The 3D Cosmic Shoreline for Nurturing Planetary Atmospheres
Journal Article
Overview
abstract
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.;
