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Some years bring
spectacular extrasolar discoveries (like the first circumbinary planet) or dramatic events
(like the breakdown of a beloved space telescope).
Others just bring more of what we’ve already seen. But 2014 fits neither description.
Although I
didn’t notice any Earth-shattering news last year, I did see a dozen or more smaller
discoveries of great significance, scattered across various sub-specialties of planetary
and exoplanetary astronomy. Some of them have already been reported in this
blog, but most came at times when I was way too busy to write about them.
So here are my
top ten news items from outer space in 2014. I’ve grouped them by affinity:
first the transiting exoplanets, then the radial velocity exoplanets, then a
directly imaged exoplanetary system, and finally some interesting developments
in – of all places! – our own Solar System.
1.
Water vapor over HAT-P-11b
For the first
time, water vapor was detected in the atmosphere of a low-mass extrasolar
planet, HAT-P-11b (Fraine et al. 2014). Although water had already been found
in several transiting Hot Jupiters, previous observations of HAT-P-11b, other Hot
Neptunes, and Hot Super Earths all returned null results. This absence of
evidence was puzzling, because recent models of planet formation predicted that
the bulk composition of many low-mass planets would include a substantial proportion of
ices. Nevertheless, after several different teams reported
featureless spectra for such promising targets as GJ 1214 (the nearest
transiting Super Earth) and GJ 436 (the nearest transiting Hot Neptune),
theorists were motivated to develop novel models in which these planets have
deep atmospheres of hydrogen and helium (H/He) directly on top of rock/metal cores
(e.g., Lopez & Fortney 2014).
Now we see that
these models aren’t a good fit for HAT-P-11b. Instead, according to the
discovery paper, this planet most likely has significant fractions of water and
other heavy molecules mixed in a deep atmosphere dominated by H/He. It remains
uncertain whether HAT-P-11b might support a mantle of high-pressure ice over a rocky
core, but it’s definitely a working hypothesis.
This detection
breathes new life into older models of Water Planets and Ice Dwarfs (e.g.,
Leger et al. 2004), and promises to reframe our current perceptions of low-mass
planets. With such a watery stratosphere, HAT-P-11b clearly did not form on its
present orbit, which whizzes the planet around its K-type parent star in just
five days. Instead, it must have accreted much farther away from the star, in a
region of the primordial nebula where icy planetesimals were available. Hence this
object’s composition provides a meaningful constraint on future work in planet
formation and structure. My impression is that this result disfavors the
hypothesis of the “minimum mass extrasolar nebula” proposed by Chiang &
Laughlin (2013), which was also critiqued on other grounds in several articles
last year (e.g., Cossou et
al. 2014, Inamdar & Schlichting 2014, Izidoro et al. 2014, Raymond & Cossou 2014, Schlaufman 2014, Schlichting 2014).
2.
Massive Kepler data dump
Although the
primary mission of the Kepler space telescope
was terminated by equipment failure in 2013, the transit data collected during three-plus
years of operation will fuel new discoveries for a long time to come. That
point was driven home in March, when Kepler scientists confirmed more than 700 planets
in one fell swoop, raising the exoplanetary census from less than 1000 to
almost 1800 planets in a single day. (By now that number has grown still
further, to 1855.) This vastly enlarged sample sharpens our view of the
exoplanetary cosmos. Instead of the gas giants that previously dominated the
census, the current sample is about 60% low-mass planets, and for the first time
transit discoveries outnumber radial velocity discoveries. Much richer data are
now available to test theories of planet structure and atmospheric composition,
not to mention system architectures and evolution.
[Here are some
definitions, in case you need them: A gas
giant is a planet more massive than 17% of Jupiter (0.17 Mjup) with a bulk
composition more than 50% H/He. A low-mass
planet has a mass smaller than 0.17 Mjup (i.e., less than 55 Earth masses
or 55 Mea) and a composition more than 50% rock/metal. Transiting planets are detected when they cross the face of their
parent stars, such that their shadows, as seen from the perspective of Earthly observers,
dim the stars’ light. Radial velocity
planets are detected by color shifts in their parent stars’ optical spectra,
which happen as the planets’ gravity tugs the stars closer or farther away from
us. Transiting planets that are massive enough or located close enough to Earth
(usually gas giants) can also be detected by radial velocity observations.]
Figure
2.
Kepler-186f is probably tidally locked and possibly terrestrial. If so, it might resemble the Eyeball Earth pictured in this artist's view. The planet imagined here is wet and fairly cold, with its icy hemisphere extending into the sunlit side. At the sub-stellar point, its atmosphere churns in a more or less permanent cyclone.
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3. Kepler-186f,
another Earth-like planet
Kepler scientists
found their Holy Grail in 2013 with the confirmation of Kepler-62f, the
first potentially rocky planet identified in a circumstellar habitable zone. They
followed up with another one last year: Kepler-186f. I
wrote extensively about that system at the time of the announcement, so I refer
you to my post for full
details. Notably, these two candidates for the status of Earth 2 bear a family
resemblance. Both orbit stars that are smaller and cooler than our Sun:
Kepler-62 is a K dwarf, Kepler-186 is an M dwarf. Both planets have radii
between 1.1 and 1.4 times Earth (1.1-1.4 Rea), implying masses between 1 and 4 Mea.
And both are predicted to have equilibrium temperatures even lower than on
Earth – quite a marvel, considering that the vast majority of low-mass exoplanets,
even in the newly enlarged census, are very hot indeed.
Figure 3. Architecture of the
Kepler-289 system, with all three transiting planets rendered at their relative
sizes. The masses of planets c and d were determined by analyzing transit
timing variations; however, that method did not yield an unambiguous mass for
the inner planet. Note that, at the time of writing, the designations of these
three planets are not set in stone. The object denoted as “d” in this figure was previously designated “c” by Kepler Mission scientists, while the Planet Hunters elected
to use their own terminology for all three objects, naming them PH b, PH c, and
PH d. In a mash-up of the Solar System and Kepler-289, Mercury’s orbit would
lie between planets c and d.
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4. Kepler-289,
another mixed-mass system
Last year I blogged
about several new examples of a rare planetary system
architecture, one that includes at least one gas giant and at least
two low-mass planets. This year, the Planet Hunters group reported another of
these mixed-mass systems: Kepler-289, which centers on a Sun-like host star located
700 parsecs away (Schmitt et al. 2014). All three detected planets are mutually
well-aligned, since all are seen in transit, and two of them (the middle and
the outer) have robust masses determined by transit timing variations. The
inner and middle planets have radii between 2 and 3 Rea and masses below 10 Mea,
while the outer giant has a radius very close to Jupiter’s, despite a mass of
only 0.42 Mjup (about 30% more than Saturn’s). Although this system isn’t a
likely candidate for habitable worlds, since none of the known planets could
support liquid water, it adds to an extremely valuable sample of exoplanets. At
my reckoning, we now have seven confirmed Kepler systems with at least one
high-mass and two low-mass planets. In addition to Kepler-289, they are
Kepler-30, -48, -68, -87, -89, and -90. We also have three radial
velocity systems that fulfill the same criteria: GJ 676A, GJ 876, and HD 10180.
To exploit a well-flogged metaphor, I believe this ensemble of twelve systems is
a cosmic Rosetta Stone holding the secrets of creation. So I’m hoping
some clever astronomers will develop a model to explain the formation of all
twelve. If they do, I think such a model will also go a long way toward
explaining the formation of planetary systems in general.
5. Celebrity planets exposed as phantoms
Now we come to
the saddest news of the year, a daunting reminder that almost all the lovely
exoplanetary data so assiduously collected by astronomers over the past two
decades remain tentative. Paul Robertson and Suvrath Mahadevan set their
sights on two nearby systems popular with the astrobiological set, GJ 581 and GJ 667C (Robertson
et al. 2014, Robertson & Mahadevan 2014). Considerable controversy has
surrounded the number and nature of the planets in each system (as reported here and here). So
Robertson, Mahadevan, and their collaborators reanalyzed existing radial
velocity data on each one and reached surprising conclusions: Instead of the
six planets proposed by Vogt et al. (2010) or the four claimed by Forveille et
al. (2011), GJ 581 has only three planets,
and none of them orbit in the habitable zone. Instead of the six planets
reported by Anglada-Escude et al. (2013), GJ 667C has only two,
and neither is a candidate for habitability. (Click the preceding links for
full blog posts on each study.) According to Robertson and Mahadevan, the data
interpreted as evidence for many of those phantom planets were simply artifacts
of the rotation periods of their respective host stars.
And so, after a
few years of great optimism, when it looked like Earth had at least two potentially
habitable counterparts within 25 light years, we’ve returned to the perspective
of 2006: none of the planets returned by radial velocity searches have masses and
temperatures consistent with surface bodies of liquid water. (Of course, it’s a
different story for transit searches; see #3 above.)
Figure 4. 30 and 31 Cygni,
shown here, comprise an especially photogenic binary star system. On closer
look this pair is actually a triple, since the yellowish star is itself binary.
Credit: Wikimedia
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6. Hot Jupiters in conjoined planetary systems
Many exoplanets
are known to orbit stars in binary systems, but until a few years ago, we knew
of no binaries in which both members of the pair supported their own families
of planets. That seemed odd, because there seemed no reason why one star would
be “planetic” while its sibling wasn’t. Then, in 2011, an unpublished
manuscript by Michel Mayor and colleagues reported two low-mass planets in
orbit around the G-type star HD 20781. This is the binary companion of another
G star, HD 20782, which was already known to harbor a gas giant planet. During
the year just ended, two more such systems were reported, this time in
peer-reviewed journals: XO-2NS (Desidera et
al. 2014) and WASP-94 AB (Neveu-VanMalle et al. 2014). The announcement of the
latter received the biggest fanfare of the lot, probably because both systems
have exactly the same architecture. Each one harbors a Hot Jupiter without any sign
of companions on cooler orbits.
Robust confirmation
of the reality of planet-rich binaries is significant for two reasons: 1) Most
Solar-type stars, which are probably the most likely to harbor habitable
planets, occur in binaries. Now that we know both members of a binary can host
planets, we can be confident that the sample of potentially habitable systems is huge. 2) From a purely science fictional viewpoint, a pair of such systems with at least one planet supporting a high-tech civilization
could become the scene of an interstellar empire - and its citizens would have no need for faster than
light travel! They could cruise from one
star to the other in ships powered by xenon ion thrusters, or similar technologies known even among primitive species like Earthlings.
A final
consideration is what to call these planet-bearing stellar couples. We have “binary” to denote a gravitationally bound pair of stars, and
“circumbinary” to denote a planet that orbits both members of a binary. But
what about systems like WASP-94AB and XO-2NS? Calling them “binary planetary
systems” is insufficient to convey the notion that both stars host their own planets.
Therefore, consistent with the biological flavor of common descriptors for
binary stars (twins, siblings), I propose the term “conjoined planetary systems.”
I concede that “Siamese systems” might be more catchy (though it’s probably
culturally inappropriate), and if you prefer a non-biological alternative, I offer “double
planetary systems.” But my money’s on “conjoined.”
Figure 5.
The
dusty protoplanetary disk around HL Tauri was photographed in spectacular
detail by ALMA. To provide a scale
for this figure, green dots mark the orbits of planets and dwarf planets in the
only two systems known to support well-constrained gas giants at wide
separations: HR 8799 and our
Solar System. Note that the protoplanetary disk around our Sun was much smaller
than HL Tauri’s disk. According to the consensus view, the present orbit of
Neptune traces its original outer edge. A full-size image of the HL Tauri disk
in all its glory, without text, is available here.
7.
Spectacular snapshot of the birth of planets
In September,
researchers using the Atacama Large Millimeter/submillimeter Array (ALMA), an
installation of telescopes in Chile’s Atacama Desert, released the most
detailed image ever obtained of a nascent planetary system. The central star is
HL Tauri, a pre-main sequence object of Solar mass that has been under
intensive study for the past 40 years. The image (available here in various sizes) depicts a large
dusty structure with a radius of about 80 AU, almost triple the radius inferred
for our Sun’s planet-forming nebula. The dust is visible at submillimeter
wavelengths as a bright disk interrupted by a series of eight darker bands that
resemble the gaps in the rings of Saturn. ALMA
astronomers identify each gap as a likely site for planet formation, hinting
that eight large planets are under construction with periods substantially
longer than Jupiter’s. As Catherine Vlahakis, an ALMA scientist, stated: “This
one image alone will revolutionize theories of planet formation.”
HL Tauri is
located at a distance of about 140 parsecs (450 light years). Estimates of its
age vary; ALMA scientists characterize the star as less than one million years
old. For such a young star, the detailed structures visible in the
protoplanetary disk, even at semimajor axes far wider than the orbit of Pluto,
came as a big surprise. Astronomers had not expected planetary accretion to
have reached such an advanced stage so far from a star.
This spectacular
photograph was the first to be obtained in ALMA’s most powerful mode, which
enables imaging at resolutions even higher than the Hubble Space Telescope. According
to the researchers, we can expect many more brilliant images from ALMA in the
next few years.
Figure 6. Enceladus, moon of
Saturn, represented at the same scale as Earth. The large body of water on the
lower right is Lake Superior, whose volume might be similar to the subsurface
lake recently identified on Enceladus.
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8.
Subsurface lake on Enceladus
As the
sixth-largest moon of Saturn, Enceladus has a diameter of only 505 kilometers
(314 miles). It consists of a rocky core supporting a thick mantle of ice. In
2005, the Cassini spacecraft confirmed that this little moon regularly vents fountains of ice, which have been
captured in photographs. The
leading explanation for this activity has been a hypothetical sea of liquid
water beneath the icy mantle.
And so it is! This
past year, Iess et al. (2014) used Cassini data on the quadrupole gravity field
of Enceladus to identify a subsurface lake or “regional sea” beneath the moon’s
south pole. This is the same area where venting is observed above the surface
features known as tiger stripes. The
research team emphasized the limited extent of this lake, ruling out a global
ocean. They found that the water is located 30 or 40 km beneath the ice, and
that its depth is about 10 km. News reports compared its volume to Lake
Superior, a large body of water in North America (though the maximum depth of Superior
is only 406 meters). The Enceladic lake vents vapor and ice particles through
fissures reaching to the surface.
This important
discovery raises the probability that other icy bodies in the Solar System –
notably Europa and Ceres – have similar morphology. It also provides a real-life
example of the structures proposed for some icy extrasolar Super Earths (Leger
et al. 2004). According to many astrobiologists, subsurface water is a
potential environment for the evolution of life.
Figure 7. Artist’s view of
watery emissions from the dwarf planet Ceres. Credit: Ron Miller
9.
Ceres emits water
Do other small
bodies in the Solar System also produce clouds of water and ice? Observations
with the Herschel Space Telescope identified emissions of water vapor from
Ceres, which is both a dwarf planet and the largest object in the Asteroid
Belt. Herschel scientists suggested that the water came from localized sources
in Ceres’ mid latitudes, but they could not be sure if it resulted from
sublimation of surface ice or from cryovolcanoes
similar to those on Enceladus (Kuppers et al. 2014).
NASA’s Dawn spacecraft, which has already visited Vesta, will soon
enter orbit around Ceres and hopefully send back our first close-up view of
this remnant from the Solar System’s remote past. Ceres is a sphere with a
diameter of about 950 km (590 mi), so it should present an excellent subject
for robo-photography. With Dawn’s data we should get a better understanding of its
internal structure and formation history. Ironically, we know much more about
the small moons of Saturn, which are only a fraction of Ceres’ size and far
more distant from Earth, than we do about Ceres, entirely because of the
various spacecraft that have visited Saturn’s photogenic entourage of rings and
moons.
10.
Chariklo, the ringed Centaur
Chariklo is a Centaur
orbiting between Saturn and Uranus. No, it’s not a mythical amalgam of a horse
and a man; in astronomical parlance, a Centaur is a small object resembling an
asteroid (or a comet or a Kuiper Belt object) orbiting outside the semimajor
axis of Jupiter. With an approximate diameter of 250 km (155 miles), Chariklo is
the largest known Centaur. As an asteroid it would be quite big, but as a
Kuiper Belt object it would be dinky. During a recent stellar occultation by Chariklo,
several “secondary events” were observed, likely evidence of
a ring system around the central object (Braga-Ribas et al. 2014). Although Chariklo
is unlikely to be spherical, its ring system is circular, consisting of two flat,
concentric rings with an outer radius of about 405 km (250 miles). By
comparison, the Saturnian ring system has an edge-to-edge diameter similar to
the separation between the Moon and Earth (240,000 miles).
The discovery
team speculated that Chariklo’s rings might be a transient phenomenon; Saturn’s rings have inspired similar
speculations, with no consensus yet. Regarding Chariklo, the team noted that some other Centaurs
are known to have tiny moons whose disintegration could create rings. If this view is
accurate, it boosts the likelihood that rings are a common feature of exoplanetary
systems (where Earthly artists already love to depict them anyway).
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And that was the
year in outer space! Lots of news on small planets, small moons, and big asteroids,
but best of all: water, water, everywhere!
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