  Various methods have been proposed to search for extrasolar planets. Compared
to the other methods, microlensing has unique applicabilities to the detections
of Earth-mass and free-floating planets. However, the microlensing method is
seriously flawed by the fact that the masses of the detected planets cannot be
uniquely determined. Recently, Gould, Gaudi, & Han introduced an observational
setup that enables one to resolve the mass degeneracy of the Earth-mass
planets. The setup requires a modest adjustment to the orbit of an already
proposed Microlensing planet-finder satellite combined with ground-based
observations. In this paper, we show that a similar observational setup can
also be used for the mass determinations of free-floating planets with masses
ranging from ~0.1 M_J to several Jupiter masses. If the proposed observational
setup is realized, the future lensing surveys will play important roles in the
studies of Earth-mass and free-floating planets, which are the populations of
planets that have not been previously probed.
