HD 219134: Take Three

Figure 1. 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

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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!

Or at least that’s the ideal.

Since last summer, we’ve seen science in action around HD 219134, an appealing amber star of 0.78 Solar masses (0.78 Msol) located very nearby in the northern constellation of Cassiopeia. That star and its planetary system have been featured in three previous blog posts (here, here, and here), which record unfolding developments both in the number of the reported planets and in the most appropriate characterization of the system architecture.

Two major studies last year presented conflicting analyses based on two different datasets. Motalebi & colleagues (2015; hereafter M15) proposed a four-planet architecture 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 through e. Almost simultaneously, Vogt & colleagues (2015; hereafter V15) proposed a six-planet architecture 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 through g in order of distance from the star, but on publication they were renamed b through h without reference to their distance, omitting e 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.

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, planet b (see Figure 1).

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.

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 planet f – 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.

Table 1. Summary of findings on the HD 21934 planetary system

Tags: P = approximate periodicity in days; PL = planet designation; d = reported period in days; Mea = reported mass in Earth units; a = semimajor axis in astronomical units (AU; Earth’s semimajor axis = 1 AU); e = orbital eccentricity.

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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 (Table 1).

In the inner system, J16 had no difficulty recovering a signal corresponding to planet 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 planet d 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.

J16 also failed to recover any periodicity corresponding to planet c. 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.

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 planet h for this object.

I say “sadly” because h implies a system with seven exoplanets, whereas only one extrasolar system with that many companions has been confirmed to date: Kepler-90. The planeticity of this remarkable system is rivaled only by HD 10180, which has six exoplanets (c through h) confirmed by radial velocity measurements and a seventh (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 planet e is a more appropriate designation for the cool gas giant in this system, on the grounds of logical consistency.

Figure 2. Selected mixed-mass planetary systems

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

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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.” Figure 2 illustrates the construct I have in mind. It updates Figure 3 from a recent posting (Almost Jupiter) by revising the orbit of the outer planet of HD 219134 and adding our Solar System.

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 g and h both have radii larger than 8 Rea.

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.

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.

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.

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.

migration through a radially structured disk

We have lots of theories about the evolution of our Solar System. These days, the most popular hinge on the unique relationship between Jupiter and Saturn. 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?

Gavin 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.

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.

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.

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.

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).

Figure 3. The protoplanetary disk around TW Hydrae

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).

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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.

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.

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REFERENCES
Coleman G, Nelson R. (2016a) On the formation of compact planetary systems via concurrent core accretion and migration. Monthly Notices of the Royal Astronomical Society 457, 2480-2500. Abstract: 2016MNRAS.457.2480C
Coleman G, Nelson R. (2016b) Giant planet formation in radially structured protoplanetary discs. Monthly Notices of the Royal Astronomical Society. In press. Abstract: 2016arXiv160405191C
Debes JH, Jang-Condell H, Schneider G. (2016) The inner structure of the TW Hya disk as revealed in scattered light. Astrophysical Journal Letters 819, L1.
Johnson MC, Endl M, Cochran WD, Meschiari S, Robertson P, MacQueen PJ, Brugamyer EJ, Caldwell C, Hatzes AP, Ramírez I, Wittenmyer RA. (2016) A 12-year activity cycle for the nearby planet host star HD 219134. Astrophysical Journal 821, 74. Abstract: 2016ApJ...821...74J
Kasting JF, 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. Proceedings of the National Academy of Sciences 111, 12641-12646. Abstract: http://adsabs.harvard.edu/abs/2014PNAS..11112641K
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. Astrophysical Journal 820, 112. Abstract: 2016ApJ...820..112K
Lovis C, 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. Astronomy & Astrophysics 528, A112. Abstract: 2011A&A...528A.112L
Motalebi F, 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. Astronomy & Astrophysics 584, A72. Abstract: 2015A&A...584A..72M
Nomura H, Tsukagoshi T, Kawabe R, Ishimoto D, Okuzumi S, Muto T, et al. (2016) ALMA observations of a gap and a ring in the protoplanetary disk around TW Hya. Astrophysical Journal Letters 819, L7.
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

 

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. 

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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, 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. Astronomy & Astrophysics 528, A112. Abstract: 2011A&A...528A.112L

Motalebi F, 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. 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.

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