Sunday, March 20, 2016

Almost Jupiter




Figure 1. 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
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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).

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 Kepler-167e. 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. 

Table 1. Characteristics of the Kepler-167 planetary system

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.
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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). 

Table 1 and Figure 2 describe a virtual twin of HD 219134 (for two different perspectives on that system, see Motalebi & al. 2015 and Wright & al. 2015). 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. 

Figure 2. Kepler-167 system architecture

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 e orbits outside the system ice line.
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This architecture has conspicuous structural similarities with the other multiplanet systems summarized in Figure 3. 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 g and h both have radii larger than 8 Rea. In Kepler-90, however, both these planets are part of the outer cluster of planets.

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. 

Figure 3. Selected mixed-mass planetary systems

Tags: ME = Earth masses; RE = Earth radii; AU = astronomical units (Earth-Sun separation = 1). Selection criteria: 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.
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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 Mass Matters). 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.

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.

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. 

planetology of Kepler-167 

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.

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.

Even without mass estimates, we can make informed guesses about the composition of the three inner planets of Kepler-167. The radius of planet d 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. 

a Jupiter analog? 

Now we come to the most remarkable claim in the discovery paper, evident in its title. The authors describe Kepler-167e as a Jupiter analog. This rare species has attracted growing interest in the past decade, along with a few competing definitions (see How Weird Is Our Solar System?). 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.

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 in situ 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.

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 in the past, therefore, I argue that a bona fide Jupiter analog must have no interior giant companions. 

Figure 4. Jupiter and Io

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 Galilean moons. 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.
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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 HD 32963 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.)

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 HD 219134 g 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 HD 219134 e, 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.

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.

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.

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. 

birth versus survival 

Let’s look again at the 10 systems summarized in Figure 3. 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.

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 Figure 3. By hybrid I mean that these architectures resemble the well-known class of compact low-mass systems, except that they add a cool gas giant to the mix.

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.

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 f (23 Mea at 0.49 AU) and planet g (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 g has a substantially more eccentric orbit than previously reported. In their analysis, planet g 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. 

mind the gap 

Most of the systems presented in Figure 3 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.

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.

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 bona fide Jupiter analogs.

Figure 5. Fifteen potential Solar System analogs 


* Not listed in Rowan et al. 2016.

Tags: 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. Selection criteria: star mass 0.7-1.2 Msol; a > 3 AU; e < 0.3, no interior giants occupying or perturbing the system habitable zone.
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how rare is Jupiter? 

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.

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.

Such a population share is much lower than I imagined a few years ago. 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).

So there’s another reason to appreciate the Earth (not to mention the Jupiter) we already know about. 


REFERENCES
Bayliss DD, Sackett PD. (2011) The frequency of Hot Jupiters in the Galaxy: Results from the SuperLupus survey. Astrophysical Journal 743, 103.
Kipping DM, 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: 2016arXiv160300042K
Kane SR, Gelino DM. (2014) On the inclination and habitability of the HD 10180 system. Astrophysical Journal 792, 111.
Lovis C, S├ęgransan 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. Astronomy & Astrophysics 528, A112. Abstract: 2011A&A...528A.112L
Motalebi F, Udry S, Gillon M, Lovis C, S├ęgransan 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. Astronomy & Astrophysics 584, A72. Abstract: 2015A&A...584A..72M
Raymond SN. (2006) The search for other Earths: Limits on the giant planet orbits that allow habitable terrestrial planets to form. Astrophysical Journal 643, L131–L134. Abstract: 2006ApJ...643L.131R 
Rowan D, 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. Astrophysical Journal, 817:104.
Tuomi M. (2012) Evidence for 9 planets in the HD 10180 system. Astronomy & Astrophysics 543, 52.
Vogt SS, 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. Astrophysical Journal 814, 12. Abstract: 2015ApJ...814...12V
Wang J , Fischer DA, Horch EP, Huang X. (2015) On the occurrence rate of Hot Jupiters in different stellar environments. Astrophysical Journal 799, 229.
Wittenmyer R, Tinney CG, O’Toole SJ, Jones HRA, Butler RP, Carter BD, Bailey J. (2011) On the frequency of Jupiter analogs. Astrophysical Journal 727, 102. Abstract: http://adsabs.harvard.edu/abs/2011ApJ...727..102W
Wittenmyer R, 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. Astrophysical Journal 819, 28.
Wright JT, Marcy GW, Howard AW, Johnson JA, Morton TD, Fischer DA. (2012) The frequency of Hot Jupiters orbiting nearby Solar-type stars. Astrophysical Journal 753, 160.

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