The multiple planets transiting Kepler-9

I. Inferring stellar properties and planetary compositions^⋆

¹ Université de Nice-Sophia Antipolis, CNRS UMR 6202, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
e-mail: mathieu.havel@oca.eu
² Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Ave, Cambridge, MA, 02139, USA

Received: 25 February 2011
Accepted: 30 March 2011

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  M_⊕  (resp.  M_⊕) of hydrogen and helium and  M_⊕  (resp.  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 Z_b/Z_c = 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_⊕.