An Evolving Cosmic Shoreline and Sandbar Bounding the Rocky Airless Valley
arXiv:2608.23912 · doi:10.3847/2041-8213/ae9743
Abstract
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.
Accepted for publication in ApJL (The Astrophysical Journal Letters), 2026. 26 pages, 3 figures, 3 tables
References in corpus (80)
- New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit
- Growth Model Interpretation of Planet Size Distribution
- Extreme Water Loss and Abiotic O Buildup On Planets Throughout the Habitable Zones of M Dwarfs
- The Extreme Ultraviolet and X-Ray Sun in Time: High-Energy Evolutionary Tracks of a Solar-Like Star
- Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST
- Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets
- Chemistry of Silicate Atmospheres of Evaporating Super-Earths
- No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c
- High Tide or Riptide on the Cosmic Shoreline? A Water-Rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ~486b from JWST Observations
- Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra
- Predictions of the atmospheric composition of GJ 1132b
- Photoevaporation vs. core-powered mass-loss: model comparison with the 3D radius gap
- A secondary atmosphere on the rocky exoplanet 55 Cancri e
- Retention of water in terrestrial magma oceans and carbon-rich early atmospheres
- Double Trouble: Two Transits of the Super-Earth GJ 1132 b Observed with JWST NIRSpec G395H
- Atmospheric Regimes and Trends on Exoplanets and Brown Dwarfs
- Atmospheres of Rocky Exoplanets
- Water on Hot Rocky Exoplanets
- Formation and composition of planets around very low mass stars
- Linking the Climate and Thermal Phase Curve of 55 Cancri e
- Pebble-driven Planet Formation around Very Low-mass Stars and Brown Dwarfs
- Pebble-driven planet formation for TRAPPIST-1 and other compact systems
- GJ 1252b: A Hot Terrestrial Super-Earth With No Atmosphere
- K2 and Spitzer phase curves of the rocky ultra-short-period planet K2-141 b hint at a tenuous rock vapor atmosphere
- The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats
- GJ 367b is a dark, hot, airless sub-Earth
- Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs
- The mass-radius relation of exoplanets, revisited
- Survival of Terrestrial N2-O2 Atmospheres in Violent XUV Environments through Efficient Atomic Line Radiative Cooling
- LHS 1140 b is a potentially habitable water world
- The Nominal Range of Rocky Planet Masses, Radii, Surface Gravities and Bulk Densities
- Redox hysteresis of super-Earth exoplanets from magma ocean circulation
- JWST Thermal Emission of the Terrestrial Exoplanet GJ 1132b
- Susceptibility of planetary atmospheres to mass loss and growth by planetesimal impacts: the impact shoreline
- Habitability of Earth-like stagnant lid planets: Climate evolution and recovery from snowball states
- Growth and evolution of secondary volcanic atmospheres: I. Identifying the geological character of hot rocky planets
- The erosion of large primary atmospheres typically leaves behind substantial secondary atmospheres on temperate rocky planets
- Atmospheric retrievals suggest the presence of a secondary atmosphere and possible sulfur species on L 98-59 d from JWST NIRSpec G395H transmission spectroscopy
- An upper limit on late accretion and water delivery in the Trappist-1 exoplanet system
- The distribution of volatile elements during rocky planet formation
- Atmospheric Erosion by Giant Impacts onto Terrestrial Planets: A Scaling Law for any Speed, Angle, Mass, and Density
- Airy worlds or barren rocks? On the survivability of secondary atmospheres around the TRAPPIST-1 planets
- Ultra-short-period Planets are Stable Against Tidal Inspiral
- Life as the Only Reason for the Existence of N2-O2-Dominated Atmospheres
- Hot Rocks Survey I : A possible shallow eclipse for LHS 1478 b
- Numerous chondritic impactors and oxidized magma ocean set Earth's volatile depletion
- Implications of different stellar spectra for the climate of tidally-locked Earth-like exoplanets
- Variability due to climate and chemistry in observations of oxygenated Earth-analogue exoplanets
- JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1 e
- Impact Induced Atmosphere-Mantle Exchange Sets the Volatile Elemental Ratios on Primitive Earths
- Hot Rocks Survey II: The thermal emission of TOI-1468 b reveals a hot bare rock
- JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1 e
- On the potentially dramatic history of the super-Earth rho 55 Cancri e
- Stabilization of dayside surface liquid water via tropopause cold trapping on arid slowly rotating tidally locked planets
- Sporadic Spin-Orbit Variations in Compact Multi-planet Systems and their Influence on Exoplanet Climate
- Improved Constraints for the XUV Luminosity Evolution of Trappist-1
- Anatomy of rocky planets formed by rapid pebble accretion III. Partitioning of volatiles between planetary core, mantle, and atmosphere
- Population-level Hypothesis Testing with Rocky Planet Emission Data: A Tentative Trend in the Brightness Temperatures of M-Earths
- Atmospheric stability and collapse on tidally locked rocky planets
- Long-term tidal evolution of the TRAPPIST-1 system
- The Cosmic Shoreline Revisited: A Metric for Atmospheric Retention Informed by Hydrodynamic Escape
- Habitable Zone and Atmosphere Retention Distance (HaZARD) Stellar-evolution-dependent loss models of secondary atmospheres
- Self-limited tidal heating and prolonged magma oceans in the L 98-59 system
- Role of Planetary Radius on Atmospheric Escape of Rocky Exoplanets
- Low 4.5 μm Dayside Emission Disfavors a Dark Bare-Rock scenario for the Hot Super-Earth TOI-431 b
- Multiple moist climate equilibrium states on arid rocky M-dwarf planets: A last-saturation tracer analysis
- Possible Evidence for the Presence of Volatiles on the Warm Super-Earth TOI-270 b
- Composition constraints of the TRAPPIST-1 planets from their formation
- Volatile-rich evolution of molten super-Earth L 98-59 d
- A formation pathway for terrestrial planets with moderate water content involving atmospheric-volatile recycling
- Novel Physics of Escaping Secondary Atmospheres May Shape the Cosmic Shoreline
- The Diffusion Limit of Photoevaporation in Primordial Planetary Atmospheres
- Born Dry or Born Wet? A Palette of Water Growth Histories in TRAPPIST-1 Analogs and Compact Planetary Systems
- Helium escaping from the atmosphere of a nearby rocky exoplanet orbiting in a habitable zone
- The tidal heating of the exoplanet 55 Cnc e. The role of the orbital eccentricity
- Evidence for an Atmosphere on the Ultra-Short Period super-Earth HD 3167 b
- Ocean Circulation on Tide-locked Lava Worlds: 3D Modeling with a Simple Boundary Iteration Method
- Atmospheric collapse and re-inflation through impacts for terrestrial planets around M dwarfs
- On the Efficacy of Ocean Formation with a Primordial Hydrogen Atmosphere
- The Effect of Tidal Heating and Volatile Budgets on the Outgassed Atmosphere of 55 Cancri e