The Radius Cliff is a Waterfall: Explaining Sub-Neptune Exoplanets with Steam Worlds

TL;DR

This paper proposes that the radius valley of Kepler exoplanets can be explained by the presence of water-rich steam worlds, supported by interior structure models, formation pathways, and a Bayesian mixture model, highlighting the role of water worlds in planetary demographics.

Contribution

It introduces a new water-rich planet formation model that reproduces the radius valley and period distributions, and combines it with a Bayesian framework to analyze Kepler planet populations.

Findings

Water worlds peak at ~7 M⊕ with 41% water fraction.

The radius cliff is explained as a 'waterfall' in water-rich planet occurrence.

At least 20% of sub-Neptunes have significant H/He atmospheres.

Abstract

The demographics of Kepler planets provide a key testbed for models of planet formation and evolution, particularly for explaining the radius valley separating super-Earths and sub-Neptunes. A primordial interpretation based on differences in bulk densities -- where rocky and water-rich planets form via migration pathways -- offers an alternative to atmospheric loss scenarios. Updated interior structure models of water worlds with adiabatic steam atmospheres reproduce the observed valley near $\sim2~R_\oplus$ more accurately. Furthermore, migration models from our Genesis library suggest that these formation pathways can also account for the distinct period distributions of super-Earths and sub-Neptunes, as well as the emergence of the hot Neptune desert. Motivated by this, we develop a Bayesian hierarchical mixture model for close-in Kepler planets ($P<100$ days), combining rocky planets and water worlds without H/He envelopes. The inferred mass distributions of rocky and water-rich planets peak at $\sim2.6~M_\oplus$ and $\sim7~M_\oplus$, respectively, with the water mass fraction of water worlds peaking at $\sim41\%$. Water worlds provide a good representation of the Kepler sub-Neptune population, with the radius cliff emerging as a ``waterfall" -- a sharp decline in their occurrence. However, our mass-radius analysis shows that water worlds alone cannot explain planets with $R \gtrsim 3~R_\oplus$, implying that at least $\sim20\%$ of sub-Neptunes in the sample are enriched in H/He gas.