The multiple planets transiting Kepler-9 I. Inferring stellar properties and planetary compositions

Mathieu Havel (1) , Tristan Guillot (1) , Diana Valencia (1, 2) , Aurélien Crida (1)

1 CASSIOPEE - Laboratoire de Cosmologie, Astrophysique Stellaire & Solaire, de Planétologie et de Mécanique des Fluides
2 EAPS - Department of Earth, Atmospheric and Planetary Sciences [MIT, Cambridge]

The discovery of multiple transiting planetary systems offers new possibilities for characterising exoplanets and understanding their formation. The Kepler-9 system contains two Saturn-mass planets, Kepler-9b and 9c. Using evolution models of gas giants that reproduce the sizes of known transiting planets and accounting for all sources of uncertainties, we show that Kepler-9b (respectively 9c) contains $45^{+17}_{-12}$\,\mearth\ (resp. $31^{+13}_{-10}$\,\mearth) of hydrogen and helium and $35^{+10}_{-15}$\,\mearth (resp. $24^{+10}_{-12}$\,\mearth) of heavy elements. More accurate constraints are obtained when comparing planets 9b and 9c: the ratio of the total mass fractions of heavy elements are $Z_{\rm b}/Z_{\rm c}=1.02\pm 0.14$, indicating that, although the masses of the planets differ, their global composition is very similar, an unexpected result for formation models. Using evolution models for super-Earths, we find that Kepler-9d must contain less than 0.1\% of its mass in hydrogen and helium and predict a mostly rocky structure with a total mass between 4 and 16\,\mearth.

Mots clés: Star: individual: Kepler-9; (Stars:) planetary systems; Planets and satellites: physical evolution

Domaines: Planétologie et astrophysique de la terre [astro-ph.EP]; Planétologie et astrophysique de la terre [astro-ph.EP]; Planétologie

Mathieu Havel, Tristan Guillot, Diana Valencia, Aurélien Crida. The multiple planets transiting Kepler-9 I. Inferring stellar properties and planetary compositions. Astronomy and Astrophysics - A&A, 2011, Kepler-9 paper I. ⟨hal-00578529v2⟩