B. Scott Gaudi

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.

Microlensing has proven to be a valuable method for detecting objects that are too faint and/or distant to detect directly. In particular, it can detect both gravitationally bound and free-floating exoplanets, and is sensitive to regions of planetary parameter space that are difficult to probe by other means. The Nancy Grace Roman Space Telescope, due to launch in September 2026, is expected to detect thousands of microlensing events, including >1400 bound exoplanetary systems, during the Roman Galactic Bulge Time Domain Survey. Planning for this mission presented the challenge of needing to analyze such a large sample of events efficiently. Historically, the analysis of even a single event could take months, due to the large number of parameters in microlensing models and multiple degeneracies. In 2018 the microlensing community conducted a data challenge to stimulate development of more efficient and scalable software tools for the analysis of the data from this and other time domain surveys. Here we present the simulated challenge dataset with analysis of the submitted entries together with a discussion of lessons learned for future data challenges.

Binary lens microlensing events in which the source crosses a caustic produce sharp, distinctive magnification peaks and can therefore be readily identified. In this paper, we explore the importance of binary orbital motion for the binary lens caustic-crossing cross section. If the orbital timescale of the binary system is smaller than the Einstein ring crossing timescale, the caustics sweep out a larger area in the source plane, generally enhancing the cross section. We find that face-on binaries in circular orbits exhibit a substantial increase (up to 4×) in the cross section for caustic crossings. However, highly inclined orbits produce a net decrease relative to a static face-on binary with the same semimajor axis. Using a sample of ∼2800 synthetic binary microlensing events drawn from a realistic Milky Way population-synthesis model, we calculate the average change in the caustic-crossing cross section for individual systems with and without orbital motion. Although orbital motion can significantly alter the cross section for specific geometries, we find that, when averaged over the full population, it produces only a small, negligible increase of ∼0.1% in the average cross section. We also compute the overall rate of caustic-crossing binary events in the simulated sample and find that, with sufficiently dense photometric sampling, 6.3% ± 0.2% of microlensing events with impact parameter u ₀ < 1 should exhibit caustic crossings. This rate depends on the distributions of binary separation, mass ratio, and multiplicity of stars and compact objects, and can therefore be used to test our understanding of these underlying properties.

Recent ground-based microlensing surveys suggest that our Galaxy may abound with small free-floating planets, potentially up to ∼21 such planets per star. We explore the implication of such possibility on the mass budget for planet formation. When the microlensing planets, both bound and free-floating, are taken into account, along with the short-period planets, T Tauri disks have insufficient mass to source the mass of known planets, even if all the solids convert into planetary bodies. Younger Class 0/I disks can help resolve the problem but generally fall short of the required mass when variable planet formation efficiency from pebble or planetesimal accretion is taken into consideration. If the free-floating planet mass function is as bottom-heavy as reported, heavier Class 0/I disks may be necessary. Alternatively, free-floaters may preferentially form in the most massive disks around massive stars consuming the majority of the mass budget, leading to a decrease in the bound planet occurrence rate for higher-mass stars, which is observed. Precise constraints on the bottom of the planet mass function are necessary: a peaked mass function may eliminate the missing mass problem; by contrast, verifying a bottom-heavy function could spell a crisis in planet formation.

We present a Hubble Space Telescope (HST) imaging survey of a 1.1 deg ² sky area toward the Milky Way Galactic bulge. This field significantly overlaps with the upcoming Nancy Grace Roman Galactic Bulge Time Domain Survey. High angular resolution imaging of this area with HST before the start of the Roman Galactic Exoplanet Survey will greatly strengthen Roman’s ability to characterize detected exoplanet systems, as well as provide a rich and wide-field archive for use as a legacy dataset toward the Galactic bulge for the broader community. We conduct coordinated parallel imaging with both wide-field cameras on HST, the Wide Field Camera 3 and Advanced Camera for Surveys, utilizing the F606W and F814W passbands. Approximately 70% of the survey was conducted during HST Cycle 32, with the remaining 30% conducted during Cycle 33. In this Letter, the first in a series, we give a general overview of the program and the observing strategy and present early results. This campaign secures HST’s lasting impact on the high-precision study of stellar populations, dynamics, exoplanet systems, interstellar extinction, metallicities, cluster associations, and more toward the center of our Galaxy.

As part of the Galactic Bulge Time Domain Survey (GBTDS), the Nancy Grace Roman Galactic Exoplanet Survey (RGES) will use microlensing to discover cold outer planets and free-floating planets not bound to stars. NASA has established several science requirements for the GBTDS to ensure RGES success. A key advantage of RGES is Roman’s high angular resolution, which will allow detection of flux from many host stars. One requirement specifies that Roman must measure the masses and distances of 40% of detected planet hosts with 20% precision or better. To test this, we simulated microlensing events toward the GBTDS fields and used Fisher matrix analysis to estimate light curve parameter uncertainties. Combining these with Roman imaging observables (lens flux and relative lens–source proper motion), we estimated the achievable precision of lens mass and distance measurements. Using pyLIMASS , a publicly available code for estimating lens properties, we applied this analysis to 3000 simulated events. Assuming the A. Cassan et al. exoplanet mass function, we find that ≥40% of host stars meet the required 20% precision threshold, confirming that the GBTDS can satisfy the mission requirement. We validated our approach by comparing our inferred lens masses and distances to empirical measurements from detailed image-constrained light curve modeling of historical microlensing events with Hubble and Keck follow-up imaging. Our results agree within roughly 1 σ , demonstrating that both approaches yield consistent and reliable mass and distance estimates, and confirming the robustness of our simulations for Roman-era microlensing science.