Abstract

Transmission spectroscopy observations allow us to probe the compositional makeup of transiting exoplanet atmospheres. Hot gas giants are ideal targets for such observations, due to their extended atmospheric scale heights, strong absorption features and frequent transits. The number of measured transmission spectra has now increased to a point where we can make the first tentative attempts at comparative planetology by scrutinising trends in the overall population. Results to date reveal a diversity of atmosphere types ranging from clear to cloudy, although the chemistry and physics of the putative aerosols remain largely unknown. Thermal emission measurements also imply a distinction between atmospheres with a thermal inversion (stratosphere) and those without, with the cause of this difference not yet understood. In this context, I will present new HST observations for the extreme planet WASP -121b, an ultra-hot gas giant in a polar orbit just beyond the Roche limit.