  Observations of accretion disks around young brown dwarfs have led to the
speculation that they may form planetary systems similar to normal stars. While
there have been several detections of planetary-mass objects around brown
dwarfs (2MASS 1207-3932 and 2MASS 0441-2301), these companions have relatively
large mass ratios and projected separations, suggesting that they formed in a
manner analogous to stellar binaries. We present the discovery of a
planetary-mass object orbiting a field brown dwarf via gravitational
microlensing, OGLE-2012-BLG-0358Lb. The system is a low secondary/primary mass
ratio (0.080 +- 0.001), relatively tightly-separated (~0.87 AU) binary composed
of a planetary-mass object with 1.9 +- 0.2 Jupiter masses orbiting a brown
dwarf with a mass 0.022 M_Sun. The relatively small mass ratio and separation
suggest that the companion may have formed in a protoplanetary disk around the
brown dwarf host, in a manner analogous to planets.
