Brown dwarfs are bodies with masses in the range between those of giant planets and the lightest stars. Some are isolated, while others orbit normal stars or exist in star clusters. Since 1995, more than 100 methane brown dwarfs, or T dwarfs, have been found with spectra similar to that of the planet Jupiter and effective temperatures in the range 500-1300 K. The detection of even cooler bodies will open up a new arena for atmospheric physics and help to determine the formation rate of stars and brown dwarfs in our Galaxy as a function of both mass and of time. Here we report the discovery in the UKIDSS Galactic Plane Survey of a brown dwarf, UGPS J0722-05, that is not only far less luminous and significantly cooler than previously known objects but also the nearest to the Solar System. The measured distance is 2.9 +/- 0.4 pc, from which we deduce an effective temperature in the range 400-500 K. The Gemini/NIRI near infrared spectrum displays deeper water vapour and methane absorption bands than the coolest known T dwarfs, and an unidentified absorption feature at 1.275 microns. Time will tell whether this object is regarded as a T10 dwarf or the first example of a new spectral type.
It is expected that NASA’s Wise space telescope will find many brown dwarfs near our solar system and possibly stars closer than Proxima Centauri. It is even possible that a companion star for our sun will be found.