The Fate of Hydrogen and Helium: From Planetary Embryos to Earth- and Neptune-like Worlds

TL;DR

This study uses first-principles simulations to explore how hydrogen and helium interact with planetary interiors, revealing their distribution and implications for planetary atmospheres and evolution.

Contribution

It provides the first detailed quantification of hydrogen and helium partitioning in planetary interiors across a range of planet sizes, linking atomic interactions to planetary-scale phenomena.

Findings

Hydrogen is siderophilic above ~25 GPa but less so beyond ~200 GPa.

Helium remains lithophilic and increasingly soluble in metal with pressure.

Most hydrogen and helium reside in planetary interiors, not atmospheres.

Abstract

Hydrogen, helium, silicates, and iron are key building blocks of rocky and gas-rich planets, yet their chemical interactions remain poorly constrained. Using first-principles molecular dynamics and thermodynamic integration, we quantify hydrogen and helium partitioning between molten silicate mantles and metallic cores for Earth-to-Neptune-mass planets. Hydrogen becomes strongly siderophilic above $\sim$25 GPa but weakens beyond $\sim$200 GPa, whereas helium remains lithophilic yet increasingly soluble in metal with pressure. Incorporating these trends into coupled structure-chemistry models suggests that majority of hydrogen and helium reside in planetary interiors, not atmospheres, with abundances strongly depending on planet mass. Such volatile exchange may influence the redox states of secondary atmospheres, longevity of primordial envelopes, predicted CHNOPS abundances, and emergence of helium-enriched atmospheres, while He 1083 nm and H Lyman-$\alpha$ lines provide potential probes of atmosphere-interior exchange. These findings link atomic-scale interactions to planetary-scale observables, providing new constraints on the origins of Earth-to-Neptune-sized worlds.