Hydrogen-water phase separation in Neptune-like exoplanets

Abstract

Uranus and Neptune could undergo hydrogen-water phase separation upon cooling, leading to a layered internal structure. Neptune-sized exoplanets are very common, and they could also undergo demixing, which might be observable through a depletion in atmospheric water abundance. Using the interior structure model TATOOINE, I set out to test this hypothesis. I assume planets with a rocky core overlaid by an initially homogeneous H-He-H2O envelope. Using recent data on the hydrogen-water system, I simulate water rain-out in dependence of key parameters affecting the thermal structure of the interior. Demixing is most likely to occur in planets with low equilibrium temperatures and low thermal opacities. For such conditions, I estimated the resulting atmospheric water abundance. Since only a few detected Neptune-sized exoplanets are sufficiently cold for demixing, future missions like PLATO and ARIEL, as well as the ongoing JWST mission, will be crucial for testing these predictions.