Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis

TL;DR

This paper explains the high abundance of sub-Neptune exoplanets with radii 2-3 Earth radii by proposing a magma-atmosphere sequestration mechanism that halts further atmospheric growth, supported by a simple model.

Contribution

It introduces a novel magma-atmosphere sequestration model to explain the sub-Neptune radius distribution, highlighting a new physical process affecting planetary atmospheres.

Findings

High base-of-atmosphere pressure causes atmosphere dissolution into magma.

Sequestration acts as a brake on planetary atmospheric growth.

Supports extensive magma-atmosphere equilibration on sub-Neptunes.

Abstract

Transiting planets with radii 2-3 $R_\bigoplus$ are much more numerous than larger planets. We propose that this drop-off is so abrupt because at $R$ $\sim$ 3 $R_\bigoplus$, base-of-atmosphere pressure is high enough for the atmosphere to readily dissolve into magma, and this sequestration acts as a strong brake on further growth. The viability of this idea is demonstrated using a simple model. Our results support extensive magma-atmosphere equilibration on sub-Neptunes, with numerous implications for sub-Neptune formation and atmospheric chemistry.