Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation

TL;DR

This study combines planetary formation models with atmospheric escape simulations to statistically analyze how evaporation shapes the size and radius distribution of exoplanets, revealing a bimodal distribution and an evaporation valley.

Contribution

It introduces a coupled approach of population synthesis and atmospheric escape modeling to explain observed planetary size distributions and the formation of the evaporation valley.

Findings

Evaporation creates a bimodal planetary size distribution.

An 'evaporation valley' separates bare cores from planets with primordial atmospheres.

Size distribution is insensitive to heating efficiency except in extreme cases.

Abstract

We apply hydrodynamic evaporation models to different synthetic planet populations that were obtained from a planet formation code based on a core-accretion paradigm. We investigated the evolution of the planet populations using several evaporation models, which are distinguished by the driving force of the escape flow (X-ray or EUV), the heating efficiency in energy-limited evaporation regimes, or both. Although the mass distribution of the planet populations is barely affected by evaporation, the radius distribution clearly shows a break at approximately 2 $R_{\oplus}$. We find that evaporation can lead to a bimodal distribution of planetary sizes (Owen & Wu 2013) and to an "evaporation valley" running diagonally downwards in the orbital distance - planetary radius plane, separating bare cores from low-mass planet that have kept some primordial H/He. Furthermore, this bimodal distribution is related to the initial characteristics of the planetary populations because low-mass planetary cores can only accrete small primordial H/He envelopes and their envelope masses are proportional to their core masses. We also find that the population-wide effect of evaporation is not sensitive to the heating efficiency of energy-limited description. However, in two extreme cases, namely without evaporation or with a 100\% heating efficiency in an evaporation model, the final size distributions show significant differences; these two scenarios can be ruled out from the size distribution of $Kepler$ candidates.