About 63 light years away from Earth, lies a very cool brown dwarf. It is approximately seven times the mass of Jupiter, and its temperature is of 30°C (86°F), making thinner and thinner the line between small stars and giant planets.
Brown dwarfs are peculiar objects: they are neither planets, nor stars. Actually, they are often referred to as “failed stars”, as their mass is too low to sustain the hydrogen fusion required to become a star: the lower limit for hydrogen fusion to start is between 75 and 80 Jupiter masses.
If it’s not a star, then why is it not a planet either? For sure, brown dwarfs and gas giant planets like Jupiter share a few characteristics, but they also have some noticeable differences: brown dwarfs are roughly the same size as Jupiter, but they are much denser. Unlike gas giants, brown dwarfs also emit radiation in the X-ray and infrared spectra.
This typical infrared spectrum allowed NASA’s Spitzer telescope to detect the new cold brown dwarf candidate, nicely named WD 0806-661B. Another pretty cool brown dwarf was also discovered in the past few weeks, by the Keck II Telescope in Hawai: CFBDSIR J1458+1013B (another cute name) is 75 light-years away, and its temperature is of less than 100°C. However, as I mentioned before, the new brown dwarf discovered by Spitzer is even cooler with its 30°C, making it the candidate for the coolest known brown dwarf.
Interestingly, WD 0806-661B is orbiting a white dwarf (a star which used to be like our Sun), making the frontier between a brown dwarf and an exoplanet even thinner. Is it a planet? Is it a brown dwarf? The thing is that WD 0806-661B (one more time, and I am sure you will perfectly remember that name!) is orbiting its star at a huge distance: it is 2,500 times the distance between Earth and the Sun, which is too far for a planet to form. Even with such an orbit, it is still unclear what the object really is: as Ian O’Neill explains, models predict that planets surviving the expansion of a dying star into a supergiant, which later collapses back into a white dwarf, will drift into wider orbits.
Further examination is needed to reveal the nature of these two objects. If they are confirmed as being brown dwarfs, then they will be the first to belong to the Y-class : their temperature is cool enough for water vapor to condense into clouds in their atmospheres. This category has only been theorized, but if the two objects are confirmed as brown dwarfs, that would definitely make them much more “planet-like” than “star-like”.