The Use of High Magnification Microlensing Events in Discovering Extra-solar Planets

Astrophys.J.500:37-50,1998 Hundreds of gravitational microlensing events have now been detected towards the Galactic bulge, with many more to come. The detection of fine structure in these events has been theorized to be an excellent way to discover extra-solar planetary systems along the line-of-sight to the Galactic center. We show that by focusing on high magnification events the probability of detecting planets of Jupiter mass or greater in the lensing zone (.6 -1.6 $R_E$) is nearly 100%, with the probability remaining high down to Saturn masses and substantial even at 10 Earth masses. This high probability allows a nearly definitive statement to made about the existence of lensing zone planets in each such system that undergoes high magnification. One might expect lightcurve deviations caused by the source passing near the small primary lens caustic to be small due to the large distance of the perturbing planet, but this effect is overcome by the high magnification. High magnification events are relatively rare (e.g. $\sim 1/20$th of events have peak magnifications greater than 20), but they occur regularly and the peak can be predicted in advance, allowing extra-solar planet detection with a relatively small use of resources over a relatively small amount of time.