Nomads of the Galaxy
  We estimate that there may be up to ~10^5 compact objects in the mass range
10^{-8} -10^{-2} solar mass per main sequence star that are unbound to a host
star in the Galaxy. We refer to these objects as nomads; in the literature a
subset of these are sometimes called free-floating or rogue planets. Our
estimate for the number of Galactic nomads is consistent with a smooth
extrapolation of the mass function of unbound objects above the Jupiter-mass
scale, the stellar mass density limit, and the metallicity of the interstellar
medium. We analyze the prospects for detecting nomads via Galactic
microlensing. The Wide-Field Infrared Survey Telescope (WFIRST) will measure
the number of nomads per main sequence star greater than the mass of Jupiter to
~ 13%, and the corresponding number greater than the mass of Mars to ~25%.
All-sky surveys such as GAIA and LSST can identify nomads greater than about
the mass of Jupiter. We suggest a dedicated drift scanning telescope that
covers approximately 100 square degrees in the Southern hemisphere could
identify nomads as small as 10^{-8} solar mass via microlensing of bright stars
with characteristic lightcurve timescales of a few seconds.
