Ground-based observations of hot exoplanet thermospheres: first insights and challenges
Unveiling the characteristics of the numerous extrasolar planets discovered every year is one of the major goal of exoplanetology. Transiting exoplanets are among the best suitable targets for atmospheric studies, particularly with transmission spectroscopy. This technique studies the light filtered through the atmosphere of an exoplanet, as it passes in front of its star. These observations have experienced a rapid development in the last few years, allowing us to precisely probe the low part of atmospheres. Despite these progresses, we are still unable to understand the link between the low and the upper part of atmospheres, with the latter undergoing evaporation. Transit observations from the ground with stabilised high-resolution spectrograph, such as HARPS, have key roles to play in this context. Indeed, while taking care of multiple challenges linked to the stellar lines variability (Rossiter-McLaughlin effect, center-to-limb variation, activity), studies of sodium lines (or other atomic transitions) deliver innovative measurements of atmospheres. The measured sodium absorptions in the atmospheres of the two hot Jupiters HD189733b and WASP-49b have revealed new informations about their thermospheres. The thermosphere is a very specific region of intermediate altitudes (very low pressure), where most of the stellar irradiation is absorbed by atoms and molecules, resulting in an upper atmospheric heating. This mechanism potentially lead to an hydrodynamical expansion of the atmosphere that may trigger the exoplanet evaporation. Henceforth, observations at high-resolution, particularly in the optical domain (e.g. with ESPRESSO), are a valuable and important resource in order to understand exoplanets atmospheres.