Direct imaging of exoplanets: why is it important to know the planet radius

Abstract

Upcoming space missions such as WFIRST and concepts like LUVOIR or HabEx will measure the starlight reflected from cold and temperate exoplanets by direct imaging. Reflected starlight is sensitive to atmospheric depths that cannot be probed in transit and provides a means for investigating non-transiting exoplanets. Directly imaged exoplanets observed in reflected starlight represents the next frontier in exoplanet atmospheres characterization. Thus, the theory for planning and interpreting future observations and the physics behind them is now in development. The goal of this work is to understand what information can be extracted from direct imaging observations of exoplanets in reflected starlight and how robust these conclusions are. Particularly, we focus on Barnard's Star b, a planet candidate announced in 2018 from the analysis of 20 years of radial velocity data of this M dwarf, the second nearest stellar system (d=5.96 ly) after alpha Centauri. Due to its proximity and given its mass and orbital distance (M·sini=3.23MEarth a=0.4 AU) this planetary system is currently one of the most promising targets for the aforementioned direct imaging missions. We simulated direct-imaging observations and performed retrievals to constrain the atmospheric properties of the exoplanet. With that, we studied how degeneracies between model parameters affect the results of the retrieval. Moreover, we analyzed how these results change when the planetary radius is unknown. This will be the case for a majority of the future directly imaged exoplanets and will substantially modify the conclusions of the characterization.