2021MNRAS.508.5991S

We study the Roman sensitivity to exoplanets in the Habitable Zone (HZ). The Roman efficiency for detecting habitable planets is maximized for three classes of planetary microlensing events with close caustic topologies. (a) The events with the lens distances of D_l >= 7 kpc, the host lens masses of M_h ≳0.6 M_☉. By assuming Jupiter-mass planets in the HZs, these events have q <= 0.001 and d >= 0.17 (q is their mass ratio and d is the projected planet-host distance on the sky plane normalized to the Einstein radius). The events with primary lenses, M_h ≲0.1 M_☉, while their lens systems are either (b) close to the observer with D_l <= 1 kpc or (c) close to the Galactic bulge, D_l >= 7 kpc. For Jupiter-mass planets in the HZs of the primary lenses, the events in these two classes have q >= 0.01, d <= 0.04. The events in the class (a) make larger caustics. By simulating planetary microlensing events detectable by Roman, we conclude that the Roman efficiencies for detecting Earth- and Jupiter-mass planets in the Optimistic HZs (OHZs, which is the region between [0.5, 2] au around a Sun-like star) are 0.01 and 5 per cent, respectively. If we assume that one exoplanet orbits each microlens in microlensing events detectable by Roman (i.e. ∼27 000), this telescope has the potential to detect 35 exoplanets with the projected planet-host distances in the OHZs with only one having a mass <=10M_⊕. According to the simulation, 27 of these exoplanets are actually in the OHZs.