A Joint Mass-Radius-Period Distribution of Exoplanets

TL;DR

This paper develops a hierarchical Bayesian framework to jointly model the mass, radius, and period distribution of Kepler exoplanets, revealing multiple planetary populations and significantly impacting estimates of Earth-like planet occurrence.

Contribution

It introduces complex mixture models that distinguish different planetary populations and demonstrates their importance in accurately characterizing exoplanet distributions and occurrence rates.

Findings

Complex models with multiple populations fit the data best.

Accounting for envelope mass loss reduces the estimated occurrence of Earth-like planets.

Many Kepler planets may be evaporated cores, affecting extrapolation to longer periods.

Abstract

The radius-period distribution of exoplanets has been characterized by the \textit{Kepler} survey, and the empirical mass-radius relation by the subset of \textit{Kepler} planets with mass measurements. We combine the two in order to constrain the joint mass-radius-period distribution of \textit{Kepler} transiting planets. We employ hierarchical Bayesian modeling and mixture models to formulate four models with varying complexity and fit these models to the data. We find that the most complex models that treat planets with significant gaseous envelopes, evaporated core planets, and intrinsically rocky planets as three separate populations are preferred by the data and provide the best fit to the observed distribution of \textit{Kepler} planets. We use these models to calculate occurrence rates of planets in different regimes and to predict masses of \textit{Kepler} planets, revealing the model dependent nature of both. When using models with envelope mass loss to calculate $\eta_\oplus$, we find nearly an order of magnitude drop, indicating that many Earth-like planets discovered with \textit{Kepler} may be evaporated cores which do not extrapolate out to higher orbital periods. This work provides a framework for higher-dimensional studies of planet occurrence and for using mixture models to incorporate different theoretical populations of planets.