  Since 1995, more than 500 exoplanets have been detected using different
techniques, of which 11 were detected with gravitational microlensing. Most of
these are gravitationally bound to their host stars. There is some evidence of
free-floating planetary mass objects in young star-forming regions, but these
objects are limited to massive objects of 3 to 15 Jupiter masses with large
uncertainties in photometric mass estimates and their abundance. Here, we
report the discovery of a population of unbound or distant Jupiter-mass
objects, which are almost twice (1.8_{-0.8}^{+1.7}) as common as main-sequence
stars, based on two years of gravitational microlensing survey observations
toward the Galactic Bulge. These planetary-mass objects have no host stars that
can be detected within about ten astronomical units by gravitational
microlensing. However a comparison with constraints from direct imaging
suggests that most of these planetary-mass objects are not bound to any host
star. An abrupt change in the mass function at about a Jupiter mass favours the
idea that their formation process is different from that of stars and brown
dwarfs. They may have formed in proto-planetary disks and subsequently
scattered into unbound or very distant orbits.
