By Dr David Whitehouse BBC News Online science editor

Astronomers have detected a brown dwarf - a "failed" star - less than 12 light-years from the Sun. It is the nearest so far discovered.

Designated Epsilon Indi B, it is a companion to a well-known bright star in the southern sky, Epsilon Indi, previously thought to be single.

The brown dwarf was discovered from its rapid motion across the sky.

The failed star and its companion form a wide binary system, separated by more than 1,500 times the distance between the Sun and the Earth.

In between planets and stars

The object was first identified from archived photographs and later confirmed with observations with the European Southern Observatory 3.5-metre New Technology Telescope at the La Silla Observatory in South America.

Astronomers estimate that Epsilon Indi B has a mass just 45 times that of Jupiter, the largest planet in our Solar System, and a surface temperature of only 1,000 Celsius.

It is the nearest star-like object to the Sun found in 15 years, and with an intrinsic brightness just 0.002% that of the Sun, one of the faintest sources ever seen outside our Solar System.

The object belongs to the so-called T dwarf category of objects, which straddle the domain between stars and giant planets.

The main star of the pair, Epsilon Indi, is one of the 20 nearest stars to the Sun at a distance of just 11.8 light-years. It is a dwarf star and with a surface temperature of about 4,000 C, somewhat cooler than the Sun.

Lingering slow-burn

Future studies of the new object will provide astronomers with clues as to the formation and evolution of brown dwarfs, and yield insights into the border zone between planets and stars.

Brown Dwarfs are thought to form in much the same way as stars, by the gravitational collapse of clumps of cold gas and dust in dense molecular clouds.

For reasons not yet entirely clear, some clumps end up with masses less than about 7.5% of that of our Sun, or 75 times the mass of planet Jupiter.

Below that boundary, there is not enough pressure in the core to initiate nuclear hydrogen fusion - the source of power for ordinary stars like the Sun.

Except for a brief early phase when some deuterium is burned, these low-mass objects simply continue to cool and fade slowly away while releasing the heat left over from their birth.