Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. III. Detectability of Giant Exomoons of Wide-separation Giant Planets

Abstract The Nancy Grace Roman Space Telescope (Roman) will conduct a Galactic Exoplanet Survey to discover bound and free-floating exoplanets using gravitational microlensing. Roman should be sensitive to lenses with mass down to ∼0.02 M ⊕ , or roughly the mass of Ganymede. Thus, the detection of moons with masses similar to the giant moons in our solar system is possible with Roman. Measuring the demographics of exomoons will provide constraints on both moon and planet formation. We conduct simulations of Roman microlensing events to determine the effects of exomoons on microlensing light curves, and whether these effects are detectable with Roman. We focus on giant planets from 30 M ⊕ to 10 M Jup on orbits from 0.3 to 30 au, and assume that each planet is orbited by a moon with moon–planet mass ratio from 10 −4 to 10 −2 and separation from 0.1 to 0.5 planet Hill radii. We find that Roman is sensitive to exomoons, although the number of expected detections is only on the order of one over the duration of the survey, unless exomoons are more common or massive than we assumed. We argue that changes in the survey strategy, in particular focusing on a few fields with higher cadence, may allow for the detection of more exomoons with Roman. Regardless, the ability to detect exomoons reinforces the need to develop robust methods for modeling triple lens microlensing events to fully utilize the capabilities of Roman.