Sub-Neptunes Are Drier than They Seem: Rethinking the Origins of Water-rich Worlds

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 H2O (>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 H2O mass fractions to below 1.5 wt%. As a result, none meet the threshold for Hycean planets. Despite that, we find H2O-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 H2 gas or sequestered into the interior, resulting in low atmospheric H2O mass fractions. Most H2O-rich envelopes are also fully miscible with H2, making a separate water layer unlikely. Our results challenge the conventional link between ice accretion and water-rich atmospheres, showing instead that H2O-dominated envelopes emerge through chemical equilibration in hydrogen-poor planets formed inside the snow line.