  We report the discovery of a very cool, isolated brown dwarf, UGPS 0722-05,
with the UKIDSS Galactic Plane Survey. The near-infrared spectrum displays
deeper H2O and CH4 troughs than the coolest known T dwarfs and an unidentified
absorption feature at 1.275 um. We provisionally classify the object as a T10
dwarf but note that it may in future come to be regarded as the first example
of a new spectral type. The distance is measured by trigonometric parallax as
d=4.1{-0.5}{+0.6} pc, making it the closest known isolated brown dwarf. With
the aid of Spitzer/IRAC we measure H-[4.5] = 4.71. It is the coolest brown
dwarf presently known -- the only known T dwarf that is redder in H-[4.5] is
the peculiar T7.5 dwarf SDSS J1416+13B, which is thought to be warmer and more
luminous than UGPS 0722-05. Our measurement of the luminosity, aided by
Gemini/T-ReCS N band photometry, is L = 9.2 +/- 3.1x10^{-7} Lsun. Using a
comparison with well studied T8.5 and T9 dwarfs we deduce Teff=520 +/- 40 K.
This is supported by predictions of the Saumon & Marley models. With apparent
magnitude J=16.52, UGPS 0722-05 is the brightest T dwarf discovered by UKIDSS
so far. It offers opportunities for future study via high resolution
near-infrared spectroscopy and spectroscopy in the thermal infrared.
