We report the discovery of the coolest field dwarf yet known , selected as a stellar object with extremely red colors from commissioning imaging data of the Sloan Digital Sky Survey .
Its spectrum from 0.8 to 2.5 \mu m is dominated by strong bands of H _ { 2 } O and CH _ { 4 } .
Its spectrum and colors over this range are very similar to those of Gliese 229B , the only other known example of a methane dwarf .
It is roughly 1.2 mag fainter than Gliese 229B , implying that it lies at a distance of roughly 10 pc .
Such a cool object must have a mass well below the hydrogen-burning limit of 0.08 M _ { \odot } , and therefore is a genuine brown dwarf , with a probable mass in the range 0.015-0.06 M _ { \odot } for an age range of 0.3-5 Gyr .