three inner planets of WASP-47 formed <i style="mso-bidi-font-style: normal;">in situ</i>, although it’s not clear how one
of them ballooned into a giant while the other two stayed small. The second, by
Hand & Alexander, reports the results of a suite of simulations of
migration scenarios for systems of compact short-period planets. In their simulations,
whole families of small planets assemble outside system snow lines and then
migrate inward to warmer orbits. If the outermost or penultimate planet
accretes enough mass during this process to grow into a gas giant (the growth mechanism
is unclear), it will sculpt the orbits of the smaller inner planets in various
ways. In extreme cases, the low-mass planets are ejected from the system or
driven into the star. In more peaceful iterations, the gas giant squeezes the
inner planets into extremely tight orbits, as we see in <a href="http://backalleyastronomy.blogspot.com/2014/02/a-new-design-for-planetary-systems.html">Kepler-90</a>.
Hand & Alexander also suggest that some compact low-mass systems harbor
invisible giants on wider orbits (a notion conveyed in their title, "There Might Be Giants"). Their celebrity candidate for such a
configuration is <a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html">Kepler-11</a>, but a similar scenario could also be applied to WASP-47, where the outer giant is actually visible. </span></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: red; font-family: "arial" , "sans-serif"; font-size: large;">Two Baby
Giants from HATS</span></b><span style="color: black; font-family: "georgia" , "serif";"> <span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;">Low-mass planets like Earth and Uranus have bulk compositions
dominated by heavy elements, whereas gas giant planets like Saturn and Jupiter consist
primarily of hydrogen. But the boundary between the two species is imprecise.
Remarkably, a program other than Kepler or K2 recently contributed critical new
data on this issue. The Hungarian Automated Telescope Network South, better
known as HATS, reported two transiting planets, HATS-7b and HATS-8b, that appear
transitional between the dwarfs and the giants. Although the two planets are similar
in mass, they differ widely in radius and composition.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black;">HATS-7b
orbits a metallic K-type star in a period of 3.2 days (Bakos et al. 2015). Transit
observations yield a radius of 6.31 Rea; radial velocity data provide a mass of
38.2 Mea. The bulk composition favored by the discovery team is about 82%
rock/metal and 18% hydrogen/helium</span></span></span><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black;"><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black;"> – </span></span></span>broadly similar to the composition of Uranus. They note that the minimum mass for a heavy
element core that is consistent with the planet’s radius is 25 Mea – much
higher than any purely rocky planet ever detected.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black;">HATS-8b
follows a similar orbit (3.5 days) around a metallic G-type star (Bayliss et
al. 2015). At 43.9 Mea, this planet is less than half as massive as Saturn (95
Mea), but its puffy radius of 9.79 Rea makes it slightly larger. The discovery
team suggests a hydrogen/helium envelope accounting for about 77% of its bulk
composition. The corresponding heavy element core of about 10 Mea is predicted by current models of gas giant formation.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black;">These
two HATS join at least two previously reported objects that lie just at the
threshold of the mass range typical of gas giant planets. They are Kepler-35b, which
has a mass of 40 Mea, a radius of 8.16 Rea, and an orbital period of 131 days; and
HAT-P-18b, which has a mass of 58 Mea, a radius of 10.6 Rea, and an orbital
period of 5.5 days. </span></span></span></div>
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<span style="font-size: medium;"><span style="font-family: "arial" , "helvetica" , sans-serif;"><b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: large;">Theory of
Small Planets</span></i></b></span></span></div>
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<br /></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">The sample of Mars- to Super Earth-size planets grew
significantly in 2015. These objects consist entirely of heavy elements and are
appropriately described as terrestrial. A selection appears in Figure 5. Although
exoplanets in this mass range (0.05–10 Mea) are known mostly through transit
observations, the full population of transiting planets is dominated by puffy gas
dwarfs with hydrogen/helium envelopes and radii of 2–4 Rea. In contrast, as
discussed in </span><a href="http://backalleyastronomy.blogspot.com/2015/12/quantifying-earth-like.html">previous
posts</a><span style="color: black; mso-themecolor: text1;">, most terrestrial
planets in the Galaxy must be smaller than 2 Rea.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">This population attracted considerable attention in the
past year, especially the subset with masses measured by transit timing or
radial velocity variations. Notable studies of small planets either published
or in press in 2015 include Dawson, Chiang & Lee, <a href="http://adsabs.harvard.edu/abs/2015MNRAS.453.1471D">A metallicity recipe for rocky planets</a>; Dressing et al., <a href="http://adsabs.harvard.edu/abs/2015ApJ...800..135D">The mass of Kepler-93b and the composition of terrestrial planets</a>; Howe
& Burrows, <a href="http://adsabs.harvard.edu/abs/2015ApJ...808..150H">Evolutionary models of Super Earths and Mini Neptunes incorporating cooling and mass loss</a>; Jontof-Hutter, Ford, et al., <a href="http://adsabs.harvard.edu/abs/2015arXiv151202003J">Robust TTV mass measurements: Ten Kepler exoplanets between 3 and 8 Mea</a>; Owen & Wu, <a href="http://adsabs.harvard.edu/abs/2015arXiv150602049O">Atmospheres of low-mass planets: The Boil-Off</a>; Rogers, <a href="http://adsabs.harvard.edu/abs/2015ApJ...801...41R">Most 1.6 Earth-radius exoplanets are not rocky</a>; and Unterborn
et al., <a href="http://adsabs.harvard.edu/abs/2015arXiv151007582U">Scaling the Earth</a>. Bibliographic details appear in the reference section below.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 5.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> Terrestrial planets near and far with
measured masses and radii</span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://1.bp.blogspot.com/-kJjL3dEPpxc/Vod8RIdNlWI/AAAAAAAAAts/ahNZAPJXG0U/s1600/Radii%2Bof%2BSmall%2BPlanets%2BDecember%2B2015.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="282" src="https://1.bp.blogspot.com/-kJjL3dEPpxc/Vod8RIdNlWI/AAAAAAAAAts/ahNZAPJXG0U/s640/Radii%2Bof%2BSmall%2BPlanets%2BDecember%2B2015.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">These imaginative renderings of eight
well-studied extrasolar planets appear alongside photographs of Mercury, Venus,
and Earth. All are represented at their relative sizes, with masses (Mea) and
radii (Rea) indicated in Earth units. 55 Cancri e is assigned the new radius
reported by Demory et al. 2015, who speculate that this Hellworld experiences frequent
volcanic eruptions. Three of the exoplanets shown here (55 Cancri e,
Kepler-10b, Kepler-78b) complete a single orbit in less than 24 hours. </span></span></div>
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<br /></div>
<div class="MsoNormal" style="page-break-after: avoid;">
<span style="font-size: medium;"><span style="font-family: "arial" , "helvetica" , sans-serif;"><b style="mso-bidi-font-weight: normal;"><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: large;">Twenty Years
of Hot Jupiters</span></i></b></span></span></div>
<div class="MsoNormal" style="page-break-after: avoid;">
<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">2015 was the 20th anniversary of the discovery of the
first Hot Jupiter, 51 Pegasi b. A Hot Jupiter is a gas giant planet that orbits
its parent star in a period shorter than 10 days. Before 51 Pegasi b was
announced, few people even dreamed that such planets were possible. Now about 300 are known.</span></span></span><br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"></span></span></span><br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">T</span></span></span><span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">heorists have been trying to explain these unexpected
objects ever since the initial detections. Until recently, all theories assumed
that they formed outside their system ice lines (at about 2 AU or so) and were
then transported somehow to their present hot orbits. Proposed mechanisms
included 1) migration by interaction with the primordial gas nebula (Type II
migration); 2) migration by gravitational interactions with another gas giant
in the same system, either during a brief period of dynamic instability or by
long-term perturbations through so-called Kozai cycles; and 3) migration by
gravitational interactions with a misaligned stellar companion of the host star
in another type of Kozai regime.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">The first mechanism has generally been the most popular,
in part because it doesn’t depend on assuming the past or present existence of
a potentially undetected (and possibly undetectable) companion planet or star.
If Type II migration is indeed the dominant delivery mechanism for Hot
Jupiters, then we would expect all Hot Jupiters that took this pathway to
follow orbits that are well-aligned with the spin axis of their parent stars.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">However, orbital misalignment appears to be quite common
among transiting Hot Jupiters (the only subset in which this phenomenon can be
observed). Accordingly, a team of astronomers launched the “Friends of Hot
Jupiter” study to test whether one of the other two mechanisms might be
responsible for most of this population. They selected a sample of 50 Hot
Jupiters in two subgroups: 23 with well-aligned orbits and 27 with misaligned
orbits.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">The results of this impressive program have now been published
in three successive articles. The first, by Knutson et al., appeared in 2014.
This installment returned the remarkable finding that half of the systems in
each group revealed radial velocity evidence for a second gas giant on a wider
orbit. Some of these were already known, but almost half were newly determined through
radial velocity trends. Nevertheless, analyses found no statistically
significant association between evidence for an outer giant and the alignment
of the known Hot Jupiter.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">The second and third installments appeared in 2015; both
involved searches for companion stars in Hot Jupiter systems (Ngo et al.,
Piskorz et al.) Although about half the host stars in the sample showed
evidence of a binary companion, the presence or absence of such a companion had
no association with the orbital alignment of the Hot Jupiter. The authors also noted
that Hot Jupiters appear to reside preferentially in binary systems, despite
the indifference of alignment to binarity.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">If nothing else, the Friends of Hot Jupiters seem to have falsified the claim
that stellar Kozai cycles are the primary cause of observed misalignments in transiting giants.
Where exactly Hot Jupiters form and how exactly they got where they are now,
however, remain open questions.</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Happy New Year!</span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;">Figure 6.</span></b><span style="color: black; font-family: "georgia" , "serif"; mso-themecolor: text1;"> The sample of Hot Jupiters studied by Sing
& colleagues</span></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://4.bp.blogspot.com/-spKSQNlB6_g/Vod8aCRYqiI/AAAAAAAAAt0/BtjuEP9MTls/s1600/Hot%2BJupiters%2Bstudied%2Bby%2BSing%2B2015.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="390" src="https://4.bp.blogspot.com/-spKSQNlB6_g/Vod8aCRYqiI/AAAAAAAAAt0/BtjuEP9MTls/s640/Hot%2BJupiters%2Bstudied%2Bby%2BSing%2B2015.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">This pretty array of Hot Jupiters was
published at the start of the holiday season, calling to mind a set of
ornaments for a cosmic Christmas tree. The image illustrates a study by Sing
and colleagues, who conducted a spectroscopic search for water in Hot Jupiter
atmospheres. </span></span></div>
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<span style="font-size: large;"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html"><i style="mso-bidi-font-style: normal;"><span style="font-family: "georgia" , "serif";">Click
here for a permanent index by topic of blog posts<br />
on Back Alley Astronomy</span></i></a></span><span style="font-family: "georgia" , "serif";">
</span></div>
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<span style="font-family: "georgia" , "serif";">-------------</span></div>
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-34655694979749390082015-12-21T18:10:00.001-08:002016-05-30T14:06:58.338-07:00Quantifying "Earth-like"<!--[if gte mso 9]><xml>
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<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="http://2.bp.blogspot.com/-HqTw_84G6b4/Vniw7cCx_-I/AAAAAAAAAq0/06JsiFDFSSc/s1600/Quantifying-Earthlike-4.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="182" src="https://2.bp.blogspot.com/-HqTw_84G6b4/Vniw7cCx_-I/AAAAAAAAAq0/06JsiFDFSSc/s640/Quantifying-Earthlike-4.jpg" width="640" /></a></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "arial" , "helvetica" , sans-serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 1.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Three
perspectives on one small planet.</span></span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">-----------</span></span></div>
<span style="font-size: large;">
</span><br />
<div class="MsoNormal">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Writers throughout the twentieth century engaged in speculation
on extraterrestrial life: not only life as we know it, but life as we never knew
it. Lately, however, interest has shifted away from such intriguing prospects as
<a href="https://en.wikipedia.org/wiki/The_Metal_Monster">crystalline colonial organisms</a><span style="color: blue;">,
<a href="https://en.wikipedia.org/wiki/The_Black_Cloud">sentient plasma clouds</a></span>, and the <a href="http://adsabs.harvard.edu/abs/1976ApJS...32..737S">ecologies</a> of <a href="https://en.wikipedia.org/wiki/The_Algebraist">gas giant</a> atmospheres, toward the more prosaic
question of <a href="https://en.wikipedia.org/wiki/Organism">carbon-based life</a> on
Earth-like worlds. Current investigations invoke <a href="https://en.wikipedia.org/wiki/Circumstellar_habitable_zone">habitable zones</a> and
habitable planets, which are defined by the conditions that enabled our own mundane
biosphere to emerge and endure (<b>Figure 1</b>).</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The hydrocarbon lakes of Titan and the subsurface oceans
of Enceladus and Europa remain topics of keen interest. For all we know, these icy
worlds might support exotic biochemistries based on carbon compounds. But we
have no secure way of detecting similar environments in exoplanetary systems,
whether by present methods or those expected for some time to come (Kasting
& Catling 2003, Kasting et al. 2014). This restriction indefinitely tethers
our extrasolar speculations to Earth. The best response to such restraint might
be simply to look in the mirror. What are the limits of “Earth-like?” Which
parameters of our own world can tell us when we’ve found an extrasolar cousin
or sibling?</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">orbit</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The most widely discussed feature of exoplanetary systems
is the <a href="https://en.wikipedia.org/wiki/Circumstellar_habitable_zone">habitable zone</a> – the range
of orbits where the host star’s flux would permit surface bodies of water on a rocky
planet with the appropriate mass and atmosphere. Obviously, Earth is such
a planet, so we know where to look for the Solar System’s habitable zone. Recent
research continues to explore this concept (Kopparapu et al. 2013, Zsom et al.
2013, Kasting et al. 2014). Kopparapu & colleagues express the more or less
standard view: The habitable zone of a G-type star with the same mass as our
Sun extends from about 0.9 to 1.5 astronomical units (AU). For a K-type star of
0.75 Solar masses (Msol) the approximate boundaries are 0.5 – 0.9 AU. For an M
dwarf of 0.4 Msol, they shrink to 0.15 – 0.30 AU.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">mass</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">It is also universally accepted that a planet needs some
minimum mass in order to sustain an active rheology, robust atmosphere, and
surface water over billions of years. Mars, at 0.11 Earth masses (Mea), is evidently
too lightweight to fulfill this condition, whereas Venus, at 0.81 Mea, would be
just right if only its orbit were wider. The lower boundary for mass must be
somewhere in between. In a classic study, James Kasting and colleagues (1993) defined
a habitable planet as “several times more massive than Mars.” They also reasoned
that larger planets “have higher internal heat flows and should therefore be
able to maintain tectonic activity” for substantial periods. As far as I know,
the only researchers who have offered a precise value for the minimum habitable
mass are Sean Raymond and colleagues (2006, 2007), who propose 0.3 Mea.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Finding the maximum mass, however, has been contentious.
At least two factors are in play: plate tectonics and atmospheric accretion.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">tectonics</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Back in 1993, Kasting & colleagues noted that the
carbon-silicate cycle is a necessary enabler of life on Earth. Although they
didn’t mention it, this cycle is supported by plate tectonics (<b>Figure 2</b>), a process foregrounded by
most subsequent discussions of extrasolar life.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">More than a decade later, when theoretical discussions of
<a href="http://backalleyastronomy.blogspot.com/2012/04/between-earth-and-uranus-part-ii.html">Super Earths</a> commenced, several
studies examined the habitability of planets in the range of 1 to 10 Mea. These
objects were typically assigned Earth-like compositions and heavy element
atmospheres – rather than, for example, extended hydrogen/helium (H/He)
envelopes. At least one study argued that plate tectonics would be “inevitable”
on such Super Earths (Valencia et al. 2007). Similar conclusions were implicit
in other literature of the time.</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://3.bp.blogspot.com/-kbOa39bMgbc/VniwsFYcoZI/AAAAAAAAAqs/nMrhaucqdV4/s1600/3-RedoubtVolcanoEruption.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="430" src="https://3.bp.blogspot.com/-kbOa39bMgbc/VniwsFYcoZI/AAAAAAAAAqs/nMrhaucqdV4/s640/3-RedoubtVolcanoEruption.jpg" width="640" /></a></div>
<div class="MsoNormal">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span><br /></div>
<div class="MsoNormal">
<span style="font-family: "arial" , "helvetica" , sans-serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 2.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Volcanic
eruptions are among the most visible indicators of the active geology that
sustains habitable conditions on Earth. This photo shows the 1990 eruption of Redoubt
Volcano along the coast of Cook Inlet in Alaska. Credit: Wikimedia</span></span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">------------------------</span></span><br />
<div style="text-align: left;">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Then came the skeptics. Some have argued that solid
objects above a few Earth masses are unlikely to support plate tectonics
(O’Neill & Lenardic 2007, Korenaga 2010), or at least that the likelihood
shrinks as mass increases (Stein et al. 2013, Stamenkovic & Breuer 2014). Others
are more optimistic, and the debate is far from over.</span></span></div>
<div style="text-align: left;">
<br /></div>
<div style="text-align: left;">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">In the meantime, it would be helpful to know exactly what
“a few Earth masses” means. If there’s an upper mass limit on tectonics, what
is it?</span></span></div>
<div style="text-align: left;">
<br /></div>
<div style="text-align: left;">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">A clue just came in a preprint by Cayman Unterborn &
colleagues (hereafter U15). Seeking to define “Earth-like,” they make plate
tectonics the primary criterion. To generalize across star systems, they propose
a three-layer model for terrestrial planets: a mostly iron <b>core</b> surrounded by magnesium silicate minerals, in proportions that
will vary from world to world, distributed in a dense <b>lower mantle</b> that transitions to a lighter <b>upper mantle</b>. In Earth, the bulk mass percentage for each layer is
respectively 32%, 51%, and 17%. U15 argue that planets with mantle fractions
and convective regimes similar to Earth will experience plate tectonics.</span></span></div>
</div>
<div class="MsoNormal" style="text-align: left;">
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">To illustrate (their Figure 8), they offer a schema of
masses, radii, and chemical compositions that meet this condition, extending as
high as an object of 5 Mea and 1.5 Rea. Although they don’t explicitly discuss
a mass limit for mantle convection, they do argue that plate tectonics is
possible on all the rocky planets encompassed by their schema. As a real-life
example they offer Kepler-36b, a classic Hot Super Earth on a 14-day orbit with
an approximate mass and radius of 4.45 Mea and 1.49 Rea, respectively.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The findings of U15 invite comparison with the recent
work of Courtney Dressing & colleagues (hereafter D15) on the structure of
terrestrial planets. D15 tried to find a single composition that could explain
the masses and radii of Earth, Venus, and five well-characterized extrasolar
terrestrials (CoRoT-7b; Kepler-10b, -36b, -78b, and -93b). By contrast, U15
tried to generalize from Earth’s parameters to construct a flexible model that would
encompass all potentially Earth-like planets. This difference in goals might
explain why D15 used a simpler model of planet structure, with only two layers:
a pure iron core accounting for 17% of the total mass and a magnesium silicate
mantle accounting for 87%.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Notably, their solution involves a much less massive core
than the one proposed by U15. Hence they find a smaller planetary mass at each
radius than do U15. As their real-life example of a terrestrial planet, D15 offer
Kepler-93b, for which they prefer a mass of about 4 Mea and a radius of 1.48
Rea. For the same radius, U15 provide a mass 10% higher. However, the mismatch between
the two studies falls within the range of uncertainties for the parameters of
the five exoplanets modeled by D15. For Kepler-93b, they defined the mass range
as 3.34-4.70 Mea, while for the same mass range U15 provided a range in radius
of about 1.35-1.50 Rea, which encompasses D15’s preferred value.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Happily, then, the results of U15 appear consistent with
the those of D15, whose model has been widely endorsed. Their conclusions have
the further appeal of supporting plate tectonics on selected planets massing up
to 5 Mea. This is a more generous upper bound than I’ve imagined <a href="http://backalleyastronomy.blogspot.com/2015/07/kepler-452b-latest-hope-for-another.html">in recent years</a>.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Nonetheless, it’s clear that an appropriate mantle
structure is insufficient by itself to guarantee either plate tectonics or
life. Water and atmosphere make critical contributions.</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://3.bp.blogspot.com/-nFze-fRs-KQ/VniwfhYM6CI/AAAAAAAAAqk/sjQAajQoJgQ/s1600/Europa%2B%252B%2BEarth%2B-%2BKPHand.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="480" src="https://3.bp.blogspot.com/-nFze-fRs-KQ/VniwfhYM6CI/AAAAAAAAAqk/sjQAajQoJgQ/s640/Europa%2B%252B%2BEarth%2B-%2BKPHand.jpg" width="640" /></a></div>
<div class="MsoNormal">
<span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"></span><br /></div>
<div class="MsoNormal">
<span style="font-family: "arial" , "helvetica" , sans-serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 3.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> The total
water content of Earth and Europa compared. Even though Europa is only about 1%
of Earth’s mass, it contains more water by mass than Earth. Credit: K. P. Hand</span></span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">---------------------------</span></span></div>
<span style="font-size: large;">
</span><br />
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<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">water</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The factors governing Earth-like physical conditions and
carbon-based life have complex interdependencies. Life needs water, and oceans
need plate tectonics, but tectonics also needs oceans (Korenaga 2010, Lammer et
al. 2009, 2010). Lubrication is required to facilitate plate movement, and apparently
ice won’t do<span style="font-family: "georgia" , "serif";">: </span>the eternal wandering of continents is <span style="font-family: "georgia" , "serif";">bo<span style="font-family: "georgia" , "serif";">rne</span></span> by water.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">For truly Earth-like conditions, however, the amount of
water requires titration. Too much is just as bad as not enough, and an excess
of water seems quite easy for young planets to accrete, at least according to
simulations of Solar System history (Raymond et al 2007). Helmut Lammer &
colleagues (2009) pointed out that a rocky planet covered by a water layer 100
km deep, as modeled by studies of “Ocean Planets” (e.g., Leger et al. 2004), would
be unsuitable for the development of life. Regardless of temperature,
high-pressure ices would form at the bottom of such an ocean and prevent
interaction between the water layer and chemicals in the crust. Without this
interaction, life could not arise, and the carbon-silicate cycle would not
emerge.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Even though we were told as children that three-quarters
of our planet is covered by oceans, that water layer is remarkably thin (<b>Figure 3</b>). The total water inventory of
Earth, including water in the mantle, is quite small: just 0.05% Mea (Raymond
et al. 2014). Yann Alibert (2014) found that a planet of Earth mass and
composition can maintain both a global ocean and a carbon cycle only if its
water content is 2% or less by mass, with the maximum percentage <span style="font-family: "georgia" , "serif";">falling</span> rapidly
with <span style="font-family: "georgia" , "serif";">rising</span> planet mass. So far, that seems to be the best available
constraint on water inventory.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">It’s worth noting that although Alibert’s upper limit is 40
times greater than Earth’s current reservoir, it’s still 5 to 25 times smaller
than the water fractions proposed for Sauna Planets and Water Worlds.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span></div>
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<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">atmosphere</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">An atmosphere is generally assumed as a prerequisite for
life. One notable exception involves airless bodies like Europa, where biochemistries
could evolve in shallow subsurface oceans. However, our focus is Earth-like
planets with masses that exceed Europa’s by more than an order of magnitude. These
objects are believed to accrete or outgas significant atmospheres during their
formation and early evolution.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Any planet above the minimum Earth-like mass (defined here
as 0.3 Mea) will likely retain its gas envelope as long as it can withstand the
high levels of extreme ultraviolet (XUV) flux emitted by young stars. Planets
orbiting near the star are the most vulnerable to atmospheric erosion, while
those with masses of several Mea have the best protection.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Pre-Kepler discussions took it for granted that any
object under 10 Mea would be incapable of supporting an H/He envelope. Then in
2011 came the discovery of the <a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html"><span style="color: black;">Kepler-11</span></a> system, where at
least four planets with masses between 2 and 8 Mea revealed puffy silhouettes
consistent with deep H/He atmospheres. Many comparable Kepler planets have been
characterized since then. It is now understood that rocky cores similar in mass
to Earth can have radically more extensive envelopes. Since H2 is a greenhouse
gas, and H/He atmospheres produce surface pressures far in excess of Earth’s,
even relatively low-mass planets with deep gas envelopes will not sustain
surface bodies of water. By definition, they are not Earth-like.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Two recent studies, led respectively by Rebekah Dawson
and Helmut Lammer, explored the conditions needed for terrestrial planets up to
5 Mea to accrete and sustain puffy envelopes. While taking different approaches,
these two groups found a similar lower boundary for envelope survival: 2 Mea.
Above that mass, unless they orbit very close to their host stars, most planets
will accrete and retain H/He atmospheres. Below that mass, even on cooler
orbits, primordial H/He will dissipate.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Dawson & colleagues (hereafter D15) conducted N-body
simulations to study the <i>in situ</i> accretion
of rocky objects in protoplanetary disks of varying metallicity, along with
their atmospheric evolution up to the dispersion of the nebular gas. They did
not address the photoevaporation of primitive atmospheres, a factor that
becomes significant only after the nebula disperses. Nonetheless, they cited
Lopez & Fortney (2014) for a discussion of atmospheric loss at later stages
of system evolution.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The central aim of D15 was to investigate the
relationship between the surface density of solid materials suspended in the
primordial nebula and the mass and atmospheric composition of the resulting planets.
Their simulations followed the evolution of rocky embryos orbiting a Sun-like
star between 0.04 and 1 AU in the presence of a dusty H/He nebula that
dissipated after 1 million years. The embryos grew by mergers and accreted gas
according to their mass. D15 found that 1) planetary cores smaller than 2 Mea
did not accrete substantial envelopes from the nebula, and 2) cores of 2 Mea or
more could form within 1 million years only in protoplanetary disks with a high
surface density of solids. Such environments are typical of highly metallic
stars even without significant gas-driven migration of planetesimals or
embryos.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">D15 also concluded that the inner nebulae of stars with
ordinary or depleted metallicity can still achieve a sufficient surface density
to build gas dwarfs if they experience migration of solids from outer orbits.
In other words, D15’s approach does not require “strict” <i>in situ</i> accretion (Chiang & Laughlin 2013). They even permit
the migration of full-formed gas dwarfs from outside 1 AU, as in Lee &
colleagues (2014, 2015).</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">This study raises interesting questions on many points,
including realistic timeframes for nebula dispersion and the effect of mixed
migration pathways on the composition of planets that achieve habitable orbits.
Nevertheless, the answers probably wouldn’t change D15’s most salient findings
on envelope accretion by young planets. They conclude that planets under 2 Mea
are unlikely to capture or sustain H/He atmospheres, while planets above that
mass will do so under typical conditions. This result provides a clear
constraint on definitions of “Earth-like.”</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://4.bp.blogspot.com/-OWCVsqjjIto/VniwVOgci4I/AAAAAAAAAqc/pQSasq2xU1c/s1600/Mars%2B%252B%2BFour%2BEarthlike%2BPlanets%2B2.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="182" src="https://4.bp.blogspot.com/-OWCVsqjjIto/VniwVOgci4I/AAAAAAAAAqc/pQSasq2xU1c/s640/Mars%2B%252B%2BFour%2BEarthlike%2BPlanets%2B2.jpg" width="640" /></a></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 4.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Since
massive rocky objects are likely to accrete deep hydrogen atmospheres, the
range of Earth-like planets is narrow, extending from about one-<span style="font-family: "arial" , "sans-serif";">third</span> to twice
the mass of Earth. Shown above are four NASA photographs of our planet scaled
according to the mass-radius relationships of Unterborn et al. (2015). For
perspective, one photogenic but out-of-range object – Mars – is included at far
left. </span></span></div>
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<span style="font-size: large;">
</span><br />
<div class="MsoNormal">
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">An earlier study by Lammer & colleagues (hereafter
L14) had already looked at the evolution of gas-enveloped planets after nebula
dispersion. They studied a single phase of the process under idealized
conditions, modeling rocky planets with masses between 0.1 and 5 Mea orbiting a
Solar twin at 1 AU. All planets were assumed to reach their final core masses in
the presence of the primordial nebula, and to accrete H/He envelopes in
proportion to their mass.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">L14 differed sharply from D15 in their handling of gas
accretion, since even sub-Earth objects in their model captured H/He. Nor did
they advance any argument regarding surface densities of solids or the formation
pathways of their theoretical planets. Their approach appears agnostic to these
factors.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">In the most favorable variations on their model, some
Super Earths of 5 Mea reached the threshold of runaway gas accretion. In real
systems these would become gas giants. The rest, along with all other planets
of lower mass, captured smaller but still substantial H/He envelopes before the
nebula dispersed. With the loss of this protective cloud, however, the effects
of XUV flux became significant.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">L14 found that rocky planets down to 2 Mea –in some cases
even as low as 1 Mea – suffered minimal atmospheric loss during the phase of
“saturated” flux in the first 100 million years of stellar evolution. These
planets were able to retain their H/He envelopes indefinitely, resembling
typical Kepler planets with masses of 2-5 Mea and radii of 2-4 Rea. Less
massive planets, however, lost their envelopes.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Considering L14’s findings alongside the other
constraints discussed so far, we see some major shrinkage in the likely mass
range of Earth-like planets around Sun-like stars. Evidently it’s about 0.3 to 2
Mea (<b>Figure 4</b>), with potential
outliers at slightly higher masses. Given the level of XUV flux typical of the
habitable zones of late F, G, and early K-type stars, all planets under 1 Mea
lose their primordial H/He, whereas most planets over 2 Mea retain it.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">L14 proposed that the relative dustiness of the nebula would
be a critical factor determining the survival of H/He envelopes around planets over
1 Mea. Some fraction of rocky planets between 2 and ~2.5 <span style="font-family: "georgia" , "serif";">M</span>ea might end up with
friendly atmospheres of nitrogen and carbon dioxide, but that outcome becomes
vanishingly less likely with increasing mass.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The only exception might be planets that reached their
final bulk during a phase of giant impacts after the nebula dissipated. This is
actually how the Solar System’s small planets formed, but <a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html">recent work</a> suggests that
our system’s <a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-i.html">evolutionary history</a> is <a href="http://backalleyastronomy.blogspot.com/2012/05/how-weird-is-our-solar-system.html">unique</a>. In other potential evolutionary
scenarios, we could imagine a collision between two Super Earths of 1.8 Mea each,
which (after the dust settles) would create a single gas-free planet of about 3.5
Mea. Since the collision happens after nebula dispersion, the new planet cannot
accrete any additional atmosphere. Thus it evolves into a truly super-sized
Earth with volcanoes, oceans, and all the rest. Scenarios like that might be
rare, though.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">On firmer ground, the complementary results of D15 and
L14 indicate that rocky planets of 0.3–2.0 Mea and 0.7–1.2 Rea will be free of
troublesome H/He envelopes. Depending on the specifics of formation pathways
and XUV flux, these planets would make plausible candidates for <a href="http://backalleyastronomy.blogspot.com/2015/01/much-ado-about-earth-2.html">Earth 2</a>.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">spectral
type</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">But don’t forget about that X-ray and XUV flux. Indeed, it’s been
getting a lot of attention over the past year or so. Most of the studies
discussed in this posting used stars of the same mass and effective temperature
as our Sun for their standard. The underlying assumption is that factors relevant
to thermal environment, such as the location of the system habitable zone, can
be scaled to fit stars of different effective temperatures, luminosities, and
colors.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">But stellar evolution places a limit on such scaling. The
earliest discussions of extrasolar life noted that stars above a certain mass –
around 1.5 times Solar (1.5 Msol) – have a main sequence lifetime too brief to
permit the evolution of life. Even if a planet of the right mass and
composition were to orbit comfortably in the habitable zone of an A-type star
of 1.8 Msol, it would barely have time to cool down and recover from asteroid
bombardment before its parent star began expanding and reddening into the subgiant
stage. Rising temperatures would then boil off the planetary ocean and sterilize
any emergent biosphere.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Fortunately, high-mass stars of spectral types A, B, and
O represent less than 1% of the stellar population in our region of the Milky
Way. What about M dwarfs, which account for three-quarters of all main sequence
stars? Questions regarding the habitability of their planets are getting
complicated.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Earlier studies noted that the close-in habitable zones
of M dwarfs would constrain planets with the appropriate insolation to be
tidally locked, without benefit of a day/night cycle. M dwarfs are also more
likely than higher-mass stars to erupt in intense flares that could destroy
volatiles and erode the atmospheres of close-in planets. Nevertheless, neither
factor seems an insurmountable barrier to the emergence of life. Presumably
organisms could evolve in non-stop daylight and eventually colonize darker
longitudes, while robust atmospheres would shield surface ecologies against
occasional flares.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Recent literature, however, finds new causes for doubt. A
study by Luger & Barnes (2015) provides several reasons for pessimism about
the habitability of M dwarf planets. In addition to the propensity of red stars
to undergo extreme flaring events, Luger & Barnes note the antagonistic
qualities of their evolutionary history. Newborn stars under about 0.65 Msol
spend several hundred million years at luminosities one to two orders of
magnitude higher than their main sequence brightness. Yet planet formation around
any star happens on a much shorter timescale, within a few tens of millions of
years after stellar ignition. Luger & Barnes demonstrate what that mismatch
in developmental histories means for water and life. Planets that form in a
young M dwarf’s habitable zone will freeze out once the star matures, whereas
planets that end up in the mature star’s habitable zone will have been roasted
for a billion years by intense X-ray and XUV flux, including violent flares. The cool
planets will be too cold, while the warm planets will be stripped of volatiles.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">These results make M dwarf stars less attractive as
potential hosts of Earth-like planets than they looked just a few years ago. It
seems that sensibly Sun-like stars in the approximate range of 0.7–1.3 Msol are
once again the best choice of parents, at least if you want to grow up to be
green.</span><b><span style="color: black; font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="color: black; font-family: "georgia" , "serif";">perspectives
on known space</span></b><span style="color: black; font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">In the context of the other constraints outlined here, the
findings of Luger & Barnes also call for a harder look at the clutch of <a href="http://backalleyastronomy.blogspot.com/2015/07/kepler-452b-latest-hope-for-another.html">small Kepler planets</a> proposed
over the past few years as potential Earth analogs. Among the six candidates
confirmed to date, four orbit stars less massive than 0.65 Msol – M dwarfs by
any other name. Ironically, these are the four smallest planets of the lot
(Kepler-438b, -186f, -395c, -442b), with radii ranging from 1.12 to 1.34 Rea. Since
all have periods shorter than 130 days, and two have periods shorter than 40
days, they all might have suffered complete desiccation. In fact, a new study
just reported that Kepler-438b, with a semimajor axis of only 0.17 AU,
experiences powerful flares from its host star that make it vulnerable to
complete loss of atmosphere (Armstrong et al. 2015). None of these candidates
seem truly Earth-like.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">The other two planets (Kepler-62f, -452b) orbit hotter
stars on longer orbital periods, so they appear to occupy their systems’
long-term habitable zones. By some definitions their radii place both of them at
or near the upper edge of the Earth-like range, but according to the criteria
established in this discussion, 452b is definitely, and 62f is probably, just too
big.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Assuming the proposed radius of 1.41 Rea, an Earth-like
composition would confer a mass of 3.5–4 Mea on Kepler-62f, following the models
of Dressing et al. (2015) and Unterborn et al. (2015), respectively. Either
value would be consistent with plate tectonics (at least according U15). But we
have no constraints on this object’s true mass. It seems just as likely to be a
less dense and thus less massive planet with a large volatile content: either a
deep global ocean, failing the criterion for water, or a remnant H/He envelope,
failing the criterion for atmosphere. According to the results of Lammer et al.
(2014) and Dawson et al. (2015), it could be a relatively hospitable rocky
planet of 3.5–4 Mea only if it formed by giant impacts after the system’s
protoplanetary nebula dissipated. Because 62f is part of compact multiplanet
system with four inner companions, however, a history of dynamical upset seems
unlikely.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Assuming a radius 1.63 Rea, Kepler-452b has a similar
range of potential compositions, although the all-rocky option is even more
unlikely. At 5–6 Mea, an ice- and hydrogen-free planet would also need to be
iron-free to achieve a radius so large. Such a composition isn’t plausible, and
even if it were, it’s hard to see how plate tectonics might develop. A water
world or an unlucky Earth-mass object with a residual H/He shroud seems more
likely.</span></span><br />
<br />
<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Given all these disappointing candidates, imaginary
Earth-like planets (<b>Figure 5</b>) will
have to do for a while longer.</span></span></div>
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<div class="separator" style="clear: both; text-align: center;">
<a href="http://3.bp.blogspot.com/-GtrdzYkcrAk/VniwBFLoP0I/AAAAAAAAAqU/nd8G_SmXqOI/s1600/Earth-like%2Bplanets%2Bacross%2Ban%2Border%2Bof%2Bmagnitude.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="472" src="https://3.bp.blogspot.com/-GtrdzYkcrAk/VniwBFLoP0I/AAAAAAAAAqU/nd8G_SmXqOI/s640/Earth-like%2Bplanets%2Bacross%2Ban%2Border%2Bof%2Bmagnitude.jpg" width="640" /></a></div>
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<span style="font-family: "georgia" , "serif";"></span><br /></div>
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<span style="font-family: "arial" , "helvetica" , sans-serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;">Figure 5.</span></b><span style="color: black; font-family: "arial" , "sans-serif"; font-size: 10pt;"> Concocted
Earth-like planets across an order of magnitude in mass, following the mass-radius
curves of Unterborn et al. 2015. For rocky worlds, radius increases more slowly
than mass, so the most massive object pictured here is not quite double the
radius of the least massive. Note that the fanciful world of 3 Mea is very
likely an outlier; most hydrogen-free planets are expected to be under 2 Mea
and 1.2 Rea, like the other five examples. (In the diagram, RE = Earth radius,
ME = Earth mass.) My gratitude to everyone who lives in these worlds.</span></span></div>
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<span style="font-size: large;"><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "georgia" , "serif";"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html">Click
here for a permanent index by topic of blog posts<span style="color: blue;"><br />
</span>on Back Alley Astronomy</a></span></i></span><span style="font-family: "georgia" , "serif";">
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<div class="MsoNormal" style="page-break-after: avoid;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">REFERENCES</span></b></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b><span style="color: black; font-size: 10pt;">Alibert Y.</span></b><span style="color: black; font-size: 10pt;"> (2014) On the radius of habitable planets. <i style="mso-bidi-font-style: normal;">Astronomy </i></span><i style="mso-bidi-font-style: normal;"><span style="color: black; font-size: 10pt;">& <span style="mso-bidi-font-weight: bold;">Astrophysics</span></span></i><span style="color: black; font-size: 10pt;"> </span><span style="color: black; font-size: 10pt;">561,
A41.</span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Armstrong DJ</span></b><span style="color: black; font-size: 10pt;">, Pugh CE, Broomhall AM, Brown DJA, Lund MN,
Osborn HP, Pollacco DL. (2015) The host stars of Kepler’s habitable exoplanets:
Superflares, rotation and activity. <i style="mso-bidi-font-style: normal;">Monthly
Notices of the Royal Astronomical Society. </i>Abstract: <a href="http://mnras.oxfordjournals.org/content/455/3/3110.abstract"><span style="color: black; mso-themecolor: text1;">http://mnras.oxfordjournals.org/content/455/3/3110.abstract</span></a>
</span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10pt;">Chiang E, Laughlin G.</span></b><span style="color: black; font-size: 10pt;"> (2013) The
minimum-mass extrasolar nebula: <i style="mso-bidi-font-style: normal;">in situ</i>
formation of close-in Super-Earths. <i style="mso-bidi-font-style: normal;">Monthly
Notices of the Royal Astronomical Society</i> 431, 3444-3455. Abstract: <a href="http://adsabs.harvard.edu/abs/2013MNRAS.431.3444C"><span style="color: black; mso-themecolor: text1;">2013MNRAS.431.3444C</span></a></span></div>
<div class="MsoNormal">
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-58657189703198254042015-12-19T13:06:00.000-08:002015-12-19T13:06:00.749-08:00Evolutionary Twist<!--[if gte mso 9]><xml>
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<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="http://2.bp.blogspot.com/-DUTtMMow_gc/VnXGpR2lXXI/AAAAAAAAAqA/RzLskQiZBRM/s1600/Transiting%2BHot%2BJupiter%2B-%2BESO.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="390" src="http://2.bp.blogspot.com/-DUTtMMow_gc/VnXGpR2lXXI/AAAAAAAAAqA/RzLskQiZBRM/s640/Transiting%2BHot%2BJupiter%2B-%2BESO.jpg" width="640" /></a></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;">Figure 1.</span></b><span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;"> The Sun-like star
WASP-47 hosts a transiting Hot Jupiter flanked by two low-mass planets, and all
three have periods shorter than 10 days. This was the first and so far the only
detection of such a configuration. Above, an artist’s rendering shows a Hot Jupiter
in transit across the face of its star. Credit: European Southern Observatory</span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;">------------------------------</span></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">A few months
back I wrote about <a href="http://backalleyastronomy.blogspot.com/2015/08/the-blazing-wasp-47s.html">WASP-47</a></span><span class="MsoHyperlink"><span style="font-family: "Georgia","serif";">b</span></span><span style="font-family: "Georgia","serif";">, the first Hot Jupiter ever found with
low-mass planets on nearby orbits. I noted my eagerness to see a theoretical
study of the possible formation mechanisms behind this unusual system, while
venturing that the orbital architecture of WASP-47 could not have resulted from
<i>in situ</i> accretion. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Then last month
I took <a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html">a
close look</a> at a study by Konstantin Batygin & Greg Laughlin (2015), who
offered a novel explanation for the formation of the inner planets of the Solar
System. Their approach folds the evolutionary history of the four terrestrial
planets into the <a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html">Nice
Model</a> and the <a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html">Grand
Tack</a>. I observed in passing that, although Laughlin was one of the earliest
proponents of <i>in situ</i> theory (which underlies
the scenario that he and Batygin proposed), he hadn’t written anything purely
theoretical on this topic for quite a while. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Two new
preprints respond serendipitously to those conjectures and observations. One is
a theoretical study by Batygin, Bodenheimer & Laughlin (2015) on the
potential for Hot Jupiters – Hot Jupiters, no less! – to form <i>in situ</i>. The authors develop a new
formation scenario for star-hugging gas giants and use numerical simulations to
test it. Then they appeal to the WASP-47 system as possible evidence that this
mechanism really works. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">The second
preprint is also highly theoretical. Nathan Kaib and John Chambers (2015) tersely
present the results of a large ensemble of numerical simulations designed to
test the Nice Model of Solar System evolution. Their findings question the utility
of this approach as an explanation for the <a href="http://backalleyastronomy.blogspot.com/2014/08/a-billion-years-of-hell-on-earth.html">Late
Heavy Bombardment</a>. They can also be interpreted as evidence that our four
terrestrial planets formed after, not before, the dynamical instability addressed
by the Nice model. Although Kaib & Chambers don’t mention the recent
articles by Batygin & Laughlin or <a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html">Volk
& Gladman</a> on the inner Solar System, their conclusions call for rethinking
both the Nice model and the Grand Tack. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><b><span style="font-family: "Georgia","serif";">a burning cradle for baby giants?</span></b></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">The new study by
Batygin & colleagues (hereafter B15) is called “<i>In situ</i> formation and dynamical evolution of Hot Jupiter systems.” The
title is a misnomer, however, as they don’t actually model the <i>in situ</i> accretion of solids on orbits
where Hot Jupiters are observed. Instead, as often happens in simulation
studies, they use simplified initial conditions to examine a narrowly defined
question: Can a dense planet of sufficient mass, orbiting at or near the inner
edge of a protoplanetary nebula, accrete enough ambient hydrogen to balloon
into a gas giant? </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">B15 offer four
representative evolutionary sequences in which solid planetary cores at a range
of masses follow circular orbits around a Sun-like star at a semimajor axis of
0.05 AU. They “purposely adopted an agnostic viewpoint” regarding formation
mechanisms, explaining that their results “are largely independent of how the
solid core arises.” The simulations seem equally friendly to x) strict <i>in situ</i> formation involving the
accretion of local mass only; y) migration of solids from cooler orbits into
the hot inner nebula and subsequent accretion at the new location; or z) migration
of fully-formed cores from cooler orbits. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">In units of
Earth mass (Mea), the core masses they select are 4 Mea, 10 Mea, and 15 Mea.
The object of 15 Mea is used in two different set-ups, one with low ambient gas
density and the other with higher density. As a reality check, I note that
atmosphere-free planets of 10 Mea or more are not attested (Jontof-Hutter et
al. 2015), while realistic models of planet formation have great difficulty
forming an object of 4 Mea or more at 0.05 AU, where silicate dust sublimates
(Dullemond & Monnier 2010).</span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Regarding gas
accretion, B15 concede that insufficient mass in ambient gas would be available
in the local feeding zone to build a gas giant at 0.05 AU. Therefore, they
assume that the local supply of hydrogen/helium is constantly replenished by
the regular flow of gas through the nebula onto the infant star. Their model seems
to hinge on the capacity of the short-period planet to capture this inrushing
gas. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Across
simulations, objects of 15 Mea were the only ones that managed to accrete gas
envelopes equal in mass to their solid cores and thereby trigger runaway gas
accretion. Thus, they find that the minimum core mass capable of growing into a
gas giant in the vicinity of a Sun-like star is 15 Mea. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal" style="page-break-after: avoid;">
<span style="font-size: large;"><b><span style="font-family: "Georgia","serif";">Figure 2.</span></b><span style="font-family: "Georgia","serif";"> Light curves of the three inner planets
of WASP-47</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://2.bp.blogspot.com/-uQ7B6iwVJgY/VnXGfyZO1TI/AAAAAAAAAp4/52SZs_oGnGE/s1600/WASP-47%2BTransit%2BLight%2BCurves%2B-%2BBecker%2B2015.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="434" src="http://2.bp.blogspot.com/-uQ7B6iwVJgY/VnXGfyZO1TI/AAAAAAAAAp4/52SZs_oGnGE/s640/WASP-47%2BTransit%2BLight%2BCurves%2B-%2BBecker%2B2015.gif" width="640" /></a></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "Georgia","serif";"></span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;">Phase-folded light
curves of the three transiting planets detected by the K2 mission: Becker et
al. 2015</span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;">---------------------------------------</span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">B15 conclude
that their scenario will regularly produce gas giants, so that <i>in situ</i> formation might “represent the
dominant channel for hot Jupiter generation.” They proceed to a discussion of
WASP-47b as an example of what this channel might bring. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">But I’m not
convinced. The likeliest origin for an object of 15 Mea with a semimajor axis
around 0.05 AU is migration from a wider orbit where accretion can more readily
produce such massive objects. As far as I know, a gas giant can migrate into a
hot orbit just as easily as a Neptune-mass object can. So why resort to a
two-step process – first migrate the core, then capture the atmosphere – when a
well-theorized one-step process (Type II migration) works even better? B15
appear to be offering a solution in search of a problem. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">They cite
WASP-47 as a rare system in which a compact family of Super Earths somehow
managed to promote one of their number to giant status without changing any
semimajor axes or otherwise disrupting the rest of the family. But it seems
that considerable acrobatics would be needed to achieve this remarkable outcome.
For example, how did only one planet get pumped up? What kept the others down? </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">I suggest that,
along with exploring novel <i>in situ</i>
explanations for the evolution of WASP-47, the exoplanet community should test
whether migration scenarios could explain <a href="http://backalleyastronomy.blogspot.com/2015/08/the-blazing-wasp-47s.html">this
architecture</a> with fewer contortions. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal" style="page-break-after: avoid;">
<span style="font-size: large;"><b><span style="font-family: "Georgia","serif";">Figure 3.</span></b><span style="font-family: "Georgia","serif";"> The wreck of the <i>Kronan</i></span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://2.bp.blogspot.com/-V915LGXSEIk/VnXGWJsHglI/AAAAAAAAApw/sjhtVv1ZLWs/s1600/Slaget_vid_%25C3%2596land_Claus_M%25C3%25B8inichen_1686.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="324" src="http://2.bp.blogspot.com/-V915LGXSEIk/VnXGWJsHglI/AAAAAAAAApw/sjhtVv1ZLWs/s640/Slaget_vid_%25C3%2596land_Claus_M%25C3%25B8inichen_1686.jpg" width="640" /></a></div>
<div class="MsoNormal">
<span style="font-family: "Georgia","serif";"></span></div>
<div class="MsoNormal">
<span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;">In
1676, the Swedish warship </span><a href="https://en.wikipedia.org/wiki/Kronan_%28ship%29"><i style="mso-bidi-font-style: normal;"><span style="font-family: "Arial","sans-serif"; font-size: 10.0pt; text-decoration: none; text-underline: none;">Kronan</span></i></a><span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;"> capsized while
trying to execute a sharp turn under too much sail, causing an explosion in the
gunpowder magazine. Few survived. Illustrated is an imaginative (and historically
inaccurate) rendering of the disaster by Claus Moinichen in 1686. </span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;">---------------------------------------</span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal">
<span style="font-size: large;"><b><span style="font-family: "Georgia","serif";">does the sailing master have no clothes?</span></b></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">The study by
Kaib & Chambers is conducted on a grander scale than B15. They ran almost
300 high-resolution simulations of Solar System evolution to test three
different versions of the Nice model and explore the effects of the resulting dynamical
instability on the four terrestrial planets. A simulation was considered
successful if the final system met three criteria after the instability concluded:
A) all four outer planets from Jupiter to Neptune survived, with the orbits of
Saturn and Jupiter lying between a 2:1 and 3:1 mean motion resonance; B) all
four inner planets also survived on stable orbits; and C) the angular motion
deficit of the four inner planets was less than or equal to its present value
after their orbits were integrated for an additional billion years. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">In all three
versions of the initial set-up, the four terrestrial planets were assigned
their present masses and orbits. In two versions, the outer system contained
Jupiter, Saturn, and three Neptune-like planets. In the third, it contained
Jupiter, Saturn, two Neptune-like planets, and two “Super Earths” of 8 Mea
each. Among all iterations of these set-ups, the success rate according to
Criterion A was 13% to 16%. However, the set-up with six outer planets failed
both of the other criteria, while the other two models produced just 1 and 2
systems, respectively, that met Criterion B. Only one iteration of one of these
models also met Criterion C. All other iterations lost at least one terrestrial
planet (usually Mercury), and many lost two (usually Mercury and Mars). </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">These results are
very likely to inform future discussions of the Nice Model. As Kaib &
Chambers conclude: </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal" style="margin-left: 0.5in;">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal" style="margin-left: 0.5in;">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">[W]</span><span style="font-family: "Georgia","serif";">e find a
probability of 1% or less that the orbital architectures of the inner and outer
planets are simultaneously reproduced in the same system. These small
probabilities raise the prospect that the giant planet instability occurred
before the terrestrial planets had formed. This scenario implies that the giant
planet instability is not the source of the Late Heavy Bombardment and that
terrestrial planet formation finished with the giant planets in their modern
configuration.</span></span></div>
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<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Notably, <a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html">the
recent study</a> by Volk & Gladman (2015) proposed an approach to the evolution
of the four terrestrial planets that dispensed with the Nice Model altogether,
suggesting an alternative explanation for the Late Heavy Bombardment. Although
I don’t find their alternative persuasive, I hope these efforts inspire further
evolutionary studies to address the problem of our denuded inner system. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">In the meantime, the findings of Kaib
& Chambers might benefit from reframing. As discussed in earlier blog posts,
the phenomenon known as the Late Heavy Bombardment or Lunar Cataclysm can be
understood in different ways. <a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html">In
one view</a>, it was a self-contained period of heavy cometary impacts that were
preceded and followed by long stretches of relative calm; its peak came about
3.8 billion years ago (Gya), at a system age of ~800 million years (Gomes et
al. 2005). In <a href="http://backalleyastronomy.blogspot.com/2014/08/a-billion-years-of-hell-on-earth.html">another</a>,
it was simply one episode in a long, saw-toothed series of bombardments that
began with Earth’s accretion and subsided only toward the end of the Archean
period, about 2.5 – 3 Gya (Morbidelli et al. 2012, Marchi et al. 2014). Instead
of triggering a late and relatively short-lived catastrophe, maybe the dynamic
instability happened in the first 100 million years of Solar System history and
inaugurated that long and violent epoch of bombardments. The Nice Model would
still explain how the massive outer planets nudged and jostled one another into
their mature (and usually wider) orbits. </span></span></div>
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</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal" style="page-break-after: avoid;">
<span style="font-size: large;"><b><span style="font-family: "Georgia","serif";">Figure 4.</span></b><span style="font-family: "Georgia","serif";"> Overview of Earth history</span></span></div>
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<br /></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://4.bp.blogspot.com/-MmDIT3WCN6E/VnXGKi8E7NI/AAAAAAAAApo/FanZWsDt4uQ/s1600/Aeons%2Bof%2BEarth%2BHistory.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="430" src="http://4.bp.blogspot.com/-MmDIT3WCN6E/VnXGKi8E7NI/AAAAAAAAApo/FanZWsDt4uQ/s640/Aeons%2Bof%2BEarth%2BHistory.gif" width="640" /></a></div>
<br />
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Again, maybe the failure of a single
model to explain the final architecture of the inner as well as the outer Solar
System is another clue that we need a new picture of inner system evolution.
What if the Grand Tack triggered an inner system catastrophe, as modeled by <a href="http://backalleyastronomy.blogspot.com/2015/11/jupiter-descending.html">Batygin
& Laughlin (2015</a>), but the resulting mess did not immediately resolve
into a four-planet system? Maybe the battle of oligarchs that followed
Jupiter’s retreat from the Sun’s inner territories was still raging while
Neptune was decimating the outer realms. Maybe lasting peace in the inner
system was contingent on resolving tensions among the outer worlds, a
resolution that evidently hinged on the exile of a Uranus-type planet. Maybe
the final detente between Jupiter and Saturn was what enabled Earth to dominate
its three smaller siblings. </span></span></div>
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<span style="font-size: large;"><i><span style="color: black; font-family: "Georgia","serif";"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html">Click
here for a permanent index by topic of blog posts<span style="color: blue;"><br />
</span>on Back Alley Astronomy</a></span></i><span style="font-family: "Georgia","serif";">
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<b><span style="font-size: 10.0pt;">REFERENCES</span></b></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt;">Batygin K, Laughlin G.</span></b><span style="color: black; font-size: 10.0pt;"> (2015) Jupiter’s decisive role in the
inner Solar System’s early evolution. <i style="mso-bidi-font-style: normal;">Proceedings
of the National Academy of Sciences</i> 112, 4214-4217. Abstract: </span><span style="font-size: 10.0pt;"><a href="http://adsabs.harvard.edu/abs/2015PNAS..112.4214B"><span style="text-decoration: none; text-underline: none;">2015PNAS..112.4214B</span></a>
</span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt;">Batygin K</span></b><span style="color: black; font-size: 10.0pt;">, Bodenheimer P, Laughlin G. (2015) <i style="mso-bidi-font-style: normal;">In situ</i> formation and dynamical
evolution of Hot Jupiter systems. In press. Abstract: </span><span style="font-size: 10.0pt;"><a href="http://adsabs.harvard.edu/abs/2015arXiv151109157B"><span style="text-decoration: none; text-underline: none;">2015arXiv151109157B</span></a></span><span class="MsoHyperlink"><span style="font-size: 10.0pt;"></span></span></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt;">Becker
J,</span></b><span style="color: black; font-size: 10.0pt;"> Vanderburg A, Adams F,
Rappaport S, Schwengler H. (2015) WASP-47: A Hot Jupiter system with two
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Journal Letters</i> 812, L18. Abstract: </span><span style="font-size: 10.0pt;"><a href="http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2015ApJ...812L..18B&db_key=AST&link_type=ABSTRACT&high=5585b8697d30707"><span style="text-decoration: none; text-underline: none;">2015ApJ...812L..18B</span></a></span><span style="color: black; font-size: 10.0pt;"> </span></div>
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<b><span style="font-size: 10.0pt;">Dullemond CP, Monnier JD.</span></b><span style="font-size: 10.0pt; mso-bidi-font-weight: bold;"> (2010). The inner regions
of protoplanetary disks. <i style="mso-bidi-font-style: normal;">Annual Review of
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<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Gomes R</span></b><span style="font-size: 10.0pt;">,
Levison HF, Tsiganis K, Morbidelli A. (2005) Origin of the cataclysmic Late
Heavy Bombardment period of the terrestrial planets. <i style="mso-bidi-font-style: normal;">Nature</i>, 435: 466-469. Abstract: <a href="http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005Natur.435..466G&db_key=AST&link_type=ABSTRACT&high=5585b8697d31051"><span style="text-decoration: none; text-underline: none;">2005Natur.435..466G</span></a>
</span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Jontof-Hutter
D</span></b><span style="font-size: 10.0pt;">, Ford EB, Rowe JF, Lissauer JJ,
Fabrycky DC, Christa Van Laerhoven5, Agol E, Deck KM, Holczer T, Mazeh T<span style="mso-bidi-font-weight: bold;">. (2015) Robust TTV mass measurements: Ten
Kepler exoplanets between 3 and 8 M<sub>earth</sub></span></span><span style="font-size: 10.0pt; mso-fareast-font-family: CMSY8;"> </span><span style="font-size: 10.0pt; mso-bidi-font-weight: bold;">with diverse densities and
incident fluxes. In press.</span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b><span style="font-size: 10.0pt;">Kaib NA, Chambers JE.</span></b><span style="font-size: 10.0pt; mso-bidi-font-weight: bold;"> (2015) The fragility of the
terrestrial planets during a giant planet instability. <i style="mso-bidi-font-style: normal;">Monthly Notices of the Royal Astronomical Society</i>. In press.
Abstract: </span><span style="font-size: 10.0pt;"><a href="http://arxiv.org/abs/1510.08448"><span style="mso-bidi-font-weight: bold; text-decoration: none; text-underline: none;">http://arxiv.org/abs/1510.08448</span></a><span style="mso-bidi-font-weight: bold;"> </span></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt;">Marchi
S</span></b><span style="color: black; font-size: 10.0pt;">, Bottke WF,
Elkins-Tanton LT, Bierhaus M, Wuennemann K, Morbidelli A, Kring DA. (2014)
Widespread mixing and burial of Earth’s Hadean crust by asteroid impacts. <i style="mso-bidi-font-style: normal;">Nature</i> 511, 578-582. </span><span style="font-size: 10.0pt;"><a href="http://adsabs.harvard.edu/abs/2014Natur.511..578M"><span style="text-decoration: none; text-underline: none;">2014Natur.511..578M</span></a></span><span style="color: black; font-size: 10.0pt;"> </span></div>
<div class="Default">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Morbidelli A</span></b><span style="font-size: 10.0pt;">, Marchi S,
Bottke WF, Kring DA. (2012) A sawtooth-like timeline for the first billion
years of lunar bombardment. <i style="mso-bidi-font-style: normal;">Earth and
Planetary Science Letters</i> 355, 144-151. Abstract: <a href="http://adsabs.harvard.edu/abs/2012E%26PSL.355..144M"><span style="text-decoration: none; text-underline: none;">http://adsabs.harvard.edu/abs/2012E%26PSL.355..144M</span></a>
</span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt;">Volk
K, Gladman B</span></b><b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">.</span></b><span style="font-size: 10.0pt;"> </span><span style="color: black; font-size: 10.0pt;">(2015) Consolidating and crushing
exoplanets: Did it happen here? <i style="mso-bidi-font-style: normal;">Astrophysical
Journal Letters</i>, 806: L26. Abstract: </span><span style="font-size: 10.0pt;"><a href="http://adsabs.harvard.edu/abs/2015ApJ...806L..26V"><span style="text-decoration: none; text-underline: none;">2015ApJ...806L..26V</span></a></span><span style="color: black; font-size: 10.0pt;"></span></div>
<div class="MsoNormal">
<br /></div>
Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-19610874656763561012015-11-08T18:17:00.000-08:002015-11-10T01:40:25.597-08:00Jupiter Descending<!--[if gte mso 9]><xml>
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<a href="http://3.bp.blogspot.com/-j2ji4JOtrpA/VkAAp6BoUhI/AAAAAAAAAo4/jUsi7oHKJ84/s1600/jupiter-courtesy-Warburg-Institute.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="480" src="http://3.bp.blogspot.com/-j2ji4JOtrpA/VkAAp6BoUhI/AAAAAAAAAo4/jUsi7oHKJ84/s640/jupiter-courtesy-Warburg-Institute.jpg" width="640" /></a></div>
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<b style="mso-bidi-font-weight: normal;"><span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;">Figure
1.</span></b><span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"> “Jupiter
rules all things in heaven with his brilliant light, and with his warmth he caresses
all creatures. With his mighty hand that God scatters cruel missiles, flashing
fire as he hurls thunderbolts from his high citadel.” Image credit: The Warburg
Institute</span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "georgia" , "serif";">----------</span></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">As the
extrasolar census approaches 2000 confirmed planets located in more than 1200 different
systems, it</span></span><span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="font-size: large;"><span style="font-family: "georgia" , "serif";">’</span></span>s no longer possible to ignore the oddity of our Solar System. About
half of all stars like our Sun host planets a few times the mass of Earth orbiting
inside circumstellar radii of 0.5 astronomical units (AU). A smaller fraction
support one or more gas giant planets on eccentric orbits substantially shorter
than Jupiter’s period of 12 years. One percent or less have gas giants (“Hot
Jupiters”) roasting inside 0.1 AU on orbits of just a few days.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Yet here at
home, the Sun’s closest offspring, Mercury, maintains an average separation of about
36 million miles (0.39 AU) from our parent star, and masses less than quintuple
our Moon. The bulk of the Solar System’s mass is located quite far from the
Sun, outside a radius of 5 AU. In that cool region our two giants, Jupiter and
Saturn, glide along slow, circular orbits with few parallels in the extrasolar catalog.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Why are the
innermost regions of the Solar System empty of planets, when so many other
stars, near and far, support either tightly packed families of low-mass planets
or smaller collections of gas giants in the same orbital space? Two studies published
earlier this year offer clever explanations. For different reasons, both
propose that our system once had additional short-period planets that were
destroyed by an inner-system catastrophe shortly after they formed. Both also assume
that the formation of low-mass planets on hot orbits is robust even in the
presence of gas giants. Subsequent evolution can then produce a variety of
system architectures.</span><b><span style="font-family: "georgia" , "serif";"> </span></b></span></div>
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<br />
<span style="font-size: large;"><b><span style="font-family: "georgia" , "serif";">a grand attack</span></b><span style="font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">From Konstantin
Batygin and Greg Laughlin, via <i>Publications
of the National Academy of Science</i>, came a scenario based on recent models
of the evolution of our system’s four inner planets. In the traditional
picture, these objects condensed near their present locations out of a broad
field of planetesimals extending outward from 0.4 AU. Recent theoretical work,
however, demonstrates that the terrestrial planets must have accreted within a
narrow ring between 0.7 and 1 AU (Hansen 2009). During their formative years,
Venus and Earth scattered Mercury inward and Mars outward. Although the widely
endorsed <a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html">Grand
Tack</a> scenario explains the outer edge of the birth ring by evoking perturbations
by migrating Jupiter, Batygin & Laughlin noted that no rationale had yet
been presented for its inner edge. Hence their study.</span></span></div>
<div class="MsoNormal">
<br /></div>
<div class="MsoNormal" style="page-break-after: avoid;">
<span style="font-size: large;"><span style="font-family: "georgia" , "times new roman" , serif;"><b>Figure
2.</b>
The Grand Tack as Grand Attack</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
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<div class="separator" style="clear: both; text-align: center;">
<a href="http://3.bp.blogspot.com/-__eTKD5uc-M/VkGO40DMV7I/AAAAAAAAApU/T3nFuPaCBUU/s1600/Grand%2BAttack2.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="600" src="http://3.bp.blogspot.com/-__eTKD5uc-M/VkGO40DMV7I/AAAAAAAAApU/T3nFuPaCBUU/s640/Grand%2BAttack2.jpg" width="640" /></a></div>
<span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;">This
figure combines the scenarios of Walsh et al. 2011 and Batygin & Laughlin
2015. <b style="mso-bidi-font-weight: normal;">Panel a</b> shows the beginning of
the Grand Tack at a system age of ~1-2 million years. The gas nebula is still
present and accretion has progressed throughout the system, with several
low-mass planets already formed on inner orbits and two gas giants growing in
the “snow region” beyond 3 AU where water freezes. (Note: BL15 place Jupiter’s
starting point at 6 AU.) <b style="mso-bidi-font-weight: normal;">Panel b</b>
shows the maximum incursion of the two gas giants into the inner system, where
they scatter planets and planetesimals as they establish an orbital resonance. Some
of the original low-mass planets have already been engulfed by the Sun. In the
outer system, two or more additional low-mass planets are growing interior to a
massive planetesimal belt. <b style="mso-bidi-font-weight: normal;">Panel c</b>
shows the retreat of proto-Jupiter and proto-Saturn as their resonant orbits
carry them back into the snow region, just as the gas nebula begins to
dissipate. Meanwhile, the planetesimals scattered inward by proto-Jupiter have
already crowded all the original low-mass planets into the Sun, leaving a ring
of colliding planetesimals and debris near the present orbit of the Earth. <b style="mso-bidi-font-weight: normal;">Panel d</b> shows the final stable
configuration of the Solar System at an age of about 1 billion years. The collisional
assembly of the four terrestrial planets has scattered residual debris into a
“garbage orbit” beyond Mars, creating the ancestral Asteroid Belt, while the
outward migration of Saturn has pushed Uranus and Neptune onto wide orbits,
outside the scale of this diagram. </span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "georgia" , "serif";">----------</span></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">They begin by
proposing that proto-Jupiter originally assembled somewhere between 3 and 10 AU
<b>(Figure 2a)</b>. Once it reached a
threshold mass, tidal interactions with the primordial gas nebula caused its
orbit to shrink in a process known as <b>Type
II migration</b>. Simultaneously, proto-Saturn began accreting in
proto-Jupiter’s wake, following its big nursery-mate on an inbound voyage.
Jupiter’s passage into the inner Solar System scattered a substantial existing population
of planetesimals and protoplanets even farther inward, pitching most of them
into the Sun.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Eventually,
tagalong Saturn was embraced by Jupiter in a <b>3:2 mean motion resonance</b>, such that Jupiter completed three orbits
for every two of Saturn. Th<span style="font-family: "georgia" , "serif";">eir</span> planetary hook-up occurred at Jupiter’s maximum
incursion into the inner system <b>(Figure
2b)</b>, at a proposed radius of 1.5 AU, the present orbit of Mars. The
marriage of Jupiter and Saturn initiated a kind of honeymoon cruise that
carried them back into the outer system on ebbing tides of nebular gas (or in
more astronomical lingo, this pair of gap-opening planets underwent resonant
migration reversal).</span></span><span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"><span style="font-size: large;"><span style="font-family: "georgia" , "serif";"> </span></span></span><br />
<br />
<span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"><span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Meanwhile, in
the inner system, a nascent family of Super Earths and assorted rocky
planetesimals w<span style="font-family: "georgia" , "serif";">ere</span> being overwhelmed by hordes of planetesimals displaced inward
by Jupiter’s aborted invasion. Batygin & Laughlin suggest that the mass
scattered by the marauding giant exceeded 10 Earth masses (10 Mea) and might
have been as <span style="font-family: "georgia" , "serif";">much</span> as 20 Mea. As soon as Jupiter began migrating outward again,
these scattered objects spiraled rapidly into the Sun. Unfortunately for the
existing Super Earths, th<span style="font-family: "georgia" , "serif";">e</span> wave of annihilation swept them along with it. As
the authors argue, “Provided that the cumulative mass of the resonant
planetesimal population is not negligible compared with the mass of the
close-in planets, the planetesimals will gravitationally shepherd the close-in
planets into the Sun.” All that remained after their engulfment was about 2 Mea
of debris, left like a cosmic bathtub ring around a radius of 1 AU <b>(Figure 2c)</b>. These leftovers were the
substrate of the four terrestrial planets <b>(Figure
2d)</b>. </span></span></span></div>
<span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"><span style="font-size: large;"></span></span><br />
<div class="MsoNormal">
<span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"><br /></span></div>
<span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;">
<span style="font-size: large;">
</span></span><br />
<div class="MsoNormal">
<span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"><span style="font-size: large;"><span style="font-family: "georgia" , "serif";">The scenario of
Batygin & Laughlin blends smoothly into the Grand Tack and provides an
apparently self-consistent model of Solar System evolution. Its mechanism is
the forward scattering of a massive population of planetesimals by Jupiter’s
inward-then-outward migration, a process that swept the inner system clean
within a radius of about 0.7 AU. The authors emphasize that their results “imply
a strong anti-correlation between the existence of multiple close-in planets
and giant planets at orbital periods exceeding ~100 days within the same system,”
an anti-correlation supported by much – but hardly all – existing exoplanetary
data. (Current exceptions are <a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html">HD
219134</a>, HD 10180, Kepler-48, Kepler-68, <a href="http://backalleyastronomy.blogspot.com/2014/02/a-new-design-for-planetary-systems.html">Kepler-87</a>,
<a href="http://backalleyastronomy.blogspot.com/2014/02/a-new-design-for-planetary-systems.html">Kepler-90</a>,
and <a href="http://backalleyastronomy.blogspot.com/2015/08/the-blazing-wasp-47s.html">WASP-47</a>.)
</span></span></span></div>
<span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;">
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Batygin &
Laughlin also draw a second and grimmer inference from their work: </span></span><span style="font-family: "georgia" , "serif";"><span style="font-size: large;">“the majority of
Earth-mass planets are strongly enriched in volatile elements and are
uninhabitable.” Although they don’t pause to detail the rationale behind that
conclusion, I believe they mean something like this: Absent dynamic
instabilities caused by migrating or scattered gas giants, inner system
evolution is likely to produce several planets of a few Mea, all of which can
accrete hydrogen atmospheres as long as they assemble before the gas nebula
dissipates Although stellar irradiation can strip this hydrogen layer from
small planets on hot orbits, planets of Earth mass or more in the system
habitable zone will <span style="font-family: "georgia" , "serif";">potentially </span>retain their primordial envelopes indefinitely, causing an
intense greenhouse effect that <span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";">rules</span> ou<span style="font-family: "georgia" , "serif";">t</span></span> surface water.</span> </span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><b><span style="font-family: "georgia" , "serif";">consolidation &
crushing</span></b></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Just two months
after the publication of this widely praised work came a preprint by Kathryn
Volk and Brett Gladman addressing the same problem from a different angle. Their
study has since been published in the <i>Astrophysical
Journal Letters</i>. The title itself is a great hook: “Consolidating and crushing
exoplanets: Did it happen here?” Like Batygin & Laughlin (hereafter BL15), Volk
& Gladman argue that most stars originally form compact systems of low-mass
planets like <a href="http://backalleyastronomy.blogspot.com/2013/04/kepler-11-revisited.html">Kepler-11</a>
and <a href="http://backalleyastronomy.blogspot.com/2013/04/holy-grail-earthman.html">Kepler-62</a>,
but then lose those planets in later stages of system evolution. Our Solar
System is just one member of the dispossessed majority. Unlike BL15, Volk &
Gladman do not make their scenario contingent on the <a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html">Grand Tack</a>. Without
explicitly saying so, they offer an alternative model whose outcome would pre-empt
the <a href="http://backalleyastronomy.blogspot.com/2012/07/solar-system-archaeology-part-ii.html">Nice
model</a> as an explanation for the <a href="http://backalleyastronomy.blogspot.com/2014/08/a-billion-years-of-hell-on-earth.html">Late
Heavy Bombardment</a> and replace the Grant Tack as an account of the sculpting
of the inner Solar System. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Volk & Gladman introduce their study
by referring to the anomaly of our system’s mass cut-off inward of 0.4 AU. But
their explanation is unrelated to a putative incursion by Jupiter and Saturn.
Instead, they argue that all compact low-mass systems persist on the brink of
chaos, and that the sample observed today represents only the survivors of an
endogenous wave of orbital disaster that destroys most such systems. As Volk
& Gladman describe it, their scenario “fits our solar system into a
framework where dynamical instability mercilessly consolidates or degrades
close-in planets.” </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">They begin with the construct of the <b>minimum mass Solar nebula</b>, which is
based on the present-day masses of the eight surviving planets. As long as we
extend that construct beyond its traditional but unmotivated inner limit, “several
Earth masses of material [would be] available” between 0.05 AU and 0.7 AU, the
present semimajor axis of Venus. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Volk &
Gladman propose that, during primordial times, at least three rocky planets with
a collective mass of 4 Mea coalesced inside Venus’ orbit. These worlds
coexisted with Venus, Earth, and Mars for many millions of orbits, making the
ancient Solar System an analog of those crowded Kepler systems. Then, at some
point between system ages of 50 million and 500 million years or more, the
innermost planets experienced a dynamical upset. The result was a cascade of
orbit crossings and high-speed, shattering collisions that disintegrated 90% of
their collective mass. Much of that mass was ground into fine dust that simply
blew away, leaving battered Mercury as “the last remaining relic” of our inner
system apocalypse. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">To test their
hypothesis of a primordial inner system of packed planets, Volk & Gladman
numerically simulated analogs of 13 Kepler systems with 5 or more low-mass planets,
eventually generating about 600 synthetic systems that they integrated for at
least 100 million years. In half of the simulated systems, two planets eventually
collided. On this evidence Volk & Gladman concluded that 90% to 95% of all
tightly packed low-mass systems experience similar dynamic instabilities. The
result is either “consolidation,” where smaller planets collide and coalesce to
form larger planets, or “destruction,” where shattering destroys colliding
planets. In their view, the low-mass, high-multiplicity systems observed by
Kepler represent the 5% to 10% that avoided such an instability, whereas our
Solar System is one of the more common cases in which destruction outpaced
consolidation. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">In their preferred
model, four Earth-mass planets formed inside 0.5 AU. The model appears to be
agnostic regarding the origin of Venus, Earth, and Mars, since it doesn’t
address their accretion. Volk & Gladman conducted test integrations indicating
that those three planets could coexist for at least half a billion years with a
hypothetical packed inner system, and would remain unaffected by the furious
collisions that eventually consumed their inner companions. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Volk &
Gladman also suggest that the destruction of the inner planets could have caused
the storm of asteroidal and cometary collisions variously known as the Lunar
Cataclysm and the Late Heavy Bombardment. This hypothesis appears to contradict
the Nice Model, which proposes that Neptune’s migration into the Solar System’s
outer debris disk (now known as the Kuiper Belt) scattered comets inward among
the planets and their moons, leaving the cratered landscapes we see today. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><b><span style="font-family: "georgia" , "serif";">more different than not</span></b></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">The models of
BL15 and Volk & Gladman 2015 (hereafter VG15) share important similarities.
Both assume that the infant Solar System contained a compact family of
short-period planets, and both argue that this ensemble was annihilated, leaving
a void inside Mercury’s present orbit. But they differ on significant points, including
the amount of mass originally available for planet formation inside 0.5 AU, the
mechanism of the dynamical instability, and the timing of the resulting
cataclysm, which has important implications for the terrestrial planets we know
today. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Regarding mass,
VG15 suggest about 4 Mea, while BL15 go an order of magnitude higher. BL15’s
assumptions depend critically on recent models of <i>in situ</i> formation, which are discussed below, whereas VG15 use the
traditional model of the minimum mass Solar nebula. They simply extend the
nebula’s inner edge, as in many other recent studies.</span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Regarding
mechanism, VG15 propose an internal trigger, while BL15 appeal to gravitational
perturbations by wandering Jupiter. For VG15, the original planets simply
self-destruct in an epoch of shattering collisions. For BL15, they are
shepherded into the Sun by the swarm of planetesimals kicked into motion by
Jupiter’s tacking maneuver.</span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">As for timing, BL15’s
scenario is constrained to play out within the first 10 million years of system
evolution, possibly within the first 2 million. This hard limit derives from
the observed lifetimes of protoplanetary nebulae around Sun-like stars, which
have a median of 2 to 3 million years (Williams & Cieza 2011). The proposed
tacking of Jupiter and Saturn from narrower to wider orbits could be sustained only
by their interaction with the nebular gas, whose dissipation closed the window
on such activity. None of this contradicts the established Grand Tack/Nice
model, which has been developed over more than a decade of theoretical testing.
Thereafter, the formation of the terrestrial planets proceeds along accepted
lines, by giant impacts among protoplanets. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">VG15 suggest a
more leisurely schedule for their catastrophe. After the initial
clutch of hot planets formed, they survived for hundreds of millions years
before an instability developed. In fact, VL15 <span style="font-family: "georgia" , "serif";">propose</span> that the catastrophe
coincided with the Lunar Cataclysm. Considerable evidence dates this event (or
series of events) to an approximate system age of 800 million years. In other
words, the Lunar Cataclysm is the same system-wide upheaval that the Nice model
was developed to explain. Thus, while BL15 explicitly fold their scenario into
the Nice/Grand Tack framework, VG15 appear to ignore or even oppose it. Their
results, therefore, seem to conflict with what we know about the assembly of
the existing terrestrial planets. </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><b><span style="font-family: "georgia" , "serif";">questions for volk & gladman</span></b></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">I find it hard
to accept that the massive catastrophe described by VG15 would leave Venus,
Earth, and Mars largely untouched. Maybe my instincts have been shaped more by
science fiction than by astrophysics, but wouldn’t an instability on the scale they
envision propagate throughout the system, destroying the three cool terrestrials
along with the hotter hypothetical planets? </span></span></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<br /></div>
<span style="font-size: large;">
</span><div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">In addition,
VG15 appear to neglect the evidence for the formation history of <span style="font-family: "georgia" , "serif";">Earth and Mars</span></span></span><span style="font-size: large;"><span style="font-family: "georgia" , "serif";">. Radiogenic isotopes indicate that Mars formed about 10 million years
after the Sun’s ignition, while the Earth-Moon system required 30 to 50 million
years. Since VG15 suggest that all three terrestrials had already formed when the innermost
planets disintegrated, something other than the birth ring presented by Hansen
(2009) is needed to explain their assembly. VG15 <span style="font-family: "georgia" , "serif";">never</span> mention the well-known mass
deficit in the space between present-day Earth and Jupiter – i.e., the “small
Mars” problem and the presence of an attenuated debris field instead of a Super
Earth inside Jupiter’s orbit. To my knowledge, the Grand Tack is the only
plausible explanation offered to date for th<span style="font-family: "georgia" , "serif";">e</span>se architectural features (Walsh
et al. 2011, Pierens & Raymond 2011). Since VG15 implicitly reject both the
Nice model and the Grand Tack, accepting their scenario means we would need to
find a new explanation for the architecture of the Solar System between 2 and 5
AU. </span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";">I</span>t looks
like VG15 raise more questions than they answer. Among them is the very plausibility
of consolidation and crushing<span style="font-family: "georgia" , "serif";"> as <span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";">a<span style="font-family: "georgia" , "serif";"> mechanism for annihil<span style="font-family: "georgia" , "serif";">ation</span></span></span></span></span>. A similar hypothesis
was explored by Anders Johansen and colleagues in 2012 to explain the observed
“Kepler dichotomy” between systems with a single transiting planet and those
with several. Like VG15, Johansen and colleagues (hereafter J12) tested dynamical
instability as an explanation for planetary depletion, and they did so by integrating
a set of synthetic planetary systems based on real Kepler systems. Despite
these similarities, the results of J13 directly contradict those of VG15. Most salient
is their finding that the synthetic systems, containing planets on circular
orbits with small mutual inclinations, were extremely stable over billions of years.
Instances of planetary collision and consolidation were too rare to explain the
observed system architectures. To assess this question in more detail, J12
integrated successively mass-boosted versions of their sample until collisions
became significant. They found that smash-ups tended to happen only when the
synthetic planets were boosted into the gas giant range, and even then,
instabilities required more than a billion years to develop. On this and
related evidence, J12 concluded that dynamical instabilities were not a
significant factor in the evolution of compact Kepler systems. [Note: another
recent simulation study by Hansen & Murray (2014) also found that, in
compact systems of planets less massive than 10 Mea, “significant dynamical
instability may not occur because, in most cases, orbit crossing cannot be
achieved.”]</span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">As a
fan instead of an astronomer, I can’t authoritatively measure VG15 and J12 against each other.
I can only note some of the ways in which they differ. One is their choice of
simulated system architectures. J12 restricted their selection to systems of
exactly three planets orbiting inside 0.5 AU, observing that the Kepler sample
of higher-order multiples available in 2012 was too small to provide a
statistically significant template for constructing synthetic systems. VG15 limited
their simulations to systems with at least five planets – a surprising choice,
since they also argued that a maximum of four hot planets formed in the
primordial Solar System. In addition, one of their 13 systems (Kepler-90)
appears to include a gas giant. Still worse, all 13 systems support planets substantially
more massive than Earth, whereas VG15 propose only planets of approximately Earth-mass
for their hypothetical Solar System ensemble. I have to wonder whether VG15 chose
the most appropriate sample to investigate their study question. </span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Another
difference between the two is that J12 addressed only ejections and collisional
consolidations, without considering collisions that cause shattering. Yet the
latter type was the explicit focus of VG15. <span style="font-family: "georgia" , "serif";">Of course, wi<span style="font-family: "georgia" , "serif";">thout collisions, <span style="font-family: "georgia" , "serif";">nothing shatters, so J12 cannot be fault<span style="font-family: "georgia" , "serif";">ed for omitting any mention of this outcome. </span></span></span></span>In this regard it seems significant
that VG15 kept their <span style="font-family: "georgia" , "serif";">investigat<span style="font-family: "georgia" , "serif";">ion</span></span> of shattering collisions separate from their simulation study. They <span style="font-family: "georgia" , "serif";">test<span style="font-family: "georgia" , "serif";">ed</span> shattering </span>only in analytic terms, and <span style="font-family: "georgia" , "serif";">noted that the<span style="font-family: "georgia" , "serif";">se results were independent of </span></span><span style="font-family: "georgia" , "serif";">the findings</span> from the simulations. Yet without a high frequency of shattering, those findings would have no bearing on the emptiness of the inner Solar System. </span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Remarkably, VG15
never comment on the major findings of J12, even though they cite that study to
support a statement about orbital inclinations. <span style="font-family: "georgia" , "serif";">Why</span> did<span style="font-family: "georgia" , "serif";">n'</span>t <span style="font-family: "georgia" , "serif";">they discuss</span> its<span style="font-family: "georgia" , "serif";"> </span>conclusions<span style="font-family: "georgia" , "serif";"><b>?</b></span></span></span><span style="font-size: large;"><b><span style="font-family: "georgia" , "serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="font-family: "georgia" , "serif";">questions for batygin & laughlin</span></b><span style="font-family: "georgia" , "serif";"> </span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">So I have to
favor BL15 over VG15. Nevertheless, despite my great respect for the work of
Batygin and Laughlin, both separately and in collaboration, I find several nits
to pick. These fall under three headings (with some overlap): their handling of
Jupiter’s original birthplace, their choice of Kepler-11 as a model for the
hypothetical inner Solar System, and their adherence to the theory of <i>in situ</i> formation.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">For the Grand
Tack, BL15 state that the exact starting point of Jupiter’s inward migration is
unimportant. I believe that’s debatable, and I note that they actually model it
as 6 AU. That semimajor axis is not only wider than Jupiter’s present orbit, but
also wider than previous proposals for the Grand Tack, which placed the
starting point between 2 and 5 AU (Walsh et al. 2011, Pierens & Raymond
2011). I can’t argue that 6 AU is implausible (in fact, it <span style="font-family: "georgia" , "serif";">would make</span> sense <span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";">as long as</span> the snow region commenced at a wider semimajor axis than it does today, as implied by theories of viscous<span style="font-family: "georgia" , "serif";"> heating)</span></span>, but I have to ask why BL15 chose
that location in particular. </span></span>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Since the
mechanism underlying the inner system cataclysm in BL15’s model is a horde of
scattered planetesimals, the horde</span></span><span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"><span style="font-size: large;"><span style="font-family: "georgia" , "serif";">’</span></span></span>s aggregate mass is a critical factor.
The wider Jupiter’s original orbit, the larger the available mass. BL15 note
that 20 Mea in planetesimals would be available inside 6 AU, while stating vaguely
that the mass in planetesimals must be “not negligible” in comparison with the
aggregate mass of the inner planets. But what does that mean? At least 10%, more
than 50%, even 100% of the total mass? It would be helpful to know both the
approximate fraction as well as the approximate mass against which it was
estimated. For example, VG15 set a low bar by positing an aggregate mass of 4
Mea for their hypothetical planets, whereas 20 Mea would be another story
entirely. </span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Another issue
raised by Jupiter’s birthplace is the nature of the region through which
proto-Jupiter and Saturn migrated on their way to sunnier orbits. If the early
Solar System had several planets inside 0.5 AU and several more outside 6 AU,
what was going on in the vast region between those endpoints? Core accretion
theory says that coagulation of solids progressed throughout the Solar nebula.
I would expect more planets to have formed in the space between, and that one
or more of them would have complicated Jupiter’s inward passage. This doesn’t
seem to be a fatal flaw in the theory, but it’s worth looking into.</span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Regarding the primordial family of
planets doomed by Jupiter, BL15 mention the likelihood that they were
“multi-Earth-mass” objects, as in so many Kepler systems. Three paragraphs
later, they offer an illustration of the potential mass involved by referring
to their simulation of the dynamical evolution of the Kepler-11 planets. In so
doing they express appropriate caution: “</span><span style="font-family: "georgia" , "serif";">We are not suggesting that a primordial
population of the Solar System</span><span style="font-family: "georgia" , "serif";">’</span><span style="font-family: "georgia" , "serif";">s close-in planets would have necessarily
borne any similarity to the Kepler-11 system.</span><span style="font-family: "georgia" , "serif";">” Nevertheless, their previous mention of “multi-Earth-mass”
planets implies a considerable aggregate mass. For the Kepler-11 system in
particular, their opening paragraph cites Lissauer et al. 2011 to define a
total in excess of 40 Mea. (That total was later revised downward to about 30
Mea by Lissauer et al. 2013.) In context, then, their “not negligible” mass in
planetesimals would be 50% of the aggregated planetary masses (20 Mea of
planetesimals/40 Mea of planets). </span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">But how could our Solar System get 30 to
40 Mea worth of planets inside 0.5 AU? </span></span></div>
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<span style="font-size: large;"><b><span style="font-family: "georgia" , "serif";">the problem with <i>in situ</i> formation</span></b></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">While conceding
that the theory is controversial, BL15 assume that their hypothetical family of
Super Earths arose by <i>in situ</i>
formation. In this model, planets of several Earth masses readily form in place
on short-period orbits, tracing the primordial distribution of solids in the
protoplanetary disk. Apart from a few earlier inquiries of limited scope (Raymond
et al. 2008, Montgomery & Laughlin 2009), this theory emerged in 2012-2013 as
a fully-fledged paradigm with various instantiations, some strict and some less
so. </span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Chiang & Laughlin (2013) presented
the “strict” version (their word). They argued that “disk-driven migration
seems too poorly understood to connect meaningfully with observations,” and deprecated
models that require this mechanism as both “premature” and “naïve.” In their
place they proposed a <b>minimum mass
extrasolar nebula (MMEN)</b>. They obtained this construct by plotting the
masses and orbits of all small Kepler planets then known, an exercise that highlights
the pile-up of mass inside 0.25 AU. They explained this pile-up by arguing that
the MMEN supports a much larger concentration of solids at small semimajor axes
than previous models. Such a large mass, they argued, readily congeals into
several Super Earths, as they illustrated with reference to the Kepler-11
system. </span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Brad Hansen & Norm Murray (2012)
presented a “less strict” variation on the model (according to Chiang &
Laughlin). They proposed that 30-100 Mea of solids could migrate from the outer
regions of the protoplanetary disk to clump within 1 AU, where the
planetesimals would stop migrating and rapidly condense into an ensemble of Super
Earths (which I would call <a href="http://backalleyastronomy.blogspot.com/2012/04/between-earth-and-uranus-part-ii.html">gas dwarfs</a>). In a subsequent publication (Hansen &
Murray 2013), the authors reduced the aggregate mass required to 20 Mea, while
arguing that migration might not be necessary after all to account for such a
concentration of solids. (Presumably th<span style="font-family: "georgia" , "serif";">eir</span> revision upped the strictness
factor.)</span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Critiques of <i>in situ</i> theory came pretty quickly. The principal objection was
that MMEN and similar models require protoplanetary disk masses much larger
than those actually observed (Raymond et al. 2014) – so large that they would
be gravitationally unstable (Schlichting 2014). Another objection was that MMEN implies a universal disk structure that is incompatible with the diversity
of known multiplanet system architectures (Raymond & Cossou 2014). Still
another was that “strict” <i>in situ</i>
models rule out disk-driven migration. Yet as Izidoro et al. (2014) argued, “</span><span style="font-family: "georgia" , "serif";">Assuming
that no migration occurs essentially ignores 30 years of disk-planet studies
that show the inevitability of orbital migration.</span><span style="font-family: "georgia" , "serif";">” Indeed, Schlaufman (2014) found that without migration,
the observed Kepler system architectures would be impossible. Additional
research (Inamdar & Schlichting 2015) argued that planets forming on
short-period orbits, even by condensation of solids originating in the outer
system, could not accrete the substantial hydrogen envelopes observed in the
Kepler sample.</span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">According to the Astrophysical Data
System, Greg Laughlin hasn</span></span><span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"><span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"><span style="font-size: large;"><span style="font-family: "georgia" , "serif";">’</span></span></span></span></span></span>t published anything theoretical on <i>in situ</i> formation since 2013. However, his
colleague Eugene Chiang has continued to expand on this approach. Chiang’s
recent work makes notable concessions to migration theory. He and his
collaborators acknowledge that strict <i>in
situ</i> approaches make the formation of gas giants at least as likely as
Super Earths, possibly requiring some fine-tuning in the time scale for nebular dissipation (Lee et al. 2014). A brand-new study (Lee & Chiang
2015) even concedes that <i>in situ</i>
models are insufficient to explain the diversity of the existing population of
low-mass planets. <span style="font-family: "georgia" , "serif";">D</span>ividing the sample into two groups – “Super-Earths,</span></span><span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="font-family: "arial" , "sans-serif"; font-size: 10.0pt;"><span style="font-size: large;"><span style="font-family: "georgia" , "serif";">”</span></span></span> which they define as planets between 1 and 4 Rea, and “Super-Puffs,” which
have radii between 4 and 10 Rea but remain in the mass range of gas dwarfs – they
offer <span style="font-family: "georgia" , "serif";">this <span style="font-family: "georgia" , "serif";">remarkable</span></span> conclusion: “Unlike Super-Earths, which can form <i>in situ</i>, Super-Puffs probably migrated
in to their current orbits.”</span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Apparently the strict <i>in situ</i> paradigm is stalled for now,
leaving us with many fluid variations on the theme of disk-driven
migration. </span></span></div>
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<span style="font-size: large;"><b><span style="font-family: "georgia" , "serif";">still
looking pretty shiny </span></b></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">From my back alley
perspective, the essential feature of Batygin & Laughlin’s scenario is its
appeal to Jupiter’s migration as the source of the catastrophe. The giant’s inward
passage was the mechanism that scattered forward a huge mass in planetesimals,
which then efficiently herded all the <span style="font-family: "georgia" , "serif";">baby </span>planets in the inner nebula into the
Sun. Although Batygin & Laughlin assume the existence of a system of
compact Super Earths (which I would call <a href="http://backalleyastronomy.blogspot.com/2012/04/between-earth-and-uranus-part-ii.html">gas
dwarfs</a>) inside 0.7 AU, their scenario doesn’t seem to require such a
configuration, especially not one with an aggregate mass of 20 to 40 Mea. Any
collection of protoplanets would do, including the modest system of terrestrial <span style="font-family: "georgia" , "serif";">objects</span> proposed by Volk & Gladman. </span></span></div>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">By now the limitations
of the MMEN and similar <i>in situ</i>
models have been discussed by numerous peer-reviewed studies, while a major
proponent of the early model (Chiang) has since revised his approach. Simultaneously,
increasingly sophisticated theories based on migration continue to proliferate.
My hunch is that the latest migratory models are largely consistent with
Batygin & Laughlin’s scenario, especially since it depends so critically on
Type II migration. Kepler-11 aside, their comprehensive approach provides the
best explanation yet for the strange things that must have happened before our
world could be born. </span></span></div>
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<span style="font-size: large;"><i><span style="color: black; font-family: "georgia" , "serif";"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html">Click
here for a permanent index by topic of blog posts<span style="color: blue;"><br />
</span>on Back Alley Astronomy</a></span></i><span style="font-family: "georgia" , "serif";">
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt;">REFERENCES</span></b></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt;">Batygin K, Laughlin G.</span></b><span style="color: black; font-size: 10.0pt;"> (2015) Jupiter’s decisive role in the
inner Solar System’s early evolution. <i style="mso-bidi-font-style: normal;">Proceedings
of the National Academy of Sciences</i> 112, 4214-4217. Abstract: </span><span style="font-size: 10.0pt;"><a href="http://adsabs.harvard.edu/abs/2015PNAS..112.4214B">2015PNAS..112.4214B</a>
</span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Chiang E, Laughlin G.</span></b><span style="font-size: 10.0pt;"> (2013) The minimum-mass extrasolar nebula: <i style="mso-bidi-font-style: normal;">in situ</i> formation of close-in
Super-Earths. <i style="mso-bidi-font-style: normal;">Monthly Notices of the
Royal Astronomical Society</i> 431, 3444-3455. Abstract: <a href="http://adsabs.harvard.edu/abs/2013MNRAS.431.3444C">2013MNRAS.431.3444C</a></span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Hansen B.</span></b><span style="font-size: 10.0pt;">
(2009) Formation of the terrestrial planets from a narrow annulus. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 703, 1131-1140.
Abstract: <a href="http://adsabs.harvard.edu/abs/2009ApJ...703.1131H">2009ApJ...703.1131H</a>
</span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Hansen B, Murray N.</span></b><span style="font-size: 10.0pt;"> (2012) Migration then assembly: Formation of Neptune-mass planets
inside 1 AU. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i>
751, 158. Abstract: <a href="http://adsabs.harvard.edu/abs/2012ApJ...751..158H">2012ApJ...751..158H</a></span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Hansen B, Murray N.</span></b><span style="font-size: 10.0pt;"> (2013) Testing <i style="mso-bidi-font-style: normal;">in situ</i>
assembly with the Kepler planet candidate sample. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 775, 53. Abstract: <a href="http://adsabs.harvard.edu/abs/2013ApJ...775...53H">2013ApJ...775...53H</a></span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Hansen B, Murray N.</span></b><span style="font-size: 10.0pt;"> (2015) Secular effects of tidal damping in compact planetary systems. <i style="mso-bidi-font-style: normal;">Monthly Notices of the Royal Astronomical
Society</i> 448, 1044-1059. Abstract: <a href="http://adsabs.harvard.edu/abs/2015MNRAS.448.1044H">2015MNRAS.448.1044H</a></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Inamdar NK,
Schlichting HE.</span></b><span style="font-size: 10.0pt;"> (2015) The formation
of Super-Earths and Mini-Neptunes with giant impacts. <i style="mso-bidi-font-style: normal;">Monthly Notices of the Royal Astronomical Society</i> 448, 1751-1760.</span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Izidoro A</span></b><span style="font-size: 10.0pt;">, Morbidelli A, Raymond SN. (2014) Terrestrial planet
formation in the presence of migrating Super-Earths. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i> 794, 11. </span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Johansen A</span></b><span style="font-size: 10.0pt;">,
Davies MB, Church RP, Holmelin V. (2012) Can planetary instability explain the
Kepler dichotomy? <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i>
758, 39. Abstract: <a href="http://adsabs.harvard.edu/abs/2012ApJ...758...39J">2012ApJ...758...39J</a></span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="font-size: 10.0pt;">Lee EJ, </span></b><span style="font-size: 10.0pt;">Chiang
E, Ormel CW. (2014 ) Make Super-Earths, not Jupiters: Accreting nebular gas
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</span>Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-90542946586976318122015-10-31T17:24:00.003-07:002015-11-13T18:08:46.119-08:00The Ghost in the Window<!--[if gte mso 9]><xml>
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<a href="http://1.bp.blogspot.com/-CoTTB6Vvav0/VjVbmTgx_rI/AAAAAAAAAok/9hpNrbh4H4Y/s1600/The%2Bghost%2Bin%2Bthe%2Bwindow.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="384" src="http://1.bp.blogspot.com/-CoTTB6Vvav0/VjVbmTgx_rI/AAAAAAAAAok/9hpNrbh4H4Y/s640/The%2Bghost%2Bin%2Bthe%2Bwindow.jpg" width="640" /></a></div>
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<span style="font-size: large;"><span style="color: black; font-family: "georgia" , "serif";">Vinesh</span><span style="font-family: "georgia" , "serif";"> Rajpaul and colleagues have forcefully
challenged the reality of <a href="http://backalleyastronomy.blogspot.com/2012/11/red-hot-planet.html">Alpha
Centauri Bb</a>, the Earth-mass Hellworld proposed as a companion to one of the
two nearest Sun-like stars. This extrasolar candidate was <a href="http://backalleyastronomy.blogspot.com/2012/10/far-centaurus.html">announced
three years ago</a> by a team led by Xavier Dumusque. Even using the
exquisitely sensitive HARPS spectrograph, Dumusque and colleagues noted that
detecting such a lightweight object stretched the limits of the radial velocity
method and required complex data analytic approaches.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">The announcement
of this planet in a familiar, nearby system generated great excitement in
the exoplanet community, along with an initially subdued but slowly growing
skepticism. Artie Hatzes first advised caution (2012) and then <a href="http://backalleyastronomy.blogspot.com/2013/06/more-backyard-controversies.html">expressed
doubt</a> (2013) about the planet’s existence. A team including Michael Endl (2015)
and Christoph Bergmann (2015) began an observational program focused on both
stars in the Alpha Centauri binary to return a definitive picture supported by
independent data. So far, however, they have not reported any conclusions.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">In their October
takedown, Rajpaul and colleagues rely on exhaustive statistical analyses
instead of new observations. They show that the existing HARPS data previously interpreted
as a terrestrial planet on a three-day orbit are just an artifact of the modeling
approach used by Dumusque and colleagues. According to their abstract:<span style="font-family: "georgia" , "times new roman" , serif;"> </span></span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "times new roman" , serif;">“</span>The </span><span style="font-family: "georgia" , "serif";">3</span><span style="font-family: "georgia" , "serif";"><span style="font-family: "georgia" , "serif";">.</span></span><span style="font-family: "georgia" , "serif";">24 </span><span style="font-family: "georgia" , "serif";">day signal observed in the Alpha Cen B
data almost certainly arises from the window function (time sampling) of the
original data. We show that when stellar activity signals are removed from the
RV variations, other significant peaks in the power spectrum of the window
function are coincidentally suppressed, leaving behind a spurious yet
apparently-significant 'ghost' of a signal that was present in the window
function’s power spectrum </span><i><span style="font-family: "georgia" , "serif";">ab
initio</span></i><span style="font-family: "georgia" , "serif";">.</span><span style="font-family: "georgia" , "serif";">”</span></span><!--[if gte mso 9]><xml>
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<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">In related news,
Rodrigo Diaz & colleagues (herafter D15) report that, with more than a
decade of HARPS radial velocity measurements in hand, they can confirm only
four of the six planets proposed for HD 40307. This system includes a single Sun-like
star located a bit farther away than Alpha Centauri, at a distance of 12.8 parsecs
(42 light years). Notably, one of the two planets phantomized by D15’s summative
analysis (HD 40307 g) was previously <a href="http://backalleyastronomy.blogspot.com/2013/01/year-of-signal.html">imagined</a>
as a “<a href="http://backalleyastronomy.blogspot.com/2012/04/between-earth-and-uranus-part-ii.html">potentially
habitable Super Earth</a>” – in other words, an object with a minimum mass
below 10 Earth masses (10 Mea) orbiting in the system habitable zone (Brasser
et al. 2014). By now, however, it seems pretty clear that the maximum planet mass
compatible with surface water in the habitable zone is closer to 5 Mea than to 10
Mea. Thus the elision of candidate g, whose minimum mass was supposed to be about 7 Mea,</span></span><span style="font-size: large;"><span style="font-family: "georgia" , "serif";"><span style="font-size: large;"><span style="font-family: "georgia" , "serif";"> provokes</span></span> no exobiological regrets.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Despite these two
subtractions, HD 40307 persists among the classic examples of <a href="http://backalleyastronomy.blogspot.com/2014/06/dwarfs-versus-giants-round-two.html">compact,
low-mass multiplanet systems</a> within 40 parsecs. According to D15, the four
robustly detected planets follow circular orbits within 0.25 astronomical units
(AU) of the star, and their minimum masses range from 3.6 to 8.7 Mea. A
dynamical integration of system elements over half a million years indicated
that the true masses of these planets could be at least twice their minimum
masses without compromising long-term stability. Similar architectures are quite
common in the large sample of Kepler multiplanet systems.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "georgia" , "serif";">Meanwhile, the
Sun’s back yard is getting crowded with ghosts!</span><i><span style="color: black; font-family: "georgia" , "serif";"> </span></i></span><br />
<br />
<div style="text-align: center;">
<span style="font-size: large;"><i><span style="color: black; font-family: "georgia" , "serif";"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html">Click
here for a permanent index by topic of blog posts<span style="color: blue;"><br />
</span>on Back Alley Astronomy</a></span></i></span><span style="font-family: "georgia" , "serif";">
</span></div>
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<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">REFERENCES</span></b></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">Bergmann C,</span></b><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">.Endl M, Hearnshaw JB, Wittenmyer RA, Wright
DJ. (2015) Searching for Earth-mass planets around Alpha Centauri: Precise
radial velocities from contaminated spectra. <i style="mso-bidi-font-style: normal;">International Journal of Astrobiology</i> 14, 173-176. Abstract: <a href="http://adsabs.harvard.edu/abs/2015IJAsB..14..173B"><span style="color: black; mso-themecolor: text1;">2015IJAsB..14..173B</span></a></span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">Brasser R</span></b><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">, Ida S, Kokubo E.
(2014) A dynamical study on the habitability of terrestrial exoplanets II: The
super Earth HD 40307 g. <i style="mso-bidi-font-style: normal;">Monthly Notices
of the Royal Astronomical Society</i> 440, 3685-3700. Abstract: <a href="http://adsabs.harvard.edu/abs/2014MNRAS.440.3685B"><span style="color: black; mso-themecolor: text1;">2014MNRAS.440.3685B</span></a></span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">Diaz RF</span></b><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">, Ségransan D, Udry
S, Lovis C, Pepe F, Dumusque X, and 19 others. (2015) The HARPS search for
southern extra-solar planets XXXVII. Bayesian re-analysis of three systems: New
super-Earths, unconfirmed signals, and magnetic cycles. <i style="mso-bidi-font-style: normal;">Astronomy & Astrophysics</i>, in press. Abstract: <a href="http://adsabs.harvard.edu/abs/2015arXiv151006446D"><span style="color: black; mso-themecolor: text1;">2015arXiv151006446D</span></a></span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">Dumusque X</span></b><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">, Pepe F, Lovis C, Ségransan D, Sahlmann J,
Benz W, Bouchy F, Mayor M, Queloz D, Santos N, Udry S. (2012) An Earth-mass
planet orbiting Alpha Centauri B. <i style="mso-bidi-font-style: normal;">Nature</i>
491, 207-211. Abstract: <a href="http://adsabs.harvard.edu/abs/2012Natur.491..207D"><span style="color: black; mso-themecolor: text1;">http://adsabs.harvard.edu/abs/2012Natur.491..207D</span></a>
</span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">Endl M</span></b><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">, Bergmann C,
Hearnshaw J, Barnes SI, Wittenmyer RA, Ramm D, Kilmartin P, Gunn F, Brogt E.
(2015) The Mt. John University Observatory search for Earth-mass planets in the
habitable zone of Alpha Centauri. <i style="mso-bidi-font-style: normal;">International
Journal of Astrobiology</i> 14, 305-312. Abstract: <a href="http://adsabs.harvard.edu/abs/2015IJAsB..14..305E"><span style="color: black; mso-themecolor: text1;">2015IJAsB..14..305E</span></a></span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">Hatzes A.</span></b><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;"> (2012) Meet Our
Closest Neighbour. <i style="mso-bidi-font-style: normal;">Nature</i> 491,
200-201.</span></div>
<div class="MsoNormal" style="mso-layout-grid-align: none; text-autospace: none;">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">Hatzes AP</span></b><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">.</span><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">
(2013)</span> Radial velocity detection of Earth-mass
planets in the presence of activity noise: The case of Alpha Centauri Bb. Abstract: <a href="http://adsabs.harvard.edu/abs/2013ApJ...770..133H"><span style="color: black; mso-themecolor: text1;">2013ApJ...770..133H</span></a></span></div>
<div class="MsoNormal">
<b style="mso-bidi-font-weight: normal;"><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;">Rajpaul V,</span></b><span style="color: black; font-size: 10.0pt; mso-themecolor: text1;"> Aigrain S, Roberts S.
(2015) Ghost in the time series: No planet for Alpha Cen B. <i style="mso-bidi-font-style: normal;">Monthly Notices of the Royal Astronomical
Society</i>, in press. Abstract: <a href="http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2015arXiv151005598R&db_key=PRE&link_type=ABSTRACT&high=5585b8697d26741"><span style="color: black; mso-themecolor: text1;">2015arXiv151005598R</span></a></span></div>
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<![endif]-->Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com0tag:blogger.com,1999:blog-7485863064099307059.post-32240655897162154972015-10-04T18:08:00.002-07:002015-10-17T17:33:29.367-07:00A Different Picture of HD 219134<!--[if gte mso 9]><xml>
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<a href="http://2.bp.blogspot.com/-zc_MrzsOpHY/Vhig7MVMztI/AAAAAAAAAnk/579keBSNrNQ/s1600/HD%2B219134%2Bsystem%2Barch%2B-%2BVogt.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="584" src="http://2.bp.blogspot.com/-zc_MrzsOpHY/Vhig7MVMztI/AAAAAAAAAnk/579keBSNrNQ/s640/HD%2B219134%2Bsystem%2Barch%2B-%2BVogt.gif" width="640" /></a></div>
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</div>
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<b style="mso-bidi-font-weight: normal;"><span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;">Figure 1.</span></b><span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;"> Schematic view of
the six-planet architecture proposed by Vogt and colleagues for HD 219134, a
small Sun-like star of spectral class K3 at a distance of only 6.53 parsecs (21
light years). Compare this model with the one proposed by <a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html">Motalebi
and colleagues</a>.</span></div>
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<span style="font-family: "Georgia","serif";">-------------</span></div>
<div class="MsoNormal">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">A few months ago
we read about <a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html">the
discovery of HD 219134</a>, a remarkable new multiplanet system in the Sun’s
back yard (Motalebi et al. 2015; hereafter M15). Its architecture is characterized by a long-period gas giant encircling several
low-mass planets. All these objects were detected through radial velocity
variations, and the innermost might also be observable in transit. In a
postscript to the discovery paper, the authors noted that another group led by
Stephen Vogt had also detected planetary signals from this star.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Vogt</span></span><span style="font-size: large;"><span style="font-family: "Georgia","serif";"><span style="font-family: Georgia,"Times New Roman",serif;"><span style="font-size: large;">’</span></span>s team just published a preprint detailing their findings (hereafter V15). <a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html" target="_blank">As I anticipated</a>, some of the new material complements and some of it conflicts
with the picture presented by M15. We still see a mixed-mass architecture with
a pronounced division between inner and outer systems (Figure 1). As in M15, the
inner system contains only low-mass planets, all orbiting starward of the
habitable zone (which for this star extends from 0.5 to 0.9 AU; see Kastings et
al. 2014). The outer system contains a single gas giant whose mass places it in
the “sub-Jovian desert” (Colon et al. 2015) and whose orbit places it outside
the habitable zone. Both teams agree on the basic characteristics of planets b
and c, and both also agree that some kind of smallish planet is present in a
period just shy of 47 days.</span><b><span style="font-family: "Georgia","serif";"> </span></b></span><br />
<br />
<span style="font-size: large;"><b><span style="font-family: "Georgia","serif";">Table 1. </span></b><span style="font-family: "Georgia","serif";">Two views of the planetary system around
HD 219134</span></span></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://1.bp.blogspot.com/-5Krdef5Smck/VhHNjMtpLlI/AAAAAAAAAmc/-KyEOY4NHUk/s1600/HD%2B219134%2B-%2BTwo%2Bdifferent%2Bpictures.gif" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="288" src="http://1.bp.blogspot.com/-5Krdef5Smck/VhHNjMtpLlI/AAAAAAAAAmc/-KyEOY4NHUk/s640/HD%2B219134%2B-%2BTwo%2Bdifferent%2Bpictures.gif" width="640" /></a></div>
<div class="MsoNormal">
<span style="font-family: "Arial","sans-serif"; font-size: 10.0pt;">Columns
headed by V present findings from Vogt et al. 2015; columns headed by M present
findings from Motalebi et al. 2015. Abbreviations: a = orbital semimajor axis;
e = orbital eccentricity.</span></div>
<div align="center" class="MsoNormal" style="text-align: center;">
<span style="font-family: "Georgia","serif";">-------------</span></div>
<div class="MsoNormal">
<span style="font-family: "Georgia","serif"; font-size: large;"><span style="font-family: "Georgia","serif";">Nonetheless, as
Table 1 shows, many results in V15 and M15 are in conflict. Here are the most salient disagreements:</span> </span><br />
<ul>
<li><span style="font-family: "Georgia","serif"; font-size: large;">Although
both groups present essentially the same picture of the two innermost planets, their
results on the outer planets differ dramatically. Both groups identify a planet
with a period just shy of 47 days, but for V15 this is <b><span style="color: #0070c0;">planet e</span></b>, with a minimum mass in
excess of 20 Mea, while for M15 it is <span style="font-family: Georgia,"Times New Roman",serif;"><b><span style="color: #c00000;">planet d</span></b>, with a minimum mass around 9 Mea.
Vogt et al. do propose a <b><span style="color: #0070c0;">planet d</span></b> of approximately 9 Mea, but they allow
it a period of only 22.8 days – about one-half the period of <b><span style="color: #c00000;">planet d</span></b>
in M15, and about one-half that of <b><span style="color: #0070c0;">planet e</span></b> in V15.</span></span><span style="font-size: large;"><span style="font-family: Georgia,"Times New Roman",serif;"> </span></span></li>
</ul>
<ul>
<li><span style="font-size: large;"><span style="font-family: Georgia,"Times New Roman",serif;">The
most substantial mismatch in planet characteristics involves the cool gas
giant. Whereas M15 indicate a sub-Saturn mass object (</span><span style="font-family: Georgia,"Times New Roman",serif;"><b><span style="color: #c00000;">planet e</span></b></span><span style="font-family: Georgia,"Times New Roman",serif;">) with a period of about
1200 days and a semimajor axis of 2.14 AU, V15 present an object slightly more
massive than Saturn (</span><span style="font-family: Georgia,"Times New Roman",serif;"><b><span style="color: #0070c0;">planet g</span></b></span><span style="font-family: Georgia,"Times New Roman",serif;">) with a period of about 2200 days and
a semimajor axis of 3.11 AU.</span></span></li>
</ul>
<ul>
<li><span style="font-family: Georgia,"Times New Roman",serif;"><span style="font-size: large;">Vogt’s
group finds two planets with periods of about 23 and 94 days, respectively, whereas
M15 find no evidence for more than three planets inside 0.4 AU. Notably, <b><span style="color: #0070c0;">planets d-f</span></b>
in V15 have period ratios that approximate a Laplace resonance of 1:2:4, while the
period of <b><span style="color: #0070c0;">planet
g</span></b> in V15 is almost twice that of <b><span style="color: #c00000;">planet e</span></b> in M15. As an amateur
who’s been following this stuff for years, I have to wonder if there’s a
problem with aliasing in one or both analyses.</span></span></li>
</ul>
<ul>
<li><span style="font-family: Georgia,"Times New Roman",serif;"><span style="font-size: large;">The
planets in V15 have much more circular orbits than those in M15. Vogt’s group reports
that while fitting the data for their inner planets (<b><span style="color: #0070c0;">b-f</span></b>), they kept the orbital
eccentricities fixed at zero to ensure a dynamically stable configuration. However,
they don’t extend this explanation to <b><span style="color: #0070c0;">planet g</span></b>, which in their model has an
eccentricity of only 0.06, very similar Saturn’s (0.056). The corresponding value
in M15 is 0.27. Nevertheless, M15 caution that the high eccentricities they
found for <b><span style="color: #c00000;">planet
d</span></b> as well as <b><span style="color: #c00000;">planet e</span></b> might stem from “un-perfect modelling
of the outer signal,” and thus might be exaggerated. (One can only hope that “un-perfect”
becomes “imperfect” on publication, unless one’s principal interest is
amusement!)</span></span></li>
</ul>
<ul>
<li><span style="font-family: Georgia,"Times New Roman",serif;"><span style="font-size: large;">M15
provide wide confidence intervals for the estimated minimum mass of each planet
candidate, while V15 provide very narrow intervals (not shown). Nevertheless,
the two teams’ estimates for planets b and c do not overlap; Vogt’s team finds
a somewhat smaller mass for planet b and a somewhat larger mass for planet c. Much
more substantial is the mismatch for the gas giant: the candidate in V15 (<b><span style="color: #0070c0;">planet g</span></b>)
is 74% more massive than the candidate in M15 (<b><span style="color: #c00000;">planet e</span></b>).</span></span></li>
</ul>
<ul>
<li><span style="font-family: Georgia,"Times New Roman",serif;"><span style="font-size: large;">V15
present a system of planets whose aggregated minimum mass (156 Mea) is twice that
of the system described by M15 (78 Mea). Nevertheless, both groups assume essentially
the same mass for the host star (0.78 Msol in M15, 0.79 Msol in V15).</span></span><span style="font-size: large;"><b><span style="font-family: "Georgia","serif";"> </span></b></span><span style="font-size: large;"><b><span style="font-family: "Georgia","serif";"> </span></b></span></li>
</ul>
<span style="font-size: large;"><b><span style="font-family: "Georgia","serif";">plus ça change . . .</span></b><span style="font-family: "Georgia","serif";"> </span></span><br />
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<br />
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">All this gives
me a sad sense of <i>déjà vu</i>. We’ve seen <a href="http://backalleyastronomy.blogspot.com/2013/01/year-of-signal.html">many similar</a>
cases of conflicting astronomical findings over the past decade, and they
typically involve observations of stars that, like HD 219134, are located <a href="http://backalleyastronomy.blogspot.com/2013/05/the-suns-back-yard.html">within
10 parsecs</a>. The most visible of these controversies relate to <a href="http://backalleyastronomy.blogspot.com/2014/07/phantom-planets-of-gliese-581.html">Gliese
581</a> and <a href="http://backalleyastronomy.blogspot.com/2013/07/welcome-our-newest-neighbors.html">Gliese
667C</a>. Both were once proposed as rich multiplanet systems, each with six low-mass
planets and at least one candidate for the coveted status of <a href="http://backalleyastronomy.blogspot.com/2015/07/kepler-452b-latest-hope-for-another.html">“habitable
Super Earth.”</a> Yet current data indicate that Gliese 581 has <a href="http://backalleyastronomy.blogspot.com/2014/07/phantom-planets-of-gliese-581.html">only
three small planets</a>, all hellishly hot, while Gliese 667C has <a href="http://backalleyastronomy.blogspot.com/2014/09/gliese-667c-just-two-planets.html">only
two</a>.</span></span></div>
</div>
<div class="MsoNormal">
<br />
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">This pair of
systems experienced their abrupt population shrinkage in 2014, when Paul Robertson
and colleagues demonstrated that the radial velocity analyses for both stars were
compromised by correlated noise related to stellar activity cycles. In each case,
the analytic breakthrough came along with a determination of the stellar rotation
period. What have exoplanetary astronomers learned since then?</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Well, M15 provide
a rotational period of 42.3 days for HD 219134 and argue that none of their periodicities
are related to this value. V15 assume (but did not measure) a rotational period of
only ~20 days for the star, and they</span></span><span style="font-size: large;"><span style="font-family: "Georgia","serif";"><span style="font-size: large;"><span style="font-family: "Georgia","serif";"> also argue that none of their periodicities are related to it. </span></span> However, the shorter period presented by V15 appears inconsistent with the stellar age they estimate: 12.46 billion years, which would make the star older than the spiral arm where it currently
resides. Stellar spin normally slows down with age (Meibom et al. 2015). As a point of reference, our Sun rotates in about 25 days, consistent with its measured age of 4.55 billion years.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Both groups
might be wrong, but it seems unlikely that both could be right.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">Although I’ve
often written about astronomical controversies, I wish they would just go
away. To break the present impasse, I look forward to a concerted, cooperative,
and international effort to obtain an authoritative perspective on HD 219134. Even though no habitable
Super Earths are at stake in this case, the proposed system architecture – a
cluster of likely gas dwarfs inside 0.5 AU and an undersized gas giant outside
2 AU – has huge significance for theories of system evolution. In addition, <a href="http://backalleyastronomy.blogspot.com/2015/08/a-transit-in-our-own-back-yard.html">as
reported by M15</a>, HD 219134 b appears to transit its parent star, a claim with
far-reaching implications that should be followed up as soon as possible.</span></span><br />
<br />
<span style="font-size: large;"><span style="font-family: "Georgia","serif";">A fascinating
and difficult case indeed!</span></span><br />
<br />
<div style="text-align: center;">
<span style="font-size: large;"><span style="font-family: "Georgia","serif";"><span style="font-size: large;"><i style="mso-bidi-font-style: normal;"><span style="color: black; font-family: "Georgia","serif";"><a href="http://backalleyastronomy.blogspot.com/2015/08/index-by-topic.html">Click
here for a permanent index by topic of blog posts<br />on Back Alley Astronomy</a></span></i></span> </span></span></div>
</div>
<div align="center" class="MsoNormal" style="text-align: center;">
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<span style="font-family: "Georgia","serif";">-------------</span></div>
<div class="MsoNormal" style="page-break-after: avoid;">
<span style="font-size: small;"><span style="font-family: Times,"Times New Roman",serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black;">REFERENCES</span></b></span></span></div>
<div class="MsoNormal">
<span style="font-size: small;"><span style="font-family: Times,"Times New Roman",serif;"><b style="mso-bidi-font-weight: normal;">Colon</b> <b style="mso-bidi-font-weight: normal;">KD</b>, Morehead RC, Ford EB. (2015) Vetting
Kepler planet candidates in the sub-Jovian desert with multiband photometry. <i style="mso-bidi-font-style: normal;"><span style="color: black;">Monthly Notices of
the Royal Astronomical Society</span></i> 452, 3001-3009. Abstract: <a href="http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2015MNRAS.452.3001C&db_key=AST&link_type=ABSTRACT&high=5585b8697d23905">2015MNRAS.452.3001C</a><b style="mso-bidi-font-weight: normal;"><span style="color: black;"> </span></b></span></span><br />
<span style="font-size: small;"><span style="font-family: Times,"Times New Roman",serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black;">Kasting JF</span></b><span style="color: black;">, Kopparapu R, Ramirez RM, Harman CE.
(2014) Remote life-detection criteria, habitable zone boundaries, and the
frequency of Earth-like planets around M and late K stars. <i style="mso-bidi-font-style: normal;">Proceedings of the National Academy of Sciences</i> 111, 12641-12646.
Abstract:</span> <a href="http://adsabs.harvard.edu/abs/2014PNAS..11112641K">http://adsabs.harvard.edu/abs/2014PNAS..11112641K</a><b style="mso-bidi-font-weight: normal;"><span style="color: black;"> </span></b></span></span><br />
<span style="font-size: small;"><span style="font-family: Times,"Times New Roman",serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black;">Motalebi
F,</span></b><span style="color: black;"> Udry S, Gillon M,
Lovis C, Ségransan D, Buchhave LA, Demory BO, Malavolta L, Dressing CD,
Sasselov D, et al. (2015) The HARPS-N Rocky Planet Search I. HD 219134 b: A
transiting rocky planet in a multi-planet system at 6.5 pc from the Sun. <i style="mso-bidi-font-style: normal;">Astronomy & Astrophysics</i>, in press.
Abstract: </span><a href="http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2015arXiv150708532M&db_key=PRE&link_type=ABSTRACT&high=5585b8697d21814">2015arXiv150708532M</a><span style="color: black; font-family: "Times New Roman","serif"; font-size: 10.0pt; mso-bidi-font-weight: normal;"> </span><b><span style="color: black; font-family: "Times New Roman","serif"; font-size: 10.0pt; mso-bidi-font-weight: normal;"> </span></b></span></span><br />
<span style="font-size: small;"><span style="font-family: Times,"Times New Roman",serif;"><b><span style="color: black; font-family: "Times New Roman","serif"; font-size: 10.0pt; mso-bidi-font-weight: normal;">Meibom S, </span></b><span style="color: black; font-family: "Times New Roman","serif"; font-size: 10.0pt; mso-bidi-font-weight: normal;">Barnes S, Platais I, Gilliland R, Latham D, Mathieu R.</span><span style="color: black; font-family: "Times New Roman","serif"; font-size: 10.0pt; mso-bidi-font-weight: normal;"> </span></span></span><span style="font-size: small;"><span style="font-family: Times,"Times New Roman",serif;"><span style="color: black; font-family: "Times New Roman","serif"; font-size: 10.0pt; mso-bidi-font-weight: normal;">(2015) A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster. <i>Nature</i> 517, 589-591. Abstract: <a href="http://www.nature.com/nature/journal/v517/n7536/full/nature14118.html">http://www.nature.com/nature/journal/v517/n7536/full/nature14118.html</a></span></span></span><br />
<span style="font-size: small;"><span style="font-family: Times,"Times New Roman",serif;"><b><span style="color: black; font-family: "Times New Roman","serif"; font-size: 10.0pt; mso-bidi-font-weight: normal;">Robertson P</span></b><span style="color: black; font-weight: normal;">,
Mahadevan S, Endl M, & Roy A. (2014) Stellar activity masquerading as
planets in the habitable zone of the M dwarf Gliese 581. <i style="mso-bidi-font-style: normal;">Science </i>345, 440-444. Abstract: <a href="http://adsabs.harvard.edu/abs/2014Sci...345..440R">http://adsabs.harvard.edu/abs/2014Sci...345..440R</a></span><b style="mso-bidi-font-weight: normal;"><span style="color: black;"> </span></b></span></span><br />
<span style="font-size: small;"><span style="font-family: Times,"Times New Roman",serif;"><b style="mso-bidi-font-weight: normal;"><span style="color: black;">Robertson
P, Mahadevan S.</span></b><span style="color: black;"> (2014)
Disentangling planets and stellar activity for Gliese 667C. <i style="mso-bidi-font-style: normal;">Astrophysical Journal Letters</i> 793, L24.
Abstract: <a href="http://adsabs.harvard.edu/abs/2014ApJ...793L..24R">http://adsabs.harvard.edu/abs/2014ApJ...793L..24R</a>
<span style="mso-spacerun: yes;"></span></span><b> </b></span></span><br />
<span style="font-size: small;"><span style="font-family: Times,"Times New Roman",serif;"><b>Vogt SS</b><span style="font-size: 10.0pt; mso-bidi-font-weight: bold;">, Burt J, Meschiari S, Butler RP, Henry GW, Wang S, Holden
B, Gapp C, Hanson R, Arriagada P, Keiser S, Teske J, Laughlin G. A six-planet
system orbiting HD 219134. <i style="mso-bidi-font-style: normal;">Astrophysical Journal</i>,
in press. Abstract: </span></span><span style="font-family: Times,"Times New Roman",serif;"><a href="http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2015arXiv150907912V&db_key=PRE&link_type=ABSTRACT&high=5585b8697d22671">2015arXiv150907912V</a></span></span></div>
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Bucky Harrishttp://www.blogger.com/profile/09846756737418704132noreply@blogger.com2