Exoplanetary Ionospheric Temperatures on the Edge of Airlessness

The pattern of airy and airless rocky planets presently being uncovered by JWST is a record of what happens when ionospheres are pushed to their limits by their host stars. Orbiting as close to a red dwarf host as the Parker Probe is to the Sun, a massive rocky planet could harbour liquid water oceans beneath an ionosphere several times hotter than its star’s effective temperature, exhibiting spectacular airglow and aurora. Not only is this a distinct and observable possibility, but planets of this kind may make up a significant fraction of habitable worlds.

What maximum temperature can a tightly bound ionosphere, composed primarily of carbon, nitrogen, and oxygen atoms, reach before escaping into space as a hydrodynamic wind? This question lies at the crux of the 500-hour Rocky Worlds DDT Program and the guiding hypothesis of a universal cosmic shoreline.

Locally, the terminal temperatures of these extreme ionospheres are determined by heating from XUV photons emitted by the star’s corona and cooling through collisional excitation of atoms that emit visible and infrared photons. Globally, the thermal structure is determined by photochemistry, fluid dynamics, and electromagnetic interactions. Additionally, stellar cycle variation of ionospheric conditions is likely key to atmospheric evolution. In this talk, we will discuss the key knowns and unknowns in predicting the “edge of airlessness” for the population of rocky exoplanets within the observational reach of the James Webb Space Telescope.