The Radius Cliff is a Waterfall: Explaining Sub-Neptune Exoplanets with Steam Worlds
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| Format: | Preprint |
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2026
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| _version_ | 1866908829776609280 |
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| author | Chakrabarty, Aritra Mulders, Gijs D. Aguichine, Artyom Batalha, Natalie |
| author_facet | Chakrabarty, Aritra Mulders, Gijs D. Aguichine, Artyom Batalha, Natalie |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2602_11923 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | The Radius Cliff is a Waterfall: Explaining Sub-Neptune Exoplanets with Steam Worlds Chakrabarty, Aritra Mulders, Gijs D. Aguichine, Artyom Batalha, Natalie Earth and Planetary Astrophysics Solar and Stellar Astrophysics 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. |
| title | The Radius Cliff is a Waterfall: Explaining Sub-Neptune Exoplanets with Steam Worlds |
| topic | Earth and Planetary Astrophysics Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2602.11923 |