Simulations of a Space-Based Microlensing Survey for Terrestrial Extra-Solar Planets

The gravitational microlensing technique can be used to carry out a sensitive survey of planets ranging in mass from giant planets down to Mars-mass planets. This requires photometric monitoring of a large number of stars ($\sim 10^8$) with high angular and temporal resolution. The Galactic Exoplanet Survey Telescope (GEST) is a 1.5m space based telescope with a large field-of-view that has recently been submitted to NASA's Discovery competition to carry out such a extra-solar planet search survey. We present a simulation of the baseline GEST mission, and we use this simulation to determine GEST's extra-solar planet detection sensitivity. We find that GEST will be sensitive to planets down to the mass of Mars, and will detect 100 Earth-mass planets at 1 AU if every star has such a planet. GEST's highest sensitivity is at separations of 0.7-10 AU, but it will also have significant sensitivity at larger separations and will be able to detect free-floating planets in significant numbers. GEST will also be able to detect $\sim 50,000$ giant planets via transits, and it is, therefore, the only proposed planet detection method that is sensitive to planets at all orbital radii. An important strength of the gravitational microlensing technique is that low-mass planets can be detected with high signal-to-noise, and we find that GEST can detect Earth-mass planets with a typical significance level of nearly $30\sigma$. This means that the planetary signals are strong enough so that there is no confusion between planetary microlensing signals and other types of perturbations to the microlensing light curves.