Princeton University/Institute for Advanced Study Planet/Exoplanet Discussion Group

Direct and statistical observational evidences suggest that photoevaporation is important in eroding the atmosphere of sub-Neptune planets. We construct full hydrodynamic simulations, coupled with consistent thermochemistry and ray-tracing radiative transfer, to understand the physics of atmospheric photoevaporation caused by high energy photons from the host star. By exploring the parameter space, we find that EUV photoionization is most important in launching photoevaporative wind. Other energetic radiation, including X-ray, are of secondary importance. This allows us to reproduce the observed bimodal radius distribution of sub-Neptune Kepler planets semi-quantitatively.