Institute for Advanced Study Astrophysics Seminar

Gravitational microlensing is sensitive to planetary systems that cannot be probed by other techniques, in particular cold, low-mass planets beyond the snow line. Ground-based microlensing surveys have discovered over 250 planets, including roughly half a dozen free-floating planet candidates. I will review the gravitational microlensing method, how it detects exoplanets, and what we have learned about the demographics of planets beyond the snow line from microlensing. I will then discuss the promise of the Roman Galactic Exoplanet Survey, which will dramatically increase the sensitivity of microlensing. It will be sensitive to analogs of all the planets in our Solar System except Mercury and is expected to detect hundreds of cold, bound planets and hundreds to thousands of free-floating planets. Roman will also detect approximately 100,000 transiting planets. Together, the transit and microlensing demographic constraints from Roman will provide the first statistical census of exoplanets within a single stellar population, complete to planets with radii or masses greater than about twice that of Earth across all semimajor axes, from zero to infinity.