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. 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.
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