Microlensing can detect and measure masses of exoplanets in a unique regime — wide-orbit, low-mass, solar system analogs. The planets found by microlensing can be characterized primarily in terms of their planet-star mass ratios. However, high resolution follow up data can measure the mass of the planets and host stars. A major advantage of the WFIRST microlensing survey is that the high angular resolution WFIRST images will enable most of the microlens exoplanet host stars to be clearly detected, and to have their mass and distance determined from the combination of high resolution imaging data and microlensing light curve constraints. The primary method of WFIRST mass measurement is being developed with the high angular resolution follow-up observations of planetary microlensing events using the Keck laser guide star (LGS) AO system and the Hubble Space Telescope (HST). Such information is crucial in order to exploit the unique sensitivity of the microlensing method to low mass planets beyond the snow line. While recent microlensing results challenge some aspects of the core accretion theory of planet formation, the comparison between observations and theory would be much more productive if the host and planet masses were known, instead of just the planet-star mass ratio. Observing time provided by NASA on the Keck Telescopes and HST has been invaluable for this ongoing effort, and we urge that these programs continue. We also support the development of advanced AO systems for the TMT and GMT that will greatly improve on the existing Keck high angular resolution observing capabilities for the faint microlens targets in the Galactic bulge.
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