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Register a new userConstraints on Super-Earths Interiors from Stellar Abundances
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Modeling the interior of exoplanets is essential to go further than the conclusions provided by mean density measurements. In addition to the still limited precision on the planets fundamental parameters, models are limited by the existence of degeneracies on their compositions. Here we present a model of internal structure dedicated to the study of solid planets up to ~10 Earth masses, i.e. Super-Earths. When the measurement is available, the assumption that the bulk Fe/Si ratio of a planet is similar to that of its host star allows us to significantly reduce the existing degeneracy and more precisely constrain the planets composition. Based on our model, we provide an update of the mass-radius relationships used to provide a first estimate of a planets composition from density measurements. Our model is also applied to the cases of two well-known exoplanets, CoRoT-7b and Kepler-10b, using their recently updated parameters. The core mass fractions of CoRoT-7b and Kepler-10b are found to lie within the 10-37% and 10-33% ranges, respectively, allowing both planets to be compatible with an Earth-like composition. We also extend the recent study of Proxima Centauri b, and show that its radius may reach 1.94 Earth radii in the case of a 5 Earth masses planet, as there is a 96.7% probability that the real mass of Proxima Centauri b is below this value.
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The latest solar atmosphere models include non-LTE corrections and 3D hydrodynamic convection simulations. These models predict a significant reduction in the solar metal abundance, which leads to a serious conflict between helioseismic data and the predictions of solar interiors models. We demonstrate that the helioseismic constraints on the surface convection zone depth and helium abundance combined with stellar interiors models can be used to define the goodness of fit for a given chemical composition. After a detailed examination of the errors in the theoretical models we conclude that models constructed with the older solar abundances are consistent (<2 sigma) with the seismic data. Models constructed with the proposed new low abundance scale are strongly disfavored, disagreeing at the 15 sigma level. We then use the sensitivity of the seismic properties to abundance changes to invert the problem and infer a seismic solar heavy element abundance mix with two components: meteoritic abundances, and the light metals CNONe. Seismic degeneracies between the best solutions for the elements arise for changes in the relative CNONe abundances and their effects are quantified. We obtain Fe/H=7.50+/-0.045+/-0.003(CNNe) and O/H=8.86+/-0.041+/-0.025(CNNe) for the relative CNNe in the GS98 mixture. The inferred solar oxygen abundance disagree with the abundance inferred from the 3D hydro models. Changes in the Ne abundance can mimic changes in O for the purposes of scalar constraints.Models constructed with low oxygen and high neon are inconsistent with the solar sound speed profile. The implications for the solar abundance scale are discussed.
Hot super-Earths likely possess minimal atmospheres established through vapor saturation equilibrium with the ground. We solve the hydrodynamics of these tenuous atmospheres at the surface of Corot-7b, Kepler 10b and 55 Cnc-e, including idealized treatments of magnetic drag and ohmic dissipation. We find that atmospheric pressures remain close to their local saturation values in all cases. Despite the emergence of strongly supersonic winds which carry sublimating mass away from the substellar point, the atmospheres do not extend much beyond the day-night terminators. Ground temperatures, which determine the planetary thermal (infrared) signature, are largely unaffected by exchanges with the atmosphere and thus follow the effective irradiation pattern. Atmospheric temperatures, however, which control cloud condensation and thus albedo properties, can deviate substantially from the irradiation pattern. Magnetic drag and ohmic dissipation can also strongly impact the atmospheric behavior, depending on atmospheric composition and the planetary magnetic field strength. We conclude that hot super-Earths could exhibit interesting signatures in reflection (and possibly in emission) which would trace a combination of their ground, atmospheric and magnetic properties.
We investigate equilibrium chemistry between a metal-core, a silicate-mantle, and a hydrogen-rich atmosphere (reactive core model) using 18 independent reactions among 25 phase components for sub-Neptune-like exoplanets. We find hydrogen and oxygen typically comprise 1-2% and ~10% by weight of the metal-core, respectively, leading to under-dense cores and thereby offering a possible alternative explanation for the densities of the Trappist-1 planets. In addition, hydrogen occurs at about 0.1% by mass in the silicate mantle, setting a maximum limit to the hydrogen-budget for out-gassing by future super-Earths. The total hydrogen-budget of most sub-Neptunes can be, to first order, well estimated from their atmospheres alone, as more than ~93% of all H resides in their atmospheres. However, reactions with the magma ocean produce significant amounts of SiO and H_2O in the atmospheres which increase the mean molecular weight averaged over the whole atmosphere, by about a factor of two, to ~4 amu. We also investigated the case where metal is excluded from the equilibrium chemistry (unreactive core model). In this case, we find most noticeably that, as the hydrogen mass fraction is reduced from 2% to 1%, the atmosphere becomes water dominated and large fractions of H are absorbed by the magma. As water dominated atmospheres appear inconsistent with observations, we conclude that either the unreactive core model does not apply to sub-Neptunes and that their evolution is better described by a reactive core, or that in-gassing of hydrogen into the mantle is much less efficient than permitted by equilibrium chemistry.
Ariel will mark the dawn of a new era as the first large-scale survey characterising exoplanetary atmospheres with science objectives to address fundamental questions about planetary composition, evolution and formation. In this study, we explore the detectability of atmospheres vaporised from magma oceans on dry, rocky Super-Earths orbiting very close to their host stars. The detection of such atmospheres would provide a definitive piece of evidence for rocky planets but are challenging measurements with currently available instruments due to their small spectral signatures. However, some of the hottest planets are believed to have atmospheres composed of vaporised rock, such as Na and SiO, with spectral signatures bright enough to be detected through eclipse observations with planned space-based telescopes. In this study, we find that rocky super-Earths with a irradiation temperature of 3000 K and a distance from Earth of up to 20 pc, as well as planets hotter than 3500 K and closer than 50 pc, have SiO features which are potentially detectable in eclipse spectra observed with Ariel.
We test the hypothesis that the sub-millimetre thermal emission and scattered light gaps seen in recent observations of TW Hya are caused by planet-disc interactions. We perform global three-dimensional dusty smoothed particle hydrodynamics simulations, comparing synthetic observations of our models with dust thermal emission, CO emission and scattered light observations. We find that the dust gaps observed at 24 au and 41 au can be explained by two super-Earths ($sim 4 mathrm{M}_{oplus}$). A planet of approximately Saturn-mass can explain the CO emission and the depth and width of the gap seen in scattered light at 94 au. Our model produces a prominent spiral arm while there are only hints of this in the data. To avoid runaway growth and migration of the planets we require a disc mass of $lesssim 10^{-2},mathrm{M}_{odot}$ in agreement with CO observations but 10$-$100 times lower than the estimate from HD line emission.
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