شلال من الكواكب: لغز اختفاء العوالم المائية سريع

الملخص الأصلي

كما كتبه المؤلفون، بالإنجليزية arXiv:2602.11923

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.