Enhanced constraints on the interior composition and structure of terrestrial exoplanets.
2018
Exoplanetinterior modelling usually makes the assumption that the elemental abundances of a
planetare identical to those of its host star. Host stellar abundances are good proxies of planetary abundances, but only for refractory elements. This is particularly true for
terrestrial planets, as evidenced by the relative differences in bulk chemical composition between the Sun and the Earth and other inner solar system bodies. The elemental abundances of a
planethost star must therefore be devolatilised in order to correctly represent the bulk chemical composition of its
terrestrial planets. Furthermore, nickel and light elements make an important contribution alongside iron to the core of
terrestrial planets. We therefore adopt an extended chemical network of the core, constrained by an Fe/Ni ratio of 18 $\pm$ 4 (by number). By applying these constraints to the Sun, our modelling reproduces the composition of the mantle and core, as well as the core mass fraction of the Earth. We also apply our modelling to four
exoplanethost stars with precisely measured elemental abundances:
Kepler-10,
Kepler-20,
Kepler-21 and
Kepler-100. If these stars would also host
terrestrial planetsin their habitable zone, we find that such
planetsorbiting
Kepler-21 would be the most Earth-like, while those orbiting
Kepler-10 would be the least. To assess the similarity of a rocky
exoplanetto the Earth in terms of interior composition and structure, high-precision host stellar abundances are critical. Our modelling implies that abundance uncertainties should be better than $\sim$ 0.04 dex for such an assessment to be made.
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