DIX Planetary Science Seminar

Sub-Neptunes and super-Earths are the most common exoplanets, yet their absence in our Solar System makes their compositions difficult to constrain. With hydrogen envelopes that trap heat for billions of years, these planets likely sustain molten interiors. The resulting magma oceans remain in constant exchange with their atmospheres, driving chemical reactions that alter both atmospheric and interior composition. In this talk, I will first introduce our global equilibrium framework which captures chemical reactions and partitioning across silicate, metal, and gas phases. I will then show how this framework reveals that the atmospheric C/O ratio of sub-Neptunes is dominated by magma ocean–atmosphere interactions rather than by formation location. Next, I will present results on the water content of sub-Neptunes, obtained by coupling population synthesis with equilibrium chemistry, demonstrating that most accreted water is destroyed and that Hycean worlds are geochemically implausible. Finally, I will discuss recent ab initio and thermodynamic studies indicating that silicates, hydrogen, and metals become miscible at high pressures and temperatures, challenging the classical picture of sub-Neptunes as layered core–mantle–envelope planets.