Exploring atmospheric escape from hot-Jupiters and water-rich sub-Neptunes with full-atmosphere models

Understanding atmospheric escape from hot Jupiters and sub-Neptunes is critical for constraining exoplanet evolution, demographics, and star-planet interactions. Observations of escaping atmospheres increasingly probe excited states of hydrogen and helium, including the He I 1083 nm triplet and H I Balmer lines. These features provide sensitive diagnostics of mass loss, thermal structure, and non-LTE level populations but have at times proven difficult to interpret. We present results from a full-atmosphere model framework that connects a multi-species thermosphere-ionosphere escape code with a 1D lower atmosphere model. For HD 209458b and HD 189733b, we show that reproducing the modest He I and H α transit depths observed alongside large UV transit depths requires relatively low mass loss rates, efficient diffusive separation, and stellar-driven variability. We incorporate updated rate coefficients and high-resolution cross-sections to predict He I absorption under varying stellar activity conditions more accurately. We extend this framework to sub-Neptunes and present predictive modeling for GJ 1214b, a warm, low-density planet with evidence for a high-metallicity, possibly water-rich atmosphere. Water-dominated thermospheres have distinct thermal structures and radiative properties, leading to reduced scale heights and potentially suppressed mass loss. These models demonstrate the importance of self-consistent, multi-layer models of atmospheric escape and underscore the need for simultaneous UV, optical, and infrared observations to disentangle the complex interplay between composition, heating, and dynamics in escaping exoplanet atmospheres.