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

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 $45_{-12}^{+17}$\,\mearth\ (resp. $31_{-10}^{+13}$\,\mearth) of hydrogen and helium and $35_{-15}^{+10}$\,\mearth (resp. $24_{-12}^{+10}$\,\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 $Z_{\mathrm{b}}/Z_{\mathrm{c}}=1.02\pm 0.14$, 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.

5 pages + 7 pages of online material ; revised article submitted to A\&A and accepted on March 30