Evaluating the Evidence for Water World Populations using Mixture Models

TL;DR

This study uses hierarchical Bayesian mixture models to analyze Kepler exoplanet data, assessing the evidence for water worlds and their distinguishability from other planetary compositions, with broad support found for icy subpopulations.

Contribution

It introduces a set of hierarchical mixture models to evaluate the presence and prevalence of water worlds among Kepler exoplanets, highlighting population degeneracies.

Findings

Broad support for icy planet subpopulations.

Estimated icy planet fraction up to 50%.

Degeneracies between water worlds and gaseous planets.

Abstract

Water worlds have been hypothesized as an alternative to photo-evaporation in order to explain the gap in the radius distribution of Kepler exoplanets. We explore water worlds within the framework of a joint mass-radius-period distribution of planets fit to a sample of transiting Kepler exoplanets, a subset of which have radial velocity mass measurements. We employ hierarchical Bayesian modeling to create a range of ten 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 different combinations in order 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, finding broad support for both. We find significant population-level degeneracies between subpopulations of water worlds and planets with primordial envelopes. Among models that include one or more icy-core subpopulations, we find a wide range for the fraction of planets with icy compositions, with a rough upper limit of 50%. Improved datasets or alternative modeling approaches may better be able to distinguish between these subpopulations of planets.