Extrasolar planet detections with gravitational microlensing

Microlensing light curves due to single stars are symmetric and typically last for a month. So far about 4000 microlensing events have been discovered in real-time, the vast majority toward the Galactic centre. The presence of planets around the primary lenses induces deviations in the usual light curve which lasts from hours (for an Earth-mass [M_⊕] planet) to days (for a Jupiter-mass [M_J] planet). Currently the survey teams, OGLE and MOA, discover and announce microlensing events in real-time, and follow-up teams (together with the survey teams) monitor selected events intensively (usually with high magnification) in order to identify anomalies caused by planets. So far four extrasolar planets have been discovered using the microlensing technique, with half a dozen new planet candidates identified in 2007 (yet to be published). Future possibilities include a network of wide-field 2m-class telescopes from the ground (which can combine survey and follow-up in the same setup) and a 1m-class survey telescope from space.