The variation of Teq for the exoplanets detectable by Roman in direct imaging: a catalogue of targets

The Nancy Grace Roman Space Telescope (previously named WFIRST) will be the first mission to analyse exoplanets by directly imaging them in reflected starlight. This technique will allow us to characterize a population of long-period planets which cannot be studied by current facilities, increasing our knowledge on the diversity of (exo)planetary atmospheres.

In this work, we studied the exoplanet detection yield of the Roman Space Telescope's coronagraph instrument (CGI). For that, we explored the NASA Exoplanet Archive and computed, for all confirmed exoplanets, 10000 possible orbital realizations based on their Keplerian parameters and corresponding uncertainties. For each realization, we checked if the planet orbits at any point within the Inner and the Outer Working Angles of the coronagraph and is also brighter than the minimum contrast of the instrument. From this statistical exercise, we obtained the probability of detecting each planet (P_detect) and found that up to 76 planets have non-zero P_detect and are therefore potentially Roman-accessible. For each of them, we computed the range of observable phase angles and equilibrium temperatures. We find that the orbits of these Roman-accessible planets have larger eccentricities than the orbits of those characterized so far in transit. This results in large variations of the equilibrium temperature along the orbit, which may affect the atmospheric structure.

Here we present a catalogue of Roman-accessible planets, including several targets with equilibrium temperatures of about 300 K. We discuss the possible atmospheric changes that exoplanets in eccentric orbits may undergo and the most favourable planets to show atmospheric variability during the time that they remain observable by Roman. The remaining years until the launch of this mission should allow to improve the orbital characterization of these planets, which will be key to refine our results.