The impact of escaping hydrogen atmospheres on super-Earth interiors

Hydrogen plays a central role in dictating the evolution of small, close-in exoplanets. For super-Earths, their primordial hydrogen-dominated atmospheres are nearly completely stripped through atmospheric escape processes such as photoevaporation, or core-powered mass-loss. However, prior to this, the hydrogen atmospheres will leave their imprint on planetary interiors via chemical reactions with molten magma ocean surfaces. In this work, we combine atmospheric evolution models with novel global chemical equilibrium models to show that hydrogen is efficiently sequestered into the planet’s interior. Once the hydrogen has escaped, we show that super-Earth bulk interiors are under-dense when compared to Earth, consistent with mass measurements of planetary systems such as TRAPPIST-1. As a natural consequence of chemical equilibrium, we predict the production of steam-dominated remnant atmospheres, which are now potentially testable in the era of JWST. We use these models to place powerful, population-level constraints on the total water content of super-Earths, leading to implications on their formation pathways. Finally, we discuss the structural impact of sequestered hydrogen in the interiors of rocky super-Earths, specifically the potential delay of iron core differentiation, and the possible implications this has on long-lived planetary magnetic fields and habitability.