The Loss of Primary Atmospheres Does Not Preclude Habitability: A Self-consistent Model of the Transition from Sub-Neptune to Terrestrial Atmospheres

Arguably, the most interesting result to emerge from exoplanet demographic studies in the last ~10 years is that there is a bimodal distribution in planet size separating genuinely rocky planets from volatile rich “sub-Neptunes”. Moreover, both types of planets seemingly derive from the same volatile-rich parent population: rocky planets arise from the complete erosion of primary (H₂-rich) atmospheres, whereas sub-Neptunes have retained their primary atmospheres. This formation pathway for terrestrial exoplanets is different from that of the Earth and other solar system terrestrial planets, for which there is no evidence of prolonged initial H₂-rich envelopes, and limited evidence for accretion of nebular volatiles. Distinct formation pathways for solar system terrestrials and exoplanets raise an important question: how might the presence of a long-lived primary atmosphere affect the subsequent evolution and habitability prospects of rocky exoplanets? Here, we answer this question with a new, self-consistent evolutionary model of the transition from primary to secondary atmospheres. The model incorporates magma ocean solidification, radiative-convective climate, thermal escape, mantle redox evolution, and the consequent speciation of Fe, C, O, H-bearing species between the atmosphere and interior. For our illustrative case study TRAPPIST-1, our model strongly favors atmosphere retention for the habitable zone planet TRAPPIST-1e. In contrast, the same model predicts a comparatively thin atmosphere for the Venus-analog TRAPPIST-1b, which would be vulnerable to complete erosion via non-thermal escape. This is consistent with JWST thermal emission observations which suggest that TRAPPIST-1b is a bare rock or has a thin atmosphere. More broadly, we conclude that the erosion of primary atmospheres typically does not preclude surface habitability, and frequently results in large surface water inventories due to the reduction of FeO by H₂. If most now-terrestrial planets formed with sizeable H₂-envelopes, then habitable zone waterworlds ought to be a common outcome of rocky planet evolution.