Unveiling the Planet Population at Birth

TL;DR

This study uses hierarchical inference and photoevaporation models to analyze the distribution of close-in exoplanets, revealing details about their core composition, mass, and atmospheric history, and challenging existing formation theories.

Contribution

It introduces a hierarchical inference approach with forward-models to constrain exoplanet core and atmospheric properties, providing new insights into planet formation and evolution.

Findings

Core-mass distribution peaks at ~4 Earth masses.

Bulk core composition is Earth-like and ice-poor.

Photoevaporation explains the majority of super-Earths formation.

Abstract

The radius distribution of small, close-in exoplanets has recently been shown to be bimodal. The photoevaporation model predicted this bimodality. In the photoevaporation scenario, some planets are completely stripped of their primordial H/He atmospheres, whereas others retain them. Comparisons between the photoevaporation model and observed planetary populations have the power to unveil details of the planet population inaccessible by standard observations, such as the core mass distribution and core composition. In this work, we present a hierarchical inference analysis on the distribution of close-in exoplanets using forward-models of photoevaporation evolution. We use this model to constrain the planetary distributions for core composition, core mass and initial atmospheric mass fraction. We find that the core-mass distribution is peaked, with a peak-mass of $\sim 4$M$_\oplus$. The bulk core-composition is consistent with a rock/iron mixture that is ice-poor and ``Earth-like''; the spread in core-composition is found to be narrow ($\lesssim 16\%$ variation in iron-mass fraction at the 2$\sigma$ level) and consistent with zero. This result favours core formation in a water/ice poor environment. We find the majority of planets accreted a H/He envelope with a typical mass fraction of $\sim 4\%$; only a small fraction did not accrete large amounts of H/He and were ``born-rocky''. We find four-times as many super-Earths were formed through photoevaporation, as formed without a large H/He atmosphere. Finally, we find core-accretion theory over-predicts the amount of H/He cores would have accreted by a factor of $\sim 5$, pointing to additional mass-loss mechanisms (e.g. ``boil-off'') or modifications to core-accretion theory.