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Register a new userUpdated Compositional Models of the TRAPPIST-1 Planets
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After publication of our initial mass-radius-composition models for the TRAPPIST-1 system in Unterborn et al. (2018), the planet masses were updated in Grimm et al. (2018). We had originally adopted the data set of Wang et al., 2017 who reported different densities than the updated values. The differences in observed density change the inferred volatile content of the planets. Grimm et al. (2018) report TRAPPIST-1 b, d, f, g, and h as being consistent with <5 wt% water and TRAPPIST-1 c and e has having largely rocky interiors. Here, we present updated results recalculating water fractions and potential alternative compositions using the Grimm et al., 2018 masses. Overall, we can only reproduce the results of Grimm et al., 2018 of planets b, d and g having small water contents if the cores of these planets are small (<23 wt%). We show that, if the cores for these planets are roughly Earth-sized (33 wt%), significant water fractions up to 40 wt% are possible. We show planets c, e, f, and h can have volatile envelopes between 0-35 wt% that are also consistent with being totally oxidized and lacking an Fe-core entirely. We note here that a pure MgSiO$_3$ planet (Fe/Mg = 0) is not the true lowest density end-member mass-radius curve for determining the probability of a planet containing volatiles. All planets that are rocky likely contain some Fe, either within the core or oxidized in the mantle. We argue the true low density end-member for oxidizing systems is instead a planet with the lowest reasonable Fe/Mg and completely core-less. Using this logic, we assert that planets b, d and g likely must have significant volatile layers because the end-member planet models produce masses too high even when uncertainties in both mass and radius are taken into account.
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The newly detected TRAPPIST-1 system, with seven low-mass, roughly Earth-sized planets transiting a nearby ultra-cool dwarf, is one of the most important exoplanet discoveries to date. The short baseline of the available discovery observations, however, means that the planetary masses (obtained through measurement of transit timing variations of the planets of the system) are not yet well constrained. The masses reported in the discovery paper were derived using a combination of photometric timing measurements obtained from the ground and from the Spitzer spacecraft, and have uncertainties ranging from 30% to nearly 100%, with the mass of the outermost, $P=18.8,{rm d}$, planet h remaining unmeasured. Here, we present an analysis that supplements the timing measurements of the discovery paper with 73.6 days of photometry obtained by the K2 Mission. Our analysis refines the orbital parameters for all of the planets in the system. We substantially improve the upper bounds on eccentricity for inner six planets (finding $e<0.02$ for inner six known members of the system), and we derive masses of $0.79pm 0.27 ,M_{oplus}$, $1.63pm 0.63,M_{oplus}$, $0.33pm 0.15,M_{oplus}$, $0.24^{+0.56}_{-0.24},M_{oplus}$, $0.36pm 0.12,M_{oplus}$, $0.566pm 0.038,M_{oplus}$, and $0.086pm 0.084,M_{oplus}$ for planets b, c, d, e, f, g, and h, respectively.
Recent observations of the potentially habitable planets TRAPPIST-1 e, f, and g suggest that they possess large water mass fractions of possibly several tens of wt% of water, even though the host stars activity should drive rapid atmospheric escape. These processes can photolyze water, generating free oxygen and possibly desiccating the planet. After the planets formed, their mantles were likely completely molten with volatiles dissolving and exsolving from the melt. In order to understand these planets and prepare for future observations, the magma ocean phase of these worlds must be understood. To simulate these planets, we have combined existing models of stellar evolution, atmospheric escape, tidal heating, radiogenic heating, magma ocean cooling, planetary radiation, and water-oxygen-iron geochemistry. We present MagmOc, a versatile magma ocean evolution model, validated against the rocky Super-Earth GJ 1132b and early Earth. We simulate the coupled magma ocean-atmospheric evolution of TRAPPIST-1 e, f, and g for a range of tidal and radiogenic heating rates, as well as initial water contents between 1 and 100 Earth oceans. We also reanalyze the structures of these planets and find they have water mass fractions of 0-0.23, 0.01-0.21, and 0.11-0.24 for planets e, f, and g, respectively. Our model does not make a strong prediction about the water and oxygen content of the atmosphere of TRAPPIST-1 e at the time of mantle solidification. In contrast, the model predicts that TRAPPIST-1 f and g would have a thick steam atmosphere with a small amount of oxygen at that stage. For all planets that we investigated, we find that only 3-5% of the initial water will be locked in the mantle after the magma ocean solidified.
The TRAPPIST-1 planetary system is an excellent candidate for study of the evolution and habitability of M-dwarf planets. Transmission spectroscopy observations performed with the Hubble Space Telescope (HST) suggest the innermost five planets do not possess clear hydrogen atmospheres. Here we reassess these conclusions with recently updated mass constraints and expand the analysis to include limits on metallicity, cloud top pressure, and the strength of haze scattering. We connect recent laboratory results of particle size and production rate for exoplanet hazes to a one-dimensional atmospheric model for TRAPPIST-1 transmission spectra. Doing so, we obtain a physically-based estimate of haze scattering cross sections. We find haze scattering cross sections on the order of 1e-26 to 1e-19 cm squared are needed in hydrogen-rich atmospheres for TRAPPIST-1 d, e, and f to match the HST data. For TRAPPIST-1 g, we cannot rule out a clear hydrogen-rich atmosphere. We also modeled the effects an opaque cloud deck and substantial heavy element content have on the transmission spectra. We determine that hydrogen-rich atmospheres with high altitude clouds, at pressures of 12mbar and lower, are consistent with the HST observations for TRAPPIST-1 d and e. For TRAPPIST-1 f and g, we cannot rule out clear hydrogen-rich cases to high confidence. We demonstrate that metallicities of at least 60xsolar with tropospheric (0.1 bar) clouds agree with observations. Additionally, we provide estimates of the precision necessary for future observations to disentangle degeneracies in cloud top pressure and metallicity. Our results suggest secondary, volatile-rich atmospheres for the outer TRAPPIST-1 planets d, e, and f.
Seven temperate Earth-sized exoplanets readily amenable for atmospheric studies transit the nearby ultracool dwarf star TRAPPIST-1 (refs 1,2). Their atmospheric regime is unknown and could range from extended primordial hydrogen-dominated to depleted atmospheres (refs 3-6). Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones (refs 6-8). An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in the near-infrared detectable during transits. Observations of the innermost planets have ruled out such signatures (ref 9). However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere (refs 10,11). Here, we report observations for the four planets within or near the systems habitable zone, the circumstellar region where liquid water could exist on a planetary surface (refs 12-14). These planets do not exhibit prominent spectroscopic signatures at near-infrared wavelengths either, which rules out cloud-free hydrogen-dominated atmospheres for TRAPPIST-1 d, e and f, with significance of 8, 6 and 4 sigma, respectively. Such an atmosphere is instead not excluded for planet g. As high-altitude clouds and hazes are not expected in hydrogen-dominated atmospheres around planets with such insolation (refs 15,16), these observations further support their terrestrial and potentially habitable nature.
TRAPPIST-1 (Gillon et al. 2017) is an extremely compact planetary system: seven earth-sized planets orbit at distances lower than 0.07 AU around one of the smallest M-dwarf known in the close neighborhood of the Sun (with a mass of less than 0.09 $M_odot$). With 3 planets within the classical habitable zone, this system represents an interesting observational target for future instruments such as the JWST (e.g. Barstow & Irwin 2016). As the planets are close-in, tidal interactions play a crucial role in the evolution of the system by controlling both orbital configurations and rotational states of the planets. For the closest planets, the associated tidal dissipation could have an influence on their internal evolution and potentially on their climate and habitability Turbet et al. (2018). Following (Tobie et al. 2005), we build multilayer models of the internal structure of the TRAPPIST-1 planets accounting for the mass and radius of Grimm et al. (2018), then we compute the tidal response and estimate the tidal heat flux of each planet as well as the profile of tidal heating with depth. Finally, we compare our results to the homogeneous model of Efroimsky (2012) and assess the impact heating rate on the thermal state of each layer of the planet.
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