The Galactic Bulge Time Domain Survey will provide an unprecedented census of extrasolar planets detected by gravitational microlensing of background stars, which is among Roman’s top level mission objectives. It will also enable exoplanet detection by the transit method, studies of variable star physics, and insights into the structure of our Milky Way Galaxy.
The observing program described below is an example of a possible survey design. The actual survey is currently being defined by a community process.
The survey will consist of repeated Wide Field Instrument (WFI) images over a set of fields selected for high stellar density and low foreground dust obscuration within the Galactic bulge region, to obtain light curve measurements at varying exoplanet, source star and host star geometries over time. The notional microlensing survey concept cycles through seven WFI fields (~2 deg2 total area) every 15 minutes, almost continuously, throughout six 72-day “bulge seasons” when the Galactic Bulge is visible given Roman’s orbital and pointing constraints.
Planets are detected via light curve deviations that differ from the normal stellar lens light curves. Usually, the signal occurs when one of the two images due to lensing by the host star passes close to the location of the planet, but planets are also detected at very high magnification where the gravitational field of the planet destroys the symmetry of the Einstein ring.
Microlensing is sensitive to a wide range of planet-star separations and host star types. The host stars for planets detected by microlensing are a random sample of stars that happen to pass close to the line-of-sight to the source stars in the Galactic bulge, so all common types of stars are surveyed, including G, K, and M-dwarfs, as well as white dwarfs and brown dwarfs. Microlensing is most sensitive to planets at a separation of ~RE (usually 2–3 AU) due to the strong stellar lens magnification at this separation, but the sensitivity extends to arbitrarily large separations. It is only planets well inside RE that are missed because the stellar lens images that would be distorted by these inner planets have very low magnifications and a very small contribution to the total brightness.
Microlensing relies upon the high density of source and lens stars towards the Galactic bulge to generate the stellar alignments needed to generate microlensing events, but this high star density also means that the bulge main sequence source stars are not generally resolved in ground-based images. This means that the precise photometry needed to detect planets of less than a tenth of the Earth's mass is not possible from the ground unless the magnification due to the stellar lens is moderately high. This, in turn, implies that ground-based microlensing is only sensitive to terrestrial planets located close to the Einstein ring (at ~2–3 AU). The full sensitivity to terrestrial planets in all orbits from the outer habitable zone to ∞ comes only from a space-based survey.
Roman observes 7 fields in the Galactic bulge on a 15 minute cadence for six 72-day seasons, interrupted only by monthly lunar avoidance cutouts. The microlensing events are continuously monitored in a single wide band to measure the basic light curve parameters. The 7 fields are monitored in the bluest filter for one exposure every 12 hours in order to measure the color of the microlensing source stars. The first and last observing seasons are separated by more than 2 years to measure lens-source relative proper motion.