An Evolving Cosmic Shoreline and Sandbar Bounding the Rocky Airless Valley

by Keith CowingAugust 27, 2026August 27, 2026

Schematic of the three atmospheric retention and loss regimes for close-in rocky planets. Illustrated diagrams appear above stacked area charts tracking the volatile reservoir evolution of representative fiducial planets (2 M⊕, 1.2 R⊕, G star host, FH2O = 10−3 , FCO2 = 10−4 , e = 0). Colored regions show retained volatile inventories in the atmosphere, magma ocean, trapped melt, and solid mantle as percentages of the initial total volatile inventory, with equivalent kg on the right axis. The black curve shows the total retained H2O, CO2, and O2-equivalent free oxygen inventory. Atmospheric O is tracked as molecular O2, while magma ocean and mantle O are bound as ferric oxide, including the initial primordial concentration. The yellow-hatched band marks the uncertainty of residual melt sealed at the rheological transition. We assume no post-solidification outgassing, so this uncertainty band represents the possible lower and upper range of total volatile retention in the interior reservoir, since compaction, geochemical cycling, tectonics, volcanism, late-stage crystallization, and other residual outgassing could further drain the interior and return volatiles to the atmosphere. (Elkins-Tanton 2008; Hirschmann 2012; Hamano et al. 2013; Lebrun et al. 2013; Schaefer et al. 2016; Hier-Majumder & Hirschmann 2017; Lichtenberg et al. 2026). Left: In the fiducial outgassingregulated sandbar example (a = 0.0125 AU), a sustained magma ocean keeps most volatiles dissolved in the melt and exposes only a thin atmosphere to escape. Middle: In the fiducial airless valley example (a = 0.05 AU), magma ocean solidification partitions volatiles between the atmosphere and sequestered interior, after which atmospheric escape outpaces replenishment. Right: For a representative planet beyond the escape-regulated shoreline (a = 0.20 AU), outgassed atmospheres survive over Gyr timescales. -- [astro-ph.EP]

[astro-ph.EP] 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 USP lava worlds to habitable zone planets, informing target selection and interpretation for TRAPPIST-1 and JWST DDT characterization.

Barron K. Nguyen, Laura K. Schaefer, Xuan Ji, Christopher A. Theissen, Fei Dai, Bo Peng, Yao Tang, Andrea Zorzi, Michelle Hill, Megan Weiner Mansfield

Comments: Accepted for publication in ApJL (The Astrophysical Journal Letters), 2026. 26 pages, 3 figures, 3 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR); Geophysics (physics.geo-ph)
Cite as: arXiv:2608.23912 [astro-ph.EP](or arXiv:2608.23912v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2608.23912
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Related DOI:
https://doi.org/10.3847/2041-8213/ae9743
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Submission history
From: Barron Nguyen
[v1] Mon, 24 Aug 2026 23:36:10 UTC (4,767 KB)
https://arxiv.org/abs/2608.23912

Astrobiology, Exoplanet,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp... More by Keith Cowing

An Evolving Cosmic Shoreline and Sandbar Bounding the Rocky Airless Valley

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