Metals in the day-side of ultra-hot Jupiter atmospheres: a key test for planetary formation

Ultra-hot Jupiters (T_eq ≥ 2,500 K) are the hottest gaseous giants known. They emerged as ideal laboratories to test theories of atmospheric structure and its link to planet formation. Indeed, because of their high temperatures, (1) they likely host atmospheres in chemical equilibrium and (2) clouds do not form in their day-side. Thousands of lines of refractory elements such as iron, normally inaccessible in planets, can be studied through high spectral resolution emission spectroscopy, providing a first look into the chemistry of refractory elements in exoplanets. In this talk we report the detection of neutral iron in the day-side emission spectrum of KELT-9b (T_day ~ 4,000  K), the first detection of an atomic species in the emission spectrum of an exoplanet, obtained with HARPS-N optical data gathered in the framework of the GAPS collaboration. Our detection unambiguously indicates the presence of a thermal inversion in the atmosphere of the planet. We also present a new technique to extract planetary parameters from the cross-correlation function in a statistically sound framework, which makes possible the combination with information from the planetary continuum that can be obtained with complementary space facilities. This is a crucial step towards the measurement of metal abundances in exoplanets, a quantity that can be compared to predictions of planet formation theories. In the near future, our technique will be extended to cooler exoplanets. In the era of EELTs and JWST, this kind of measurements could ultimately open a new window on exoplanet formation and evolution.