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Evolving Gas-Dwarf Exoplanets Including Dissolution of Hydrogen into their Rocky Cores

Presentation #619.04 in the session Interior Structure Modeling.

Published onApr 03, 2024
Evolving Gas-Dwarf Exoplanets Including Dissolution of Hydrogen into their Rocky Cores

Gas-dwarfs are low-mass planets with rocky cores and primordial hydrogen-helium envelopes. Most gas-dwarfs have massive magma oceans that are in direct contact with their atmospheres for timescales of Gyr. While the magma persists, the interface between the core and the atmosphere on warm gas-dwarfs is both chemically reactive and permeable. We have extended the open-source Modules for Experiments in Stellar Astrophysics (MESA) code to account for the dissolution of hydrogen into the rocky cores of gas-dwarf planets. We have included into MESA a sophisticated model for rocky cores that calculates both the depth of the magma ocean and the overall core radius and adjusts these properties at each timestep as the planet cools and the pressure-temperature conditions at the core-envelope boundary change. With a model for the solubility of hydrogen in the magma as a function of pressure and temperature, our new routines track the amount of hydrogen dissolved in the core and adjust the mass of the planet envelope as hydrogen dissolves into/exsolves out of the core as the planet evolves. We find that including the dissolution of hydrogen in the core has an order unity effect on the model gas-dwarf planet radii, and may cause the radii of some sub-Neptune-size planets to increase over time. The core hydrogen reservoir has so far been neglected in studies interpreting the slope of the radius valley; inferences drawn from comparisons to gas-dwarf planet evolution models that assume inert cores may be suspect. By combining our MESA extensions treating the dissolution of hydrogen in the planet core with extensions modeling atmospheric escape from the planet—due to both photoevaporation and core-powered mass loss—we quantify how these two processes together sculpt super-Earth and gas-dwarf planet populations.

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