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

TL;DR

This paper uses evolution models to characterize the compositions and properties of the Kepler-9 planetary system, revealing similar heavy element fractions in the gas giants and constraining the rocky nature of the super-Earth.

Contribution

It introduces detailed evolution modeling to infer exoplanet compositions and provides new constraints on the internal structure of the Kepler-9 planets.

Findings

Kepler-9b contains approximately 45 Earth masses of hydrogen and helium.

Kepler-9c contains approximately 31 Earth masses of hydrogen and helium.

Kepler-9d is likely a rocky planet with less than 0.1% hydrogen and helium.

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 $45^{+17}_{-12}$\,\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.