Revealing thermospheres of exoplanets from high-resolution spectroscopy
The sodium doublet in the optical is one of the most powerful probes of exoplanet atmospheric properties, when observed in transmission spectroscopy during transits. Absorption in the sodium line cores originates from the upper atmospheric regions, where the stellar energetic radiation is absorbed, giving rise to a thermosphere characterised by intense heating and strong winds. Recent high-spectral resolution observations of the sodium doublet in hot gas giants allowed us to resolve the line shape, opening the way for extracting thermospheric properties using line-profile fitting. I will present a retrieval method fully exploiting the resolution of the line profile to determine the temperature-pressure profile and the velocity of high-altitude winds in the thermosphere of the prototypical hot gas giant HD 189733b. The method could be applied to the whole sample of planets from the on-going HEARTS & SPADES survey for hot exoplanetary atmospheres at high-spectral resolution with HARPS and HARPS-N, and to the upcoming observations with next generation spectrographs like ESPRESSO. The interpretation of these observations will draw the first picture of the impact of stellar irradiation on planetary upper atmospheres.