Institute of Astronomy

How Rocky Are They? The Composition Distribution of Kepler's Sub-Neptune Planet Candidates within 0.15 AU

SpeakerTalk DateTalk Series
Angie Wolfgang1 August 2014Across HR 2014 Talks


The Kepler Mission has found thousands of planetary candidates with radii between 1 and 4 Rearth. As these planets have no analogues in our own Solar System, they provide an unprecedented opportunity to understand the range and distribution of planetary compositions allowed by planet formation and evolution. Although a mass measurement is usually required to constrain the possible compositions of an individual super-Earth-sized planet, we can address this question for the Kepler sample without them. This is possible with a statistical approach that leverages interior structure models which, by accounting for the thermal evolution of a gaseous envelope around a rocky core, yield radii largely independent of mass. In particular, we apply Hierarchical Bayesian Modeling to a complete subsample of Kepler planet candidates to find the current-day composition distribution, which shows that gaseous envelopes are most likely to be ~ 1% of the planet's total mass. We also address the gaseous/rocky transition and illustrate its sensitivity to the uncertainty in the planets' radii. Finally, we illustrate that this composition distribution does not result in a one-to-one relationship between the population of sub-Neptune masses and radii; accordingly, dynamical studies which wish to use Kepler data must adopt a probabilistic approach to accurately represent the range of possible masses at a given radius.