Detecting earth-mass planets with gravitational microlensing

We show that Earth-mass planets orbiting stars in the Galactic disk and bulge can be detected by monitoring microlensed stars in the Galactic bulge. The star and its planet act as a binary lens which generates a light curve that can differ substantially from the light curve due only to the star itself. We show that the planetary signal remains detectable for planetary masses as small as an Earth mass when realistic source star sizes are included in the light curve calculation. These planets are detectable if they reside in the ''lensing zone,'' which is centered between 1 and 4 AU from the lensing star and spans about a factor of 2 in distance. If we require a minimum deviation of 4% from the standard point-lens microlensing light curve, then we find that more than 2% of all M(+) planets and 10% of all 10 M(+) in the lensing zone can be detected. If a third of all lenses have no planets, a third have 1 M(+) planets, and the remaining third have 10 M(+) planets then we estimate that an aggressive ground-based microlensing planet search program could find one Earth-mass planet and half a dozen 10 M(+) planets per year.