The Evolution of Atmospheric Escape of Highly Irradiated Gassy Exoplanets

Abstract Atmospheric escape has traditionally been observed using hydrogen Lyman-α transits, but more recent detections utilise the metastable helium triplet lines at 1083nm. Capable of being observed from the ground, this helium signature offers new possibilities for studying atmospheric escape. Such detections are dependent however on the specific high-energy flux received by the planet. Previous studies show that the extreme-UV band both drives atmospheric escape and populates the triplet state, whereas lower energy mid-UV radiation depopulates the state through photoionisations. This is supported observationally, with the majority of planets with 1083nm detections orbiting a K-type star, which emits a favourably high ratio of EUV to mid-UV flux. The goal of our work is understanding how the observability of escaping helium evolves. We couple our one-dimensional hydrodynamic non-isothermal model of atmospheric escape with a ray-tracing technique to achieve this. We consider the evolution of the stellar radiation and the planet’s gravitational potential.