Core-envelope miscibility in sub-Neptunes and super-Earths

Sub-Neptunes and super-Earths, the most abundant types of planet in the galaxy, are unlike anything in the Solar System, with radii between those of Earth and Neptune^1,2. Fundamental questions remain regarding their structure and origin. Although super-Earths have a rocky composition³, sub-Neptunes form a distinct population at larger radii and are thought to consist of a rocky core overlain by a hydrogen-rich envelope^4,5. At the extreme conditions of the core-envelope interface (exceeding several gigapascals and several thousand kelvin^4,6), reaction between core and envelope seems possible, but the nature and extent of these reactions are 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 plausible 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 observable with ongoing or future missions. Core-envelope miscibility profoundly affects the evolution of sub-Neptunes and super-Earths, by dissolving a large fraction of the hydrogen of the planet in the core and driving exchange of hydrogen between core and envelope as the planet evolves.