Probing Atmospheric Escape Through the Near-Infrared Helium Triplet

The most productive tracer of exoplanetary atmospheric escape is the measurement of excess absorption in the near-infrared metastable helium triplet during transits. Atmospheric escape of a close-in planet's atmosphere plays a role in its evolutionary pathway, but to which extent remains unknown. It could explain demographic features like the radius valley and Neptunian desert. We will describe the development of instrumental, reduction, and modelling techniques to study exoplanetary atmospheric escape, focusing on the helium triplet. One such development is the NIGHT spectrograph, intended to provide the first survey of escaping atmospheres. NIGHT spectra will be processed with ANTARESS, a state-of-the-art workflow for reducing high-resolution spectral time-series of exoplanet transits and computing transmission spectra in a robust and reproducible way. Transmission spectra contain the potential signature of the planetary atmosphere as well as distortions induced by the occultation of local regions of the stellar surface along the transit chord. Transmission spectra cannot be corrected for those stellar distortions without biasing the planetary signal. They must instead be directly interpreted using a numerical model like the EvE code, which generates realistic stellar spectra that account for the system's 3D architecture, the planet's atmospheric structure, and its local occultation of the stellar disc. This global approach, from the measurement and computation of transmission spectra to their interpretation, will be a legacy of the NCCR PlanetS, becoming the standard procedure to study high-resolution spectroscopy of planetary transits.