To Cool is to Keep: Residual H/He Atmospheres of Super-Earths and sub-Neptunes

Current theory predicts that observed rocky super-Earths accreted large nebular hydrogen/helium envelopes before disk dispersal. These atmospheres have since been mostly lost through hydrodynamic outflows. Such super-Earth atmospheres may soon be observable, but their mass, composition, and redox state resulting from their evolution are largely unexplored, despite these processes’ potential impact on habitability. I will present the observable outcomes of the evolution of super-Earths from their initial states since disk dispersal. Using theoretical models, I will demonstrate that loss of the primordial atmosphere can be incomplete, leading to a thin residual H/He envelope. The masses of these remnant atmospheres vary by orders of magnitude depending on the planet’s mass and the flux it receives from its host star. Super-Earths finish mass loss with atmospheric masses ranging from 10^-9 to 10^-2 planet masses for typical parameters. I will discuss the implications of this residual hydrogen for subsequent secondary atmospheres, including their masses, composition, and observational signatures.