Microlensing by multiple planets in high-magnification events

Microlensing is increasingly gaining recognition as a powerful method for the detection and characterization of extrasolar planetary systems. Naively, one might expect that the probability of detecting the influence of more than one planet on any single microlensing light curve would be small. Recently, however, Griest & Safizadeh have shown that, for a subset of events, those with minimum-impact parameter u(min) less than or similar to 0.1 (high-magnification events), the detection probability is nearly 100% for Jovian-mass planets with projected separations in the range 0.6-1.6 of the primary Einstein ring radius R-E and remains substantial outside this zone. In this Letter, we point out that this result implies that, regardless of orientation, all Jovian-mass planets with separations near 0.6-1.6R(E) dramatically affect the central region of the magnification pattern and thus have a significant probability of being detected (or ruled out) in high-magnification events. The joint probability, averaged over all inclinations and phases, of two planets having projected separations within 0.6-1.6R(E) is substantial: 1%-15% for two planets with the intrinsic separations of Jupiter and Saturn orbiting around 0.3-1.0 M. parent stars. We illustrate by example the complicated magnification patterns and light curves that can result when two planets are present, and we discuss the possible implications of our result on detection efficiencies and the ability to discriminate between multiple and single planets in high-magnification events.