2021A&A...651A...7C

Context. Reflected starlight measurements will open a new path in the characterization of directly imaged exoplanets. However, we still lack a population study of known targets to which this technique can be applied.
Aims. We investigate which of the about 4300 exoplanets confirmed to date are accessible for the Roman Space Telescope coronagraph in reflected starlight at reference wavelengths λ=575, 730, and 825nm. We carry out a population study and also address the prospects for phase-curve measurements.
Methods. We used the NASA Exoplanet Archive as a reference for planet and star properties and explored the effect of their uncertainties on the exoplanet detectability by applying statistical arguments. We defined a planet as Roman-accessible on the basis of the inner and outer working angles of the instrument and its minimum planet-to-star contrast (IWA, OWA, and C_min). We adopted three plausible configurations for these technical specifications, labeled pessimistic, intermediate, and optimistic. Our key outputs for each exoplanet are its probability of being Roman-accessible (P_access), the range of observable phase angles, the evolution of its equilibrium temperature, the number of days per orbit for which it is accessible, and its transit probability.
Results. In the optimistic scenario, we find 26 Roman-accessible exoplanets with P_access>25% and host stars brighter than V=7mag. This population is biased towards planets more massive than Jupiter, but also includes the super-Earths tau Cet e and f, which orbit near the habitable zone of their star. Thirteen planets are part of multi-planetary systems. Three of these planets have known transiting companions, which offers opportunities for a contemporaneous atmospheric characterization. The intermediate and pessimistic scenarios yield ten and three Roman-accessible exoplanets, respectively. We find that inclination estimates (e.g. with astrometry) are required to refine the detectability prospects.
Conclusions. A science phase of the coronagraph instrument has a remarkable potential for characterizing the atmospheres of exoplanets that cannot be studied with other techniques.