Planet radii of Kepler Object of Interest

These planets have no analogues in our own solar system, providing an unprecedented opportunity to understand the range and distribution of planetary compositions allowed by planet formation and evolution. A precise mass measurement is usually required to constrain the possible composition of an individual super-Earth-sized planet, but these measurements are difficult and expensive to make for the majority of Kepler planet candidates (PCs).

Fortunately, adopting a statistical approach helps us to address this question without them. In particular, we apply hierarchical Bayesian modeling to a subsample of Kepler PCs that is complete for P<25 days and R_pl_>1.2R_{Earth}_ and draw upon interior structure models that yield radii largely independent of mass by accounting for the thermal evolution of a gaseous envelope around a rocky core. Assuming the envelope is dominated by hydrogen and helium, we present the current-day composition distribution of the sub-Neptune-sized planet population and find that H+He envelopes are most likely to be ~1% of these planets' total masses with an intrinsic scatter of +/-0.5 dex.