The 3D Cosmic Shoreline for Nurturing Planetary Atmospheres
- 1University of Colorado Boulder
Abstract
Various “cosmic shorelines” have been proposed to delineate which planets have atmospheres. The fates of individual planetary atmospheres may be set by a complex sea of growth and loss processes, driven by unmeasurable environmental factors or unknown historical events. Yet, defining population-level boundaries helps illuminate which processes matter and identify high-priority targets for future atmospheric searches. Here, we provide a statistical framework for inferring the position, shape, and fuzziness of an instellation-based cosmic shoreline, defined in the 3D space of planet escape velocity, planet bolometric flux received, and host-star luminosity. We circumvent the need to estimate each individual host star’s historical X-ray and extreme ultraviolet fluences by including luminosity in the definition of the shoreline, explicitly modeling how sharply such drivers of atmospheric escape intensify toward lower-luminosity M dwarf stars and marginalizing over the associated uncertainties. Using solar system and exoplanet atmospheric constraints, under the assumption that one planar boundary applies across a wide parameter space, we find the critical flux threshold for atmospheres scales with escape velocity with a power-law index of p = 5 . 90 − 0.43 + 0.61 , steeper than the canonical literature slope of p = 4, and scales with stellar luminosity with a power-law index of q = 1.1 7 − 0.20 + 0.28 , steep enough to disfavor atmospheres on Earth-sized planets out to the habitable zone for stars less luminous than log 10 ( L ⋆ / L ⊙ ) = − 2.2 3 − 0.21 + 0.18 (roughly spectral type M4V). This model provides quantitative predictions for the probability that any given planet possesses an atmosphere, which will be rigorously tested by upcoming JWST Rocky Worlds observations.
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BibTeX
@article{bertathompson2026cosmic,
title = {{The 3D Cosmic Shoreline for Nurturing Planetary Atmospheres}},
author = {Zach K. Berta-Thompson and Patcharapol Wachiraphan and Catriona A. Murray},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/ae77fd},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/ae77fd},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - The 3D Cosmic Shoreline for Nurturing Planetary Atmospheres
AU - Zach K. Berta-Thompson
AU - Patcharapol Wachiraphan
AU - Catriona A. Murray
PY - 2026
DA - 2026/10/29
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/ae77fd
UR - https://iopscience.iop.org/article/10.3847/1538-4357/ae77fd
AB - Various “cosmic shorelines” have been proposed to delineate which planets have atmospheres. The fates of individual planetary atmospheres may be set by a complex sea of growth and loss processes, driven by unmeasurable environmental factors or unknown historical events. Yet, defining population-level boundaries helps illuminate which processes matter and identify high-priority targets for future atmospheric searches. Here, we provide a statistical framework for inferring the position, shape, and fuzziness of an instellation-based cosmic shoreline, defined in the 3D space of planet escape velocity, planet bolometric flux received, and host-star luminosity. We circumvent the need to estimate each individual host star’s historical X-ray and extreme ultraviolet fluences by including luminosity in the definition of the shoreline, explicitly modeling how sharply such drivers of atmospheric escape intensify toward lower-luminosity M dwarf stars and marginalizing over the associated uncertainties. Using solar system and exoplanet atmospheric constraints, under the assumption that one planar boundary applies across a wide parameter space, we find the critical flux threshold for atmospheres scales with escape velocity with a power-law index of p = 5 . 90 − 0.43 + 0.61 , steeper than the canonical literature slope of p = 4, and scales with stellar luminosity with a power-law index of q = 1.1 7 − 0.20 + 0.28 , steep enough to disfavor atmospheres on Earth-sized planets out to the habitable zone for stars less luminous than log 10 ( L ⋆ / L ⊙ ) = − 2.2 3 − 0.21 + 0.18 (roughly spectral type M4V). This model provides quantitative predictions for the probability that any given planet possesses an atmosphere, which will be rigorously tested by upcoming JWST Rocky Worlds observations.
SN - 1538-4357
SN - 0004-637X
ER -