Two new brown dwarfs have been discovered relatively close to to our solar system. Spotted by astronomers from the Leibniz Institute for Astrophysics Potsdam (AIP), the “failed stars”* are only 15 and 18 light-years from the sun.

15 and 18 light-years may not seem that close — after all, the nearest bona fide star to the sun, red dwarf Proxima Centauri, is a mere four light-years away. But if these discoveries continue it may not be long until a brown dwarf, and not Proxima, is found to be our nearest stellar neighbor.

ANALYSIS: Record Breaker: ‘Very Cold’ Brown Dwarf Discovered

These two brown dwarfs, called WISE J0254+0223 and WISE J1741+2553, are in addition to the AIP team’s 2003 discovery of another two brown dwarfs orbiting the star Epsilon Indi, 12 light-years from Earth. This new double discovery was made during analysis of recently published data from NASA’s Wide-field Infrared Survey Explorer (WISE).

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The new objects, radiating brightly in infrared wavelengths, grabbed the team’s attention as both are moving at high speed across the sky — this was an indication that they may be fairly close to us. Later, their close vicinity was confirmed after comparing their color and magnitude with other known brown dwarfs.

ANALYSIS: Ultra-Cold Brown Dwarf Discovered?

Brown dwarfs are often referred to as “failed stars” as they are not massive enough to support nuclear fusion in their cores, and yet they cannot be called “planets” as they don’t exhibit chemical differentiation with depth and have convective flows — a very star-like quality. Therefore, they exist in a stellar hinterland, where they are neither a star or a planet, and yet exhibit characteristics of both.

But astronomers still classify brown dwarfs by their spectral type (a scale of letters assigned to the luminosity of stars), which relates to their temperature. At the lowest, coolest end of the scale, radiating in infrared wavelengths, are the oddball brown dwarfs.

So far, the coolest brown dwarfs observed exist at the lowest end of the scale, with a spectral class of “T.” However, there is a theoretical class “Y” that is even cooler than the T-class brown dwarfs — they are predicted to have a temperature less than 225 degrees Celsius (440 F).

Both of these new brown dwarfs discovered by the AIP team appear to belong to the coolest type of T-class brown dwarfs, very close to the boundary with the theoretical Y-class brown dwarfs.

There have been a few Y-class “candidates” discovered and observatories such as the Keck telescopes in Hawaii are forming tighter and tighter constraints on when a T-type brown dwarf becomes a Y-type.

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The AIP press release ponders an interesting scenario: “It cannot be excluded that ultracool brown dwarfs surround us in similar high numbers as stars and that our nearest known neighbor will soon be a brown dwarf rather than Proxima Centauri.”

If this did happen, as pointed out by Paul Gilster, Project Icarus consultant [see: "Tau Zero Takes Aim at Interstellar Propulsion"], an interesting possibility would present itself:

Do brown dwarfs, hitherto undetected, surround us in large numbers? We certainly can’t rule out the possibility, and we can expect much more data mining from the riches WISE has accumulated. And yes, the case for a brown dwarf closer than the Alpha Centauri stars is still open, making the brown dwarf hunt of unusual interest for identifying potential targets for future probes.

When considering the vast distances between the stars, could such brown dwarfs even act as “stepping stones” through interstellar space? Perhaps our future interstellar probes, such as the Project Icarus starship, could use these ultracool brown dwarfs as refueling stations during their epic treks to the stars.

They may be considered “failed stars,” but these objects are of huge scientific interest and may even trump distant stars as the more interesting interstellar destination to visit.

*Or “overachieving Jupiters.” Whichever term you prefer.

Publication: “Two very nearby (d ~ 5 pc) ultracool brown dwarfs detected by their large proper motions from WISE, 2MASS, and SDSS data,” Scholz et al., 2011. arXiv:1105.4059v2 [astro-ph.GA]