Two ultracool brown dwarfs located only 15 and 18 light years away from the Sun have been discovered using the Wide-Field Infrared Survey Explorer.

The brown dwarfs were discovered by astronomers at the Leibniz Institute for Astrophysics Potsdam (AIP) using the Wide-Field Infrared Survey Explorer (WISE). The duo, dubbed WISE J0254+0223 and WISE J1741+2553, lie at 18 and 15 light years away respectively. While the closest known brown dwarf is only 12 light years away, the discovery of two new neighbours could mean that we are surrounded by these “failed stars” and that red dwarf star Proxima Centauri, at 4.2 light years away, might eventually have competition as the Sun’s nearest companion.

Ralf-Dieter Scholz and his team at AIP used WISE to detect the brown dwarfs via their strong presence in the infrared. The brown dwarf pair also exhibit a high proper motion, that is, their position in the sky has changed from our point of view in a relatively short period of time – objects close to us are more likely to have an easily detectable proper motion.

Brown dwarfs are failed stars which were never hot enough and massive enough to initiate nuclear fusion. The letters OBAFGKM are assigned to ordinary stars in order to classify their spectral type, with O being the hottest and M being the coolest. Our Sun is a G type star with a temperature of 6,000 Kelvin. Brown dwarfs are cooler than M dwarfs and the letters L and T now follow M in the spectral sequence. The newly discovered brown dwarfs are “late type” T dwarfs, with a spectral type of between T8 and T10, meaning that they lie towards the cooler end of the T-class brown dwarfs. Another nearby brown dwarf, UGPS 0722, allows the temperature of the new brown dwarfs to be estimated. “The spectra of one of our objects, WISE 1741, and of the comparison T10 dwarf UGPS 0722 are very similar,” Scholz tells Astronomy Now. “Therefore we can assume a similar temperature [of] about 500 Kelvin. For our second object, WISE 0254, we have no spectroscopic observations so far, but I expect it to have a similar spectral type, hence a similar temperature.”

At a temperature of only 500 Kelvin, which is a typical oven temperature of 230 degrees Celsius, these brown dwarfs are so cool that they may even belong to the proposed Y-class brown dwarfs. “I would not exclude that these objects will later be classified as early Y dwarfs, once there are more of these ultracool brown dwarfs that will have been observed and the classification system for Y dwarfs will be established,” explains Scholz.

It is thought that brown dwarfs could contribute to some of the missing mass in the Universe, as they are very difficult to detect. However, Scholz says that this would not be a significant contribution. “I think that the contribution of brown dwarfs to baryonic dark matter remains very small (at most a few per cent), even if we will find in the future that the number of brown dwarfs is comparable to the number of stars. At the moment, we know about ten times less brown dwarfs than stars in the immediate solar neighbourhood.”