Characterizing rocky exoplanets without atmospheres

Recent observations of the super-Earths LHS 3844b, GJ 1252b, and TRAPPIST-1b suggest many short-period rocky exoplanets do not have an atmosphere, and instead resemble bare rocks like the Moon or Mercury. These planets offer a unique opportunity to study exoplanet surface geology and probe how exoplanet surfaces are shaped by interaction with a planet’s orbital evolution, tidal heating, and other dynamics. Here we focus on two processes, tidal heating and space weathering, and simulate their impact on exoplanet observations with a new bare rock model. We show that the observed lack of large tidal heating on LHS 3844b requires that the planet must be tidally locked and its orbital eccentricity has to be less than ~0.001 (more circular than Io’s orbit), thereby giving first support to tidal evolution models for small exoplanets. Turning to space weathering, we find that LHS 3844b’s thermal emission is hotter than expected for a fresh basaltic surface, but is matched well by a surface that was darkened by space weathering, indicating that space weathering is likely a widespread process on short-period bare rock exoplanets. Our results underline that bare rock exoplanets provide a unique opportunity to study exo-geology, test tidal evolution models, and probe planetary surface processes in a novel regime.