Core-Envelope Miscibility in Sub-Neptunes and Super-Earths

Sub-Neptunes and super-Earths are the most abundant types of plan- ets in the galaxy [1, 2], yet fundamental questions remain regarding their structure and evolution. Sub-Neptunes are thought to consist of a rocky core overlain by a hydrogen-rich envelope with pressure- temperature conditions at the interface exceeding several GPa and several thousand K [3, 4]. At these extreme conditions reaction between core and envelope seems possible, but the nature and extent of such reactions is unknown. Here we use first principles molecular dynamics driven by density functional theory to show that silicate and hydrogen are completely miscible over a wide range of plausi- ble core-envelope pressure-temperature conditions. We find the origin of miscibility in extensive chemical reaction between hydrogen and silicate, producing silane, SiO, and water species, which may be observ- able with ongoing or future missions. Core-envelope miscibility will profoundly affect the evolution of sub-Neptunes and super-Earths, by dissolving a large fraction of the planet’s hydrogen in the core, and driving exchange of hydrogen between core and envelope as the planet evolves.