Transmission Spectroscopy of the Highly Inflated Hot Saturn WASP-21b

Hot gas giant exoplanets provide the best opportunity for characterizing the structure and composition of exoplanet atmospheres by the use of transmission spectroscopy, owing to their often inflated atmospheres and deep transits. However, clouds and hazes within exoplanet atmospheres can mute, and sometimes mask entirely, absorption lines of interest. Before dedicating expensive JWST time to the study of a cloudy exoplanet, it would be valuable to predict a priori whether an exoplanet is likely clear or cloudy. This is one of the goals of the Low Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy (LRG-BEASTS). We present the LRG-BEASTS optical transmission spectrum of the highly inflated hot Saturn WASP-21b, obtained via fitting spectroscopic transit light curves of the exoplanet from three separate transit events. WASP-21b has a similar equilibrium temperature and surface gravity to the cloud-free exoplanet WASP-39b. These two parameters are expected to influence the degree of cloudiness of an exoplanet atmosphere. If we find WASP-21b to have a feature-rich optical transmission spectrum, similar to WASP-39b, this would be a demonstration that this region of parameter space is more likely home to cloud-free exoplanets.