Direct-imaging of cold exoplanets: The importance of knowing the planet radius

Direct imaging of exoplanets in reflected starlight will be a major step forward in atmospheric characterization. Nancy Grace Roman Space Telescope (former WFIRST) or concepts such as LUVOIR or HabEx will directly observe the starlight reflected from cold and temperate exoplanets. This will probe atmospheric depths that are not accessible in transit measurements. Direct-imaging technique will also allow us to study the atmospheres of non-transiting exoplanets. Here we present results from a recent work (1) where we aim to understand what information can be extracted from such direct imaging observations. We simulated direct-imaging measurements and performed retrievals to constrain the atmospheric properties of the exoplanet. We find that knowing the planet radius will be key to improve the atmospheric characterization. This will affect the predictions about the science outcome of direct-imaging missions, because most of the exoplanets accessible to direct imaging will lack a measurement of the radius. We apply our study to Barnard's Star b and discuss the prospects for characterizing the atmosphere of this nearby (d=1.8pc) super-Earth. (1) https://doi.org/10.1051/0004-6361/202038101