Detecting Earth‐Mass Planets with Gravitational Microlensing

We show that Earth mass planets orbiting stars in the Galactic disk and bulgecan be detected by monitoring microlensed stars in the Galactic bulge. The starand its planet act as a binary lens which generates a lightcurve which candiffer substantially from the lightcurve due only to the star itself. We showthat the planetary signal remains detectable for planetary masses as small asan Earth mass when realistic source star sizes are included in the lightcurvecalculation. 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 afactor of 2 in distance. If we require a minimum deviation of 4\% from thestandard point-lens microlensing lightcurve, then we find that more than 2\% ofall $\mearth$ planets and 10\% of all $10\mearth$ in the lensing zone can bedetected. If a third of all lenses have no planets, a third have $1\mearth$planets and the remaining third have $10\mearth$ planets then we estimate thatan aggressive ground based microlensing planet search program could find oneearth mass planet and half a dozen $10\mearth$ planets per year.Comment: 9 pages, AAS latex, epsf, 5 figures (postscript), 2 additional color postscript figures available at ftp://igpp.llnl.gov/pub/bennett/em_plane