Robert Hurt/JPL, Janella Williams/Penn State University. Contrary to their name, brown dwarfs are not brown. The sun is immediately to WISE J085510.83-071442.5's right in this artist's impression

The sun has a new neighbor , one so faint we have not noticed it before, despite being the fourth closest system to our own. In addition to its significance from being so close, the new object is also the coolest member of its category ever observed.

Brown dwarfs fill the gap between large planets and the smallest stars. They start out collapsing from patches of gas just as stars do , but never get big enough to initiate fusion of ordinary hydrogen. While they do generate a little heat from fusing small amounts of deuterium, and in some cases lithium, their main source of heat is the from gravitational collapse . This slowly dies away with time, and the deuterium burning also runs out in timescale measured in millions, rather than billions, of years.

Until the new discovery, which for the moment bares the romantic name WISE J085510.83-071442.5, the coolest known brown dwarfs had temperatures close to the terrestrial global average – comfortable places to be if you could ignore the crushing gravity and the fact there's no ground to stand on. However, the new discovery falls somewhere between -38°C and -13°C. Planets, should they exist, will naturally be much colder.

The discovery was made by Associate Professor Kevin Luhman of Penn State University, using the NASA's Wide-field Infrared Survey Explorer and announced in The Astrophysical Journal . Luham is searching images taken by WISE for objects that have moved between observations. If an object is close it will appear to move against the background stars as the Earth shifts from one side of the sun to the other, and is also likely to appear to be moving from year to year much faster than something more distant.

"This object appeared to move really fast in the WISE data," said Luhman. "That told us it was something special.” While WISE is good for picking up objects worthy of further exploration, follow up requires something more powerful. Luham used images taken by the Spitzer space telescope and the Gemini South telescope, Chile. As well as confirming WISE J085510.83-071442.5's rapid movement Spitzer was able to measure the wavelength at which its radiation peaks. Since peak wavelength for objects of this sort is dependent on temperature this provides us with the ability to calculate its exceptionally cold status.

WISE J085510.83-071442.5 has mass low enough to raise tricky questions of categorization. Current estimates are 3-10 times the mass of Jupiter. Since some definitions hold that objects must be at least 10 Jupiter masses to earn the title of Brown dwarf it may eventually find itself demoted in the records, just as Pluto was. However, while it is possible this is a large planet that was ejected from its solar system at some point, it is considered more likely to have formed on its own, enough to earn brown dwarf status on other definitions.

Contrary to their name, brown dwarfs are not generally brown. As noted, their radiation is brightest in the infrared range, where our eyes cannot detect it. Their color varies, depending on the chemicals present in their outer atmosphere.

It is only a year since the third closest star star system turned up 6.5 light years away , two brown dwarfs also found by Luhman, and now dubbed Luhman 16. It is now clear that our patch of space is much more crowded than we realized.

"It is remarkable that even after many decades of studying the sky, we still do not have a complete inventory of the Sun's nearest neighbors," said Michael Werner, NASA's project scientist for Spitzer. "This exciting new result demonstrates the power of exploring the universe using new tools, such as the infrared eyes of WISE and Spitzer."