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 \mu m 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 ( A _ { V } \sim 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 \sim 1400 K and suggests a mass of \sim 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 ( \sim 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 .