Directly imaged exoplanets in reflected starlight: The importance of knowing the planet radius

Abstract

Direct-imaging will be a key technique to characterize the atmospheres of non-transiting exoplanets. Space telescopes using this method are planned to be launched in this decade and several works are trying to predict their science outcome through retrieval exercises. Here we present an atmospheric model and a retrieval package that we developed to study how different planet and atmospheric parameters affect the reflected-light spectra that will be measured. We find that our knowledge of the planet radius will play a major role in the atmospheric characterization. For instance, if the value of the planet radius (Rp) is unknown, it will be challenging to distinguish between cloudy and cloud-free atmospheres. We therefore conclude that Rp, a parameter which was not considered in previous studies of direct-imaging observations, should be included in future retrieval exercises of this type.