An Evolving Cosmic Shoreline and Sandbar Bounding the Rocky Airless Valley

Recent JWST observations challenge the traditional “cosmic shoreline” from both sides, revealing thick volatile atmospheres on the hottest close-in “lava worlds,” where irradiation should drive the most extreme escape, and bare rocky surfaces on cooler terrestrial planets around M dwarfs, where atmospheres would be expected to survive. Using a coupled atmosphere-interior evolution model, we show that atmosphere retention is governed not by a single escape boundary but by two: a hot, outgassing-regulated “cosmic sandbar” and a cooler, escape-regulated “cosmic shoreline,” separated by an “airless valley” that may mark a graveyard of stripped sub-Neptune cores. The sandbar arises because long-lived magma oceans, sustained further by tidal heating from secular eccentricity excitation in multi-planet systems, keep most volatiles dissolved and expose only a small atmospheric reservoir to escape, whereas cooler planets solidify, sequestering volatiles in the deep solid mantle while overexposing the rest to loss. This two-regime structure recasts the single cosmic shoreline as two boundaries set by distinct physics: outgassing and escape. We provide time-evolving fits for both boundaries across G, K, and M stellar types as a function of volatile inventory, planetary mass, age, and tidal heating. Lava worlds with thick atmospheres are unlikely around stars cooler than K-type unless sustained by extreme tidal and/or other interior heating. This framework links atmosphere survival from ultra-short-period planet lava worlds to habitable zone planets, informing target selection and interpretation for TRAPPIST-1 and JWST Director’s Discretionary Time characterization.