Institute of Astronomy

Most 1.6 Earth-Radii Planets are not Rocky

SpeakerTalk DateTalk Series
Leslie Rogers28 July 2014Across HR 2014 Posters


The Kepler Mission, combined with ground based radial velocity follow-up and TTV dynamical analyses, has revolutionized the observational constraints on sub-Neptune-size planet compositions. The results of an extensive Kepler follow-up program, which includes multiple Doppler measurements for 22 KOIs (Marcy et al. 2014), more than double the population of sub-Neptune-sized transiting planets with radial velocity mass constraints. This unprecedentedly large and homogeneous sample of planets with both mass and radius constraints opens the possibility of a statistical study of the underlying population of planet compositions. We focus on the intriguing transition between rocky exoplanets (comprised of iron and silicates) and planets with voluminous layers of volatiles (H/He and astrophysical ices). Applying a hierarchical Bayesian statistical approach to the sample of Kepler transiting sub-Neptune planets with Keck radial velocity follow-up, we constrain the fraction of planets that are dense enough to be rocky (as a function of planet radius). We show, with 95% statistical confidence, that the majority of 1.6 Earth-radii planets are too low density to be comprised of Fe and silicates alone. At larger radii, the constraints on the fraction of rocky planets are even more stringent. These insights into the size demographics of rocky and volatile-rich planets offer empirical constraints for planet formation theories, and guide the range of planet radii to be considered in studies of the occurrence rate of ""Earth-like"" planets.