New spectral windows into the escaping atmospheres of exoplanets

Atmospheric escape is expected to have important consequences for the evolution of planets and has been suggested to create the observed radius valley and hot Neptune desert. To date, escaping exoplanet atmospheres have typically been probed with a handful of spectral lines, such as Lyman alpha, the metastable helium triplet and UV lines of metals. Inferring important characteristics such as the outflow geometry and mass-loss rate from these observations has been difficult due to differing theoretical predictions and model degeneracies. Expanding on the suite of tracers used to probe escaping atmospheres would help to mitigate these challenges. We post-process hydrodynamic outflow models with NLTE photoionization code Cloudy to predict the transit spectrum of typical gas giant planets and we find new spectral lines in the UV and optical that can potentially be used to study their upper atmospheres. By indicating the atmospheric altitude each of these lines probe, we can identify complementary lines which will allow us to better constrain the outflow properties.