The missing stellar physics component for atmospheric evaporation of exoplanets

The aim of this ambitious project is to use stellar physics to overcome a roadblock in our understanding of how exoplanets lose their primary hydrogen-helium atmospheres and make space for secondary, life-enabling ones. The best observing window for evaporating primary atmospheres are transit observations in the near-infrared metastable helium lines. However, correctly interpreting the observed absorption is extremely challenging. How much of the planetary helium is in the metastable state is controlled by a very small, but incredibly hard to observe part of the host star's extreme-ultraviolet spectrum. Not knowing this controlling factor makes quantitative estimates of the planet's evaporating atmosphere currently impossible. In this project my team and I will put helium triplet spectroscopy on a solid stellar physics footing for the first time, leveraging my extensive background in stellar X-ray astronomy. We will develop a procedure to reconstruct the crucial part of the stellar extreme-UV spectrum accurately from observed coronal X-ray spectra of stars, vastly outperforming the current highly uncertain estimation methods. We will model the outflowing planetary atmosphere to predict helium absorption as a function of stellar extreme-UV irradiation, and for the first time add accurate physics of X-ray bright stellar flares. We will use an unprecedented approach to test our predictions: We will conduct transit observations of exoplanetary helium during different phases of a host star's activity cycle, i.e. observationally isolating the changing stellar extreme-UV irradiation as a driver of helium absorption for the first time. This high risk - high gain project will turn the phenomenology of the helium window to the exoplanetary atmosphere into true understanding, by paying equal attention to the stellar window in the extreme-UV that controls how much we can see of the planet's escaping atmosphere.