Extra-solar planets can be discovered by gravitational lensing when a foreground star with a bound planet passes across the line of sight and amplifies the light of a background star. The highest density of suitable background stars is provided by the bulge stars that swarm around the center of our own galaxy. The detection probability scales as the square-root of the planet mass. For Jupiter-mass planets the detection probability is of order 10 per cent per stellar lensing event, and 50-100 such events can be monitored each year. Earth-mass planets can be detected by this method.
The St Andrews/SAAO Robotic Telescope Initiative aims to establish in
South Africa a 1-metre Robotic Telescope equipped with a simultaneous
optical/infrared camera to discover extra-solar planets by monitoring
the gravitational lensing light curves. We expect to discover new
planets around other stars at a rate of at least 5-10 per year, to
build up a knowledge of the mass and orbit radius distributions of
planets around different types of stars.