To cool is to keep: residual H/He atmospheres of super-Earths

Super-Earths will constitute a large portion of the small exoplanets well-suited for detailed atmospheric characterization that TESS aims to discover. Current theory predicts these planets accreted large nebular hydrogen/helium envelopes before disk dispersal, which have since been mostly lost through hydrodynamic outflows. The effects of this early evolution on super-Earths’ long-term atmospheric mass, composition, and redox state are largely unexplored, despite potential ramifications for both the habitability and atmospheric observability of this common class of planets. I present the observable outcomes of the atmospheric evolution of super-Earths undergoing core-powered mass loss. Using theoretical models, I demonstrate that loss of the primordial atmosphere can be incomplete, leading to a small residual H/He envelope. The masses of these remnant atmospheres vary by orders of magnitude depending on the planet's mass and the flux received from its host star. Super-Earths finish mass loss with retained atmospheric masses ranging from 10^-8 to 10^-3 planet masses for typical super-Earth parameters. I discuss how this residual hydrogen affects the composition and enhances potential observational signatures of these atmospheres.