Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds

TL;DR

This study challenges the idea that water-rich atmospheres on sub-Neptunes originate from ice accretion beyond the snow line, showing instead that interior-atmosphere interactions in hydrogen-poor planets formed inside the snow line produce H2O-dominated envelopes.

Contribution

The paper introduces a new model of chemical equilibrium in sub-Neptunes, demonstrating that water-rich atmospheres are more likely to form inside the snow line through interior processes, not ice accretion.

Findings

Most water initially accreted is destroyed by interior-atmosphere interactions.

H2O-dominated atmospheres are found only on planets formed inside the snow line.

Water-rich envelopes are mostly miscible with H2, making separate water layers unlikely.

Abstract

Recent claims of biosignature gases in sub-Neptune atmospheres have renewed interest in water-rich sub-Neptunes with surface oceans, often referred to as Hycean planets. These planets are hypothesized to form beyond the snow line, accreting large amounts of H$_2$O (>10 wt%) before migrating inward. However, current interior models often neglect chemical equilibration between primordial atmospheres and molten interiors. Here, we compute global chemical equilibrium states for a synthetic population of sub-Neptunes with magma oceans. Although many initially accrete 5-30 wt% water, interior-atmosphere interactions destroy most of it, reducing final H$_2$O mass fractions to below 1.5 wt%. As a result, none meet the threshold for Hycean planets. Despite that, we find H$_2$O-dominated atmospheres exclusively on planets that accreted the least ice. These planets form inside the snow line, are depleted in carbon and hydrogen, and develop small envelopes with envelope mass fractions below 1%, dominated by endogenic water. In contrast, planets formed beyond the snow line accrete more volatiles, but their water is largely converted to H$_2$ gas or sequestered into the interior, resulting in low atmospheric H$_2$O mass fractions. Most H$_2$O-rich envelopes are also fully miscible with H$_2$, making a separate water layer unlikely. Our results topple the conventional link between ice accretion and water-rich atmospheres, showing instead that H$_2$O-dominated envelopes emerge through chemical equilibration in hydrogen-poor planets formed inside the snow line.