Between Steam and Stone: The Role of Volatile Delivery, Phase, and Partitioning in Setting Planetary Radii

Owing to observational advances, exoplanets are no longer treated as simple point masses in planet formation theory - their masses, radii, and thus densities are now measurable. Combined with atmospheric composition data, the characterization of exoplanets has revealed distinct populations of exoplanets, ranging from rocky super Earths to gaseous giant planets. However, the nature of the intermediate-sized sub-Neptunes remains mysterious.

In this talk, I will review why formation models predict the migration of sub-Neptunes from cold regions in the disk, implying a water-rich (or generally volatile-rich) composition. The final planetary mass and radius is only determined after thermodynamic and photoevaporative evolution over Gyr timescales. I will show how the partitioning and phase of the volatile species affects the sub-Neptune population and their transition to become a rocky planet. While a treatment of water as steam mixed with H/He is relatively successful in reproducing the observed exoplanet demographics, the expected sequestration…