The multiple planets transiting Kepler-9

Abstract

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 M⊕ (resp. 31+13-10 M⊕) of hydrogen and helium and 35+10-15 M⊕ (resp. 24+10-12 M⊕) 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 Zb/Zc = 1.02 ± 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 M⊕. Appendix is available in electronic form at http://www.aanda.org