We report the discovery of a second field methane brown dwarf from the commissioning data of the Sloan Digital Sky Survey ( SDSS ) .
The object , SDSS J134646.45–003150.4 ( SDSS 1346–00 ) , was selected because of its very red color and stellar appearance .
Its spectrum between 0.8–2.5 \mu m is dominated by strong absorption bands of H _ { 2 } O and CH _ { 4 } and closely mimics those of Gliese 229B and SDSS 162414.37+002915.6 ( SDSS 1624+00 ) , two other known methane brown dwarfs .
SDSS 1346–00 is approximately 1.5 mag fainter than Gliese 229B , suggesting that it lies about 11 pc from the sun .
The ratio of flux at 2.1 \micron to that at 1.27 \micron is larger for SDSS 1346–00 than for Gliese 229B and SDSS 1624+00 , which suggests that SDSS 1346-00 has a slightly higher effective temperature than the others .
Based on a search area of 130 deg ^ { 2 } and a detection limit of z ^ { * } = 19.8 , we estimate a space density of 0.05 pc ^ { -3 } for methane brown dwarfs with T _ { eff } \sim 1000 K in the 40 pc ^ { 3 } volume of our search .
This estimate is based on small-sample statistics and should be treated with appropriate caution .