Roman: Direct-imaging in Reflected Starlight with NGRST: Detectability of Confirmed Exoplanets and Population Analysis

The Nancy Grace Roman Space Telescope (NGRST, formerly named WFIRST) will be the first mission to directly image exoplanets in reflected starlight. This will allow us to analyse cold and temperate exoplanets, which cannot be accessed by current facilities. So far, atmospheric characterization is achieved mainly through transit and occultation measurements, which biases these studies towards hot planets in close-in orbits. Direct-imaging observations of long-period planets in reflected starlight will increase our knowledge on the diversity of exoplanets and their atmospheres. This will also affect the theories explaining the formation and evolution of such planetary systems and their architectures. In this work, we studied the exoplanet detection yield of NGRST and future concepts such as LUVOIR or HabEx. For that, we explored the NASA Exoplanet Archive and computed, for all confirmed exoplanets, a range of possible orbital solutions based on their Keplerian parameters and corresponding uncertainties. From that, we obtained the probability of detection and the observational configurations in each case. We analysed the particularities of this subset of detectable exoplanets in comparison with other populations such as the transiting planets. In addition, we discussed the possibilities of retrieving atmospheric properties from direct-imaging measurements of these exoplanets and identified the most favourable targets for such studies. Direct-imaging observations in reflected starlight are expected to be available in this decade. Here we conclude that NGRST will be able to detect a set of long-period exoplanets which is large enough to begin statistical studies of this population. This will help complete the big picture of exoplanet diversity. The coming years until this mission is launched should allow the community to define the most interesting targets to be observed and improve their orbital solutions