Resolving the Microlens Mass Degeneracy for Earth-Mass Planets

Of all planet-finding techniques, microlensing is potentially the most sensitive to Earth-mass planets. However, microlensing lightcurves generically yield only the planet-star mass ratio: the mass itself is uncertain to a factor of a few. To determine the planet mass, one must measure both the ``microlens parallax'' and source-lens relative proper motion. Here we present a new method to measure microlens masses for terrestrial planets. We show that, with only a modest adjustment to the proposed orbit of the dedicated satellite that finds the events, and combined with observations from a ground-based observing program, the planet mass can be measured routinely. The dedicated satellite that finds the events will automatically measure the proper motion and one projection of the ``vector microlens parallax.'' If the satellite is placed in an L2 orbit, or a highly elliptical orbit around the Earth, the Earth-satellite baseline is sufficient to measure a second projection of the vector microlens parallax from the difference in the lightcurves as seen from the Earth and the satellite as the source passes over the caustic structure induced by the planet. This completes the mass measurement.