Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey

Context . The Nancy Grace Roman Space Telescope will expand the reach of gravitational microlensing surveys by increasing the number of events monitored and the precision of their light curves. Aims . We investigated Roman ’s ability to detect triple-lens microlensing systems, cases where a foreground star with two bound exoplanets produces detectable anomalies in a microlensing event, using its planned high-cadence observations toward the Galactic bulge. Methods . We simulated a large set of high-magnification microlensing light curves based on Roman ’s expected survey characteristics. A detection criterion, based on a required χ 2 improvement for a two-planet model, was applied to determine whether the second planet can be reliably distinguished from a single-planet (binary-lens) model. Results . Our simulations show that the majority of two-planet microlensing events would be detectable with Roman . Events in which both planets are relatively massive (planet–star mass ratios of order 10 −3 ), or in which the more massive planet occupies a favorable resonant configuration, produce strong central perturbations and result in detection efficiencies of roughly 90%. By contrast, systems with only low-mass planets ( q ∼ 10 −4 ) or with less favorable alignments generate much weaker signals, which often fall below the detection threshold. In general, the planetary mass ratios and the resulting caustic geometry (e.g., central caustic size in resonant versus wide–close orbits) are the dominant factors governing detectability. Conclusions . Taking into account the expected frequency of planetary systems and the fraction of high-magnification events, we estimate that Roman will detect a high-magnification triple-lens event in approximately 4.1% of multiplanet microlensing events, corresponding to an upper limit of about 58 events, derived under the assumption that all simulated high-magnification binary events host an additional planet, over the course of the full survey.