2017ApJ...843..122Z

The planets of the solar system are neatly divided between those with atmospheres and those without when arranged by insolation (I) and escape velocity (v_esc). The dividing line goes at I∝v_esc⁴. Exoplanets with reported masses and radii are shown to crowd against the extrapolation of the solar system trend, making a metaphorical cosmic shoreline that unites all the planets. The I∝v_esc⁴ relation may implicate thermal escape. We therefore address the general behavior of hydrodynamic thermal escape models ranging from Pluto to highly irradiated extrasolar giant planets (EGPs). Energy-limited escape is harder to test because copious XUV radiation is mostly a feature of young stars, and hence requires extrapolating to historic XUV fluences (I_xuv) using proxies and power laws. An energy-limited shoreline should scale as I_xuv∝v_esc³sqrt(ρ), which differs distinctly from the apparent I_xuv∝v_esc⁴ relation. Energy-limited escape does provide good quantitative agreement to the highly irradiated EGPs. Diffusion-limited escape implies that no planet can lose more than 1% of its mass as H₂. Impact erosion, to the extent that impact velocities v_imp can be estimated for exoplanets, fits a v_imp~4–5 v_esc shoreline. The proportionality constant is consistent with what the collision of comet Shoemaker-Levy 9 showed us we should expect of modest impacts in deep atmospheres. With respect to the shoreline, Proxima Centauri b is on the metaphorical beach. Known hazards include its rapid energetic accretion, high impact velocities, its early life on the wrong side of the runaway greenhouse, and Proxima Centauri's XUV radiation. In its favor is a vast phase space of unknown unknowns.