  The gravitational microlensing light curves that reveal the presence of
extrasolar planets generally yield the planet-star mass ratio and separation in
units of the Einstein ring radius. The microlensing method does not require the
detection of light from the planetary host star. This allows the detection of
planets orbiting very faint stars, but it also makes it difficult to convert
the planet-star mass ratio to a value for the planet mass. We show that in many
cases, the lens stars are readily detectable with high resolution space-based
follow-up observations in a single passband. When the lens star is detected,
the lens-source relative proper motion can also be measured, and this allows
the masses of the planet and its host star to be determined and the star-planet
separation can be converted to physical units. Observations in multiple
passbands provide redundant information, which can be used to confirm this
interpretation. For the recently detected super-Earth planet,
OGLE-2005-BLG-169Lb, we show that the lens star will definitely be detectable
with observations by the Hubble Space Telescope (HST) unless it is a stellar
remnant. Finally, we show that most planets detected by a space-based
microlensing survey are likely to orbit host stars that will be detected and
characterized by the same survey.
