Princeton University Extrasolar Planet Discussion Group

Abstract: Host metallicity and system multiplicity encode fundamental information about planet formation, yet both remain poorly constrained for low-mass planets beyond the snow line. Gravitational microlensing is uniquely sensitive to this regime, and the Roman Galactic Bulge Time-Domain Survey (GBTDS) will discover a large sample of such planets extending into the inner Galaxy. Turning this sample into demographic measurements along these two dimensions requires two capabilities that are currently missing: metallicity measurements for lenses and systematic searches of the triple-lens parameter space. I will present two methods that I developed to address these challenges. 

First, I will present the photometric–microlensing metallicity method, which combines multi-band lens photometry with the angular Einstein radius to measure metallicities of M-dwarf lenses out to bulge distances. Applied to the Roman GBTDS, the method could yield host metallicities for roughly 150 planetary systems with a typical precision of about 0.25 dex, enabling the first measurement of the occurrence–metallicity relation for cold low-mass planets and constraining the low-metallicity cutoff for their formation. 

Second, I will present AdaMGrid, a new framework that makes full parameter-space searches for triple-lens solutions computationally feasible. Applied to the Roman GBTDS, which is expected to yield of order 100 triple-lens events, AdaMGrid should enable robust inference of lens configurations across the full sample, enabling demographic studies of planetary multiplicity and planets in stellar binaries in the cold low-mass regime. 

Together, these methods would turn Roman’s cold-planet census into new measurements of how planet formation depends on host composition and system architecture across the Galaxy.