  We report the discovery of a young planetary-mass brown dwarf in the rho Oph
cloud core. The object was identified as such with the aid of a 1.5-2.4 micron
low-resolution spectrum obtained using the NIRC instrument on the Keck I
telescope. Based on the COND model, the observed spectrum is consistent with a
reddened (Av ~ 15-16) brown dwarf whose effective temperature is in the range
1200-1800 K. For an assumed age of 1 Myr, comparison with isochrones further
constrains the temperature to ~ 1400 K and suggests a mass of ~ 2-3 Jupiter
masses. The inferred temperature is suggestive of an early T spectral type,
which is supported by spectral morphology consistent with weak methane
absorption. Based on its inferred distance (~ 100 pc) and the presence of
overlying visual absorption, it is very likely to be a rho Oph cluster member.
In addition, given the estimated spectral type, it may be the youngest and
least massive T dwarf found so far. Its existence suggests that the initial
mass function for the rho Oph star-forming region extends well into the
planetary-mass regime.
