Constraining Water World Populations from Exoplanet Demographics

Water worlds have been hypothesized as an alternative to photo-evaporation to explain the gap in the radius distribution of exoplanets. We explore water worlds within the framework of a joint mass-radius-period distribution of planets. We employ hierarchical Bayesian modeling to create a range of mixture models that include multiple compositional subpopulations of exoplanets. We model these subpopulations — including planets with gaseous envelopes, evaporated rocky cores, evaporated icy cores, intrinsically rocky planets, and intrinsically icy planets — in various combinations to assess which combinations are most favored by the data. Using cross-validation, we evaluate the support for models that include planets with icy compositions compared to the support for models that do not. We present current Kepler-based constraints on the overall contribution of each compositional sub-population to the total planet distribution and offer projections for TESS and PLATO’s capabilities to distinguish between water worlds and planets with primordial envelopes. We conclude with a thorough investigation of the population-level degeneracies that persist when interpreting a statistical sample of planet mass, radius, and orbital period measurements.