Do Rocky Planets around M Stars Have Atmospheres? A Statistical Approach to the Cosmic Shoreline

Abstract Answering the question “do rocky exoplanets around M dwarfs have atmospheres?” is a key science question to be answered with JWST, with a large community effort aimed to address it, including 500 hr of Director’s Discretionary Time. Theoretically, the so-called “Cosmic Shoreline” developed for the solar system may not hold around M dwarfs unchanged due to their harsher extreme-UV (XUV) environment, possibly resulting in most M dwarf rocky planets lacking significant atmospheres—a hypothesis that remains to be statistically tested empirically through judicious target selection. We identify target selection as a combinatorial optimization problem (“knapsack problem”). We develop a statistical framework to test population-level hypotheses from observations and combine a formation and evolution model, 1D-RCE atmosphere model, and genetic algorithm to simulate populations and find the optimized set of observations. We find that, first, if all rocky planets around M dwarfs are indeed bare rocks, JWST can efficiently place an upper bound on the atmosphere occurrence rates to less than one in eight, even without optimized target selection, but further improvements are cost-prohibitive. Second, if the Cosmic Shoreline hypothesis (XUV or bolometric) holds true for M dwarfs, strong evidence (ΔBIC> 5) can be found within ∼500 observing hours using the optimal strategy of a “wide and shallow” approach. Our statistical framework can be directly applied to upcoming observations to robustly identify the Cosmic Shoreline and to optimize target selection for determining other trends in exoplanet atmosphere observations, including those from future missions.