Transmission spectroscopy, which entails the detection of minute wavelength-dependent variations of the inferred planetary radius due to an optically thick atmosphere, provides us with a unique opportunity of detecting and characterising atmospheres of exoplanets. This is essential to understanding the formation of these alien worlds and the evolutionary paths that they embark on. Ground-based observatories (VLT and Gemini for instance) have played a vital role, in will continue to do so, in complementing results from space-bound telescopes (HST, JWST).
I will present recent results from ESO's FORS2 instrument at UT1 of the VLT. These include the improvements made to differential spectrophotometric observations with this instrument after the upgrade of the atmospheric dispersion corrector prisms. I will present results such as those by Nikolov et al. (2016), Sedaghati et al. (2015,2016,2017) which show significant detections of atmospheric properties. Our on-going analysis of WASP-19b, from multi-epoch and multi-grism observations, allows us to make very detailed observations of its atmosphere covering the entire visible wavelength domain, including characterising haze composition, detection of a large Rayleigh slope, water absorption in the near-IR, as well as multiple other molecules.
These results have been driving the theory of exoplanetary atmosphere science, where inclusion of a whole host of new molecular species into retrieval models, has lead to the first discovery of a number of molecules in the atmosphere of an exoplanet. I will also present results from archival data analysis (WASP-80b; Sedagahti et al. 2017) and how to overcome systematic effects in the data that were taken prior to the upgrade in 2014. These results show that with careful consideration of the optical state parameters that cause systematic trends, one is able to model transit light curves that include correlated noise, and very precise transmission spectra can be obtained.
For instance, in the case of WASP-80b, we report the detection of significant absorption from the potassium doublet and the more general deviation from a flat spectrum, as it has been reported previously.