Wednesday, May 3, 2017

TRAPPIST-1 and Kepler-11: Revised Masses

Figure 1. 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.

A few months ago, Michaël Gillon & colleagues reported a remarkable seven-planet architecture for a nearby ultra-cool red dwarf, TRAPPIST-1. 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.

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.

The periods, semimajor axes, and equilibrium temperatures of the TRAPPIST planets are unchanged, and only two of them have smaller radii. As shrinkage goes, this is rather slight. Nevertheless, many planetary masses, and all densities, are dramatically different. Figure 1 and Table 1 contrast the results of Wang & colleagues with those of Gillon & colleagues. Figure 2 depicts the planets at their relative sizes and densities according to Wang’s group (revised from the first figure in my previous post on TRAPPIST-1).

Table 1. Comparison of TRAPPIST-1 parameters from Wang et al. and Gillon et al.

Period is expressed in Earth days; radius, mass, and density are expressed in Earth units.
(W) = Wang et al. 2017; (G) = Gillon et al. 2017.

a super mercury among super ganymedes?

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 e, 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 h, for which Gillon’s group reported no mass at all. Moreover, for the three innermost planets (b, c, d), the densities estimated by Wang’s group are formally consistent with a rocky composition like Earth’s, again within uncertainties.

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 c requiring major enrichment in iron to explain its large mass. Planets b and d, 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.

Figure 2. Revised densities for the planets of TRAPPIST-1

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 c, 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 previous post on the same system.)

The next three planets (e, f, g), 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.

For planet h, 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 h 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.

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 (c) 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 (e, f, g) 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).

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.

Figure 3. Exotic aquatic life on Earth

Nembrotha cristata (left), a tropical sea slug, by Chriswan Sungkono; Hapalochlaena lunulata (right), a highly venomous octopus native to the Philippines, photographer unknown.

younger star, fatter planets

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 benchmark system 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.

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 (blogged here).

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.”

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.

Table 2. Comparison of Kepler-11 parameters from Bedell et al. and Lissauer et al.

Notes: a = semimajor axis in astronomical units; period = orbital period in days.
B 17 = Bedell et al. 2017; L 13, L 11 = Lissauer et al. 2013, 2011.

Table 2 compares three generations of data on Kepler-11. 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 g, where we see no change. Yet in comparison with the 2013 update, the latest estimates have less extreme consequences for planetary composition.

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 c through f are unambiguously puffy, requiring hydrogen envelopes to bring their ample radii in line with their relatively puny masses (all <10 Mea).

The most consequential change involves planet 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.

The newly estimated density of 0.445 Earth strengthens the argument that planet 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.

Bedell M, 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. Astrophysical Journal 839, 94.
Gillon M, 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. Nature 533, 221-224. Abstract: 2016Natur.533..221G
Gillon M, 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. Nature 542, 456-460. Abstract: 2017Natur.542..456G
Hussmann H, Sotin C, Lunine J. (2015) Interiors and evolution of icy satellites. In Treatise on Geophysics, Volume 10: Physics of Terrestrial Planets and Moons, ed. G. Schubert. Elsevier B.V.
Kuchner MJ. (2003) Volatile-rich Earth-mass planets in the habitable zone. Astrophysical Journal 596, L105-L108.
Leger A, Selsis F, Sotin C, et al. (2004) A new family of planets? “Ocean Planets.” Icarus 169, 499-504.
Lissauer JJ, 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. Astrophysical Journal 770, 131. Abstract: 2013ApJ...770..131L
Luger R, 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.
Noack L, 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? Icarus 277, 215-236.
Wang S, Wu DH, Barclay T, Laughlin GP. (2017) Updated masses for the TRAPPIST-1 planets. In press. Abstract: 2017arXiv170404290W

Tuesday, April 18, 2017

HD 219134 Scorecard: 5 planets, 2 transiting

Figure 1. 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 and c 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 d and e are assigned the same radii as Neptune and Saturn, respectively). See Table 1.
Fresh from their discovery of four new Earth-size planets 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 here) that the innermost planet (b) is visible in transit, while announcing that the second planet (c) also transits. In addition, they present radial velocity masses for both transiting planets (hereafter tranets), as well as a refined radius for planet b. For both objects, these values are consistent with a purely rocky composition.

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 here) and Johnson & colleagues (hereafter J16, blogged here). 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.

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. Table 1 summarizes their findings on planets b through f, supplemented by the findings of J16 on planet e (which J16 called planet h, even though no one has proposed as many as seven planets – i.e., b through h – for this system).

Table 1. Revised system parameters of HD 219134

Tags: 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 e are based on those for planet h in Johnson et al. 2016. All other data are based on Gillon et al. 2017.

conflicts resolved?

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 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 c, we can also be confident of that planet’s mass, radius, and density, with no change in its estimated period. Even planet d retains virtually the same period reported by M15 and V15, although its mass falls between the values they provided.

Although planet f 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 f 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 f appears to place it just outside a 2:1 mean motion resonance with planet d. Similar period ratios have appeared in several compact low-mass systems discovered by the Kepler Telescope.

Consistent with M15 and J16, Gillon & colleagues implicitly reject V15’s proposed planet g 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.

mass distribution

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 Table 1). Gillon & colleagues argue that the two innermost planets, b and c, 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, f and d, 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.

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. 

stingy spitzer scheduling

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 and c. 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

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.

architecture and habitability

As I discussed in an earlier post 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.)

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.

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 here).

beware of outsiders

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). 

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).

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.

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: WASP-47, Kepler-11, Kepler-62, Kepler-90, and Kepler-20.

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 g. Becker & Adams propose that an undetected gas giant on a cool orbit might be responsible for this configuration.  

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.

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: Beware of Outsiders. Once we have more data on HD 219134, comparable analyses should be able to constrain the alignment of planet e.

tranets in near space

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 Gliese 436 at 10.2 parsecs (one hot tranet more massive than Neptune), Gliese 1132 at 12 parsecs (one hot terrestrial tranet), TRAPPIST 1 at 12.1 parsecs (seven terrestrial tranets), 55 Cancri at 12.3 parsecs (one very hot, massive terrestrial tranet), LHS 1140 at 12.47 parsecs (one temperate terrestrial tranet), Gliese 1214 at 13 parsecs (one hot, puffy tranet about half the mass of Uranus), HD 189733 at 19.2 parsecs (one transiting Hot Jupiter), HD 97658 at 21 parsecs (one hot tranet more massive than Neptune), Gliese 3470 at 29 parsecs (one hot Uranus-mass tranet), HAT-P-11 at 38 parsecs (one hot tranet more massive than Neptune), and Kepler-42 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).

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.

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.

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Motalebi F, Udry S, Gillon M, Lovis C, Ségransan D, Buchhave LA, Demory BO, Malavolta L, Dressing CD, Sasselov D, Rice K, Charbonneau D, Collier Cameron A, Latham D, Molinari E, Pepe FA, Affer L, Bonomo AS, Cosentino R, Dumusque X, .Figueira P, Fiorenzano A, Gettel S, Harutunyan A, Haywood RD, Johnson J, Lopez E, Lopez-Morales M, Mayor M, Micela G, Mortier A, Nascimbeni V, Philips D, Piotto G, Pollacco D, Queloz D, Sozzetti A Venderburg A, Watson CA. (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
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Saturday, March 18, 2017

TRAPPIST-1 and the Seven Dwarfs

Figure 1. 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 f is the most similar to Earth in radius, but not in mass or density (see Table 1). The density of planet h has not been estimated yet.

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 bona fide planets. Michaël Gillon & colleagues initially reported three Earth-sized objects (b, c, d) 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 tranets), as the system is located only 39 light years (12 parsecs) away in the Sun’s back yard. 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.

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 protoplanetary disks 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.

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.

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 h, the outermost. Not only do the six inner tranets resemble Earth and Venus in size and mass – three of them (e, f, g) occupy the system habitable zone (see Figure 1).

As a colorful header in The Sun proclaimed, “Nasa says TRAPPIST-1 solar system could be teeming with ‘exotic’ alien life forms!”

Figure 2. Comparing TRAPPIST-1 to our Solar System
Image Credit: European Southern Observatory

three terrestrial planets in the liquid water zone

If TRAPPIST-1 were a G-type star like our Sun, or a K-type star like Epsilon Eridani or HD 219134, 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 Proxima Centauri. It’s so lightweight that it barely crosses the threshold between stars, which are the sites of nuclear fusion, from brown dwarfs, which are not.

But the problem isn’t the diminutive size or puny luminosity of TRAPPIST-1 (see Figure 2 for perspective). It’s the evolutionary history of M dwarfs, as detailed in an earlier blog post. 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.

Even the writers for The Sun 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:

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.”

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.

“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)


Table 1. TRAPPIST-1 system parameters

Tags: 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).

system architecture

Now let’s get behind the headlines and put this discovery in context (see Table 1). 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 Kepler-90, HD 10180, 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 tweens. 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.

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 Kepler-11 and Kepler-20 have semimajor axes of 0.46 AU and 0.35 AU, respectively. By contrast, the semimajor axis of the seventh planet (h) 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 Kepler-444, 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.

Because we can observe all seven planets in transit, we know that the TRAPPIST-1 system is completely “flat” or co-planar: 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 to planet h, the ratios are 8:5 (b/c), 5:3 (c/d), 3:2 (d/e), 3:2 (e/f), 4:3 (f/g), and 3:2 (g/h) (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.

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 Laplace resonance, in honor of the first astronomer to describe it (Pierre-Simon de Laplace) This and similar instances of interlocking resonances are likely to enhance the long-term stability of any system architecture (Gillon et al. 2017).

Nonetheless, the unparalleled intricacy of the architecture of TRAPPIST-1 takes this configuration to a whole new level (Figure 3), with several critical implications for our understanding of the system. First, in situ formation 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 h). Subsequent interactions with the primordial nebula caused them to migrate inward to their present locations very early in system history.

Figure 3. A triple transit of TRAPPIST-1
This figure illustrates the light curve of TRAPPIST-1 on a day when three of its planets – c, e, and f – transited almost simultaneously. Given the system’s multi-resonant orbital architecture, simultaneous transits must be frequent. Image credit: Gillon et al. 2017.

Second, these tightly packed orbits give rise to powerful gravitational interactions that cause transit timing variations (TTVs), as in the classic case of Kepler-11. 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.

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.


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).

As shown in Table 1 and Figure 2, 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.

On theoretical grounds, however, Gillon & colleagues prefer a mixed composition of rock and ice for planets b through g. 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.

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 (c and g) are also the densest – we also see anomalies. Planet b is hotter and less massive than planet c, 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.

Luger & colleagues remind us that tidal heating is yet another consequence of the interlocking resonances that structure the TRAPPIST-1 system. It is also likely to be a factor in the thermal evolution of these seven planets. Luger’s group argues that planet b could have a tidal flux similar to that of Io (which also participates in a multi-resonant chain), potentially generating intense volcanism. Thus, it might be no coincidence that the density of planet b (66% Earth) is also very similar to the density of Io (64% Earth). Planets c through e experience lesser tidal effects, but still much stronger than the heat flux caused on Earth by our homeworld’s radioactive core. These planets might also experience volcanism comparable to Io’s. The more distant planets f through h appear to have avoided such a sizzling history.

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).

exotic alien life-forms?

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 The Sun 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).

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.

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 g and h 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.

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.

celebrity septets

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:

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 recherché 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?

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 through Sunday for tranet h, 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.”)

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, Friends (which I confess I’ve never seen), plus another septet based on the titles of the first seven movies in the Fast & Furious franchise (ditto).

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 (Transiting Planets and Planetesimals Small Telescope), 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 Challenger. You might still be able to check out the March 2 issue of The Telegraph and the March 3 issue of The Daily Mail to see a selection of these candidates.

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 Furious 7!

Barnes R, 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: 2016arXiv160806919B
Bolmont E, 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. Monthly Notices of the Royal Astronomical Society 464, 3728-3741. Abstract: 2017MNRAS.464.3728B
Gillon M, 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. Nature 533, 221-224. Abstract: 2016Natur.533..221G
Gillon M, 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. Nature 542, 456-460. Abstract: 2017Natur.542..456G
Hamill J. “Life, But Not As We Know It.” The Sun, 24 February 2017.
Hussmann H, Sotin C, Lunine J. (2015) Interiors and evolution of icy satellites. In Treatise on Geophysics, Volume 10: Physics of Terrestrial Planets and Moons, ed. G. Schubert. Elsevier B.V.
Luger R, 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.