Title: From hot gas giants to cooler exo-Earths: Exploring the diversity of exoplanet atmospheres
Abstract: Transiting exoplanets play a key role in advancing our understanding about the atmospheric composition, thermal structure and weather of these distant alien worlds. Over the past decade, transit spectroscopy of close-in exoplanets has started to unveil an emerging diversity of atmospheres and a prevalence of clouds and hazes, ranging in composition from alkali volatiles to metal oxide refractories. Currently, no obvious pattern of atmospheric chemistry has emerged to show how it links with the occurrence of clouds and hazes, planet formation and the physical properties of the host stars. Only large surveys, combined with previous results, will allow us to establish correlations and help elucidate the main processes responsible for the formation and evolution of the overall exoplanet population. While HST continues to play a pioneering role in the atmospheric exploration of exoplanets, significant progress has also been made from the ground, notably with VLT FORS2. I will present results from the first large-scale ground-based transmission spectral survey from hot gas giants down to cooler Earth-mass worlds. With scarce optical spectrographs suitable for transmission spectroscopy on the ground and in space beyond HST, VLT and FORS2 play a leading role in exploring the diversity of exoplanet atmospheres by providing new targets and highly-complementary optical transmission spectroscopy for the upcoming infrared JWST and ARIEL enabling absolute atmospheric abundances.