| Abstract: The first major result from James Webb Space Telescope surveys of rocky exoplanets is that many appear to have lost their atmospheres completely, even across a wide range of planetary masses and temperatures. Escape is fuelled by ionising radiation from the host star, but its efficiency depends on cooling lines from the hot soup of atoms and molecules in the upper atmosphere, demanding more sophisticated non-LTE energy balance models. Equally pressing is the need to fill the gap between the traditional models of escape. In the Jeans limit the atmosphere stays hydrostatic up to the exobase, where energetic particles can escape directly to space, while in the limit analogous to Parker's solar wind the atmosphere accelerates through the sound speed in the strongly collisional region below. But an atmosphere on the edge of survival escapes at a rate between these limits, where the outflow is weakly collisional and the radial and transverse temperatures can diverge. I'll present a first-principles solution built on a neat closure of the Boltzmann equation: a drifting bi-thermal Maxwellian, yielding a ballistic escape rate that varies with the bulk motion and non-equilibrium thermal structure, and so spans the escape regimes continuously. Remarkably, a unique Parker-style transonic solution survives even as the fluid assumption breaks down, under new critical conditions set by the departure from equilibrium, before then giving way to subsonic evaporation. I'll close with what this new theory means for the prevalence of exo-Earths and exo-Venuses in the galaxy. |