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From hot gas giants to cooler exo-Earths: A pioneering transmission spectral survey of exoplanet atmospheres

Presented by: Nikolay Nikolov (Johns Hopkins University)
Category: Special Interest   Duration: 1 hour   Broadcast date: January 17, 2019
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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 reveals a large 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 and Spitzer continue 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 discuss results from the first comparative ground-based multi-object spectroscopy follow up of cloud-free, cloudy and hazy exoplanets with atmospheric features detected with HST. By comparing and contrasting the VLT spectra with HST spectroscopy, we find that VLT FORS2 is an ideal instrument for transmission spectroscopy. I will further present results from the first large-scale ground-based transmission spectral survey from hot gas giants down to cooler Earth-mass worlds. In particular, I will discuss the optical transmission spectrum of the ‘hot Saturn’ WASP-96b, which exhibits the complete pressure-broadened profile of the sodium absorption feature enabling a precise absolute sodium abundance and atmospheric metallicity from the ground. By teaming up HST with VLT and including previous observations, I ultimately aim to increase the number of characterized exoplanet atmospheres by a factor of five, enabling the first serious statistical investigation into the relationship between clouds/hazes and fundamental properties such as planet mass, radius, temperature, and atmospheric metallicity.