Understanding what helium absorption tells us about atmospheric escape

Atmospheric escape is now considered the major contributing factor in shaping the demographic of detected exoplanets. However, inferences about the exoplanet populations strongly depend on the accuracy of the models. Direct observational tests of atmospheric models are still in their infancy. Helium escape from planetary atmospheres has rapidly become the primary observational probe, having been observed in ~20 exoplanets. Grounding our understanding in the basic physics of atmospheric escape I will present a new theoretical model to predict the excess absorption from helium absorption line. I will argue that the helium triplet population rapidly approaches a statistical equilibrium between population by recombination and depopulation by electron collisions. In this limit the helium triplet fraction is primarily set by the gas temperature. By using insights from hydrodynamic outflow models that constrain the outflows temperatures I will show how mass-loss models can be directly tested using a population of escaping helium observations. Interestingly, I will show that once these effects are taken into account, the helium absorption (appropriately scaled by the planet’s hill sphere) scales with incident irradiation. I will demonstrate this population-level prediction can be used to probe how the mass-loss efficiency scales with planetary and stellar properties. This strengthen the idea of how important is helium for understanding atmospheric escape, and eventually in shaping the exoplanet population that we see today. I will use this work to demonstrate the first pathway to directly observationally constraining atmospheric escape and its impact on the exoplanet population.