Evolutionary Analysis of Gaseous Sub-Neptune-mass Planets with MESA

Sub-Neptune-sized exoplanets represent the most common types of planets in the Milky Way, yet many of their properties are unknown. Here, we present a prescription to adapt the capabilities of the stellar evolution toolkit Modules for Experiments in Stellar Astrophysics to model sub-Neptune-mass planets with H/He envelopes. With the addition of routines treating the planet core luminosity, heavy-element enrichment, atmospheric boundary condition, and mass-loss due to hydrodynamic winds, the evolutionary pathways of planets with diverse starting conditions are more accurately constrained. Using these dynamical models, we construct mass-composition relationships of planets from 1-400 M⊕ and investigate how mass-loss impacts their composition and evolution history. We demonstrate that planet radii are typically insensitive to the evolution pathway that brought the planet to its instantaneous mass, composition and age, with variations from hysteresis ≾2%. We find that planet envelope mass-loss timescales, τ_(env), vary non-monotonically with H/He envelope mass fractions (at fixed planet mass). In our simulations of young (100 Myr) low-mass (M_p ≾ 10M⊕) planets with rocky cores, τ_(env) is maximized at M_(env)/M_p = 1% to 3%. The resulting convergent mass-loss evolution could potentially imprint itself on the close-in planet population as a preferred H/He mass fraction of ~1%. Looking ahead, we anticipate that this numerical code will see widespread applications complementing both 3D models and observational exoplanet surveys