An empirical determination of the Cosmic Shoreline

Abstract

The cosmic shoreline concept was introduced to separate planets with atmospheres from those without, by relating the cumulative X-ray and extreme-ultraviolet (XUV) instellation (integrated over the planet's lifetime) to the planetary escape velocity, using the Solar System planets to anchor the empirical relation. The exoplanet community has since attempted to refine the cosmic shoreline to provide a consistent ranking or prioritisation tool for exoplanet observations - i.e., to quickly identify which small planets are most likely to have retained an atmosphere and therefore merit expensive follow-up with facilities such as JWST or the upcoming ELTs. Here, we use an empirical approach to refine the Cosmic Shoreline concept. In particular, we used the data provided by the ExoAtmospheres database, and the NASA Exoplanet Archive, along with solar system data. We reconcile limitations in the classical shoreline definition by anchoring our Empirical Exoplanet Cosmic Shoreline (EECS) simultaneously to Mars, GJ 9827 d, L 98-59 d, GJ 3090 b, and Pi Mensae c (all having tentative atmospheric detections). The EECS exhibits a significantly steeper slope than previously theorized, while consistently categorising Solar System moons and dwarf planets according to their atmospheric properties. Applied to planets orbiting M dwarfs, the EECS suggests that a larger fraction retain atmospheres than predicted by classical models, but incorporating revised XUV fluence histories for low-mass M dwarfs reveals severe atmospheric vulnerability. We finally identify high-priority targets for the JWST Rocky Worlds survey and future ELTs observations based on their EECS positioning and Transmission/Emission Spectroscopy Metrics.