Unlike normal stars, brown dwarfs are objects that are not
massive enough to sustain hydrogen fusion in their cores. Disks of material are
known to exists around young brown dwarfs and this have led to speculation that
planetary systems similar to those around normal stars can also form around
brown dwarfs. In fact, there have been several detections of planetary-mass
companions around brown dwarfs. However, these brown dwarfs and their planetary-mass
companions have large mass ratios (companion comparable in mass to the brown
dwarf) and large companion-brown dwarf separations. For example, the
planetary-mass companions around brown dwarfs 2MASS 1207-3932 and 2MASS
0441-2301 have large mass ratios of ~ 0.16 for 2MASS 1207-3932 and ~ 0.25 - 0.5
for 2MASS 0441-2301. In addition, they also have large separations of ~ 15 AU
for 2MASS 0441-2301 and ~ 45 AU for 2MASS 1207-3932. This suggests a formation
scenario similar to a binary system rather than a star-planet system.
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A recent paper by Han et al. (2013) reports on the discovery
of a low mass ratio planetary-mass object in a close orbit around a brown
dwarf. This planetary-mass object was discovered via a gravitational
microlensing event in 2012. Microlensing is the astronomical phenomenon wherein
the brightness of a background star is magnified due to the bending of light by
the gravity of a passing foreground object. In this case, the foreground object
was the brown dwarf and its planetary-mass companion. This brown dwarf is
designated OGLE-2012-BLG-0358L and it is estimated to be ~ 24 Jupiter masses
while its planetary-mass companion is ~ 2 Jupiter masses. OGLE-2012-BLG-0358L
and its planetary-mass companion have a small mass ratio of ~ 0.08 and a small
separation of ~ 0.87 AU.
The mass ratio of OGLE-2012-BLG-0358L and its planetary-mass
companion is a factor of 2 to 3 times smaller the mass ratios of 2MASS
1207-3932 and 2MASS 0441-2301. In particular, the separation of OGLE-2012-BLG-0358L
and its planetary-mass companion is ~ 15 and ~ 40 times smaller than those of
2MASS 1207-3932 and 2MASS 0441-2301 respectively. As a result, the small mass
ratio and small separation suggests that the planetary-mass companion around OGLE-2012-BLG-0358L
may have formed from a disk of material around the brown dwarf, in a manner
similar to the formation of planets around normal stars.
Reference:
Han et al. (2013), “Microlensing Planet Around Brown-Dwarf”,
arXiv:1307.6335 [astro-ph.EP]