The role of interior-atmosphere partitioning of hydrogen in shaping the radius cliff in the exoplanet frequency-radius distribution

The radius cliff is a prominent feature in the radius–occurrence distribution of close-in exoplanets, yet its physical origin remains uncertain. Understanding this feature is crucial for constraining the formation and evolutionary pathways of sub-Neptune planets, which constitute the majority of the observed exoplanet population. Proposed explanations include runaway gas accretion, atmospheric mass loss, and the dissolution of hydrogen into the planetary interior. However, none of these mechanisms has yet reproduced the observed radius cliff robustly. The hydrogen dissolution hypothesis is currently limited by uncertainties in the thermochemical properties of materials under the high-pressure and high-temperature conditions characteristic of sub-Neptune interiors.

We develop equilibrium chemistry models of sub-Neptunes that incorporate hydrogen dissolution into their magma mantles, informed by constraints from recent high-pressure, high-temperature laboratory experiments. The primary objective is to assess whether hydrogen partitioning between the interior and the atmosphere can account for the observed radius cliff and to quantify its role relative to competing formation and evolution scenarios.

Our results show that a large fraction of total hydrogen partitions into the interior, thereby increasing atmospheric metallicity, which can be constrained observationally. We also evaluate the effect of H2-H2O miscibility in the atmosphere on the partitioning of hydrogen. These results will improve the physical interpretation of exoplanet population statistics and contribute to a more unified theoretical framework for explaining the observed radius distribution.