In collaboration with Penn State colleagues Kevin Luhman and John Bochanski, we have uncovered a remarkably cold brown dwarf whose atmosphere appears to be a mere 300 K. This is the equivalent of a warm day at the beach, not what one expects from the surface of a star. In fact, the properties of this brown dwarf are so unusual that it may actually be a room temperature superplanet on an incredibly long orbit around a dead star.
The source, GJ 3483B, is a co-moving companion to the nearby white dwarf GJ 3483 (aka WD 0806-661), a DQ4 at a distance of 19.2 pc (62.6 lightyears). It was detected by the Spitzer Space Telescope in the mid-infrared 4.5 µm band in 2004 and again in 2009, and the motion of the brown dwarf agrees exactly with that of the white dwarf, validating its physical association. Because we know the distance to the white dwarf, we are able to measure the absolute brightness of GJ 3483B in the mid-infrared. It turns out to be about 2.5 magnitudes, or a factor of 10 times, fainter than the coldest brown dwarf known at the time, UGPS 0722-05. Moreover, it has been undetectable in several other photometric bands to strict limits, reaffirming its very dim nature. Using the mid-infrared brightness, the age of the white dwarf (about 1.5 billion years – a third of the age of our Sun) and brown dwarf evolutionary models, we find an estimated atmospheric temperature of 300 K – the same as the Earth – and a mass about 7 times that of Jupiter.
The temperature, mass and association of GJ 3483B with another star makes it seem very planet-like in appearance. However, it has an incredibly wide orbit – about 2500 Astronomical Units (1 AU is equal to the Earth-Sun distance) – implying an orbit period of at least 150,000 years. Is this a planet that has been pushed into a wide orbit after its host star, originally twice the mass of the Sun, shed 70% of its mass at the end of its stellar lifetime? Or is this simply a double star system, with one star happening to have a very low, planetary-like mass?
The low temperature inferred for GJ 3483B is itself remarkable, as the only equivalent-temperature atmosphere we know of is that of Earth. It is so cold that ice clouds are expected to be present in its upper atmosphere – an ice star! However, GJ 3483B would not itself be “habitable” – its hydrogen- and helium-rich atmosphere would be poisonous to life as we know it.
GJ 3483B is probably not the only one of its kind. Similarly cool brown dwarfs – referred to as “Y dwarfs” – are being found as companions to other brown dwarfs, and as isolated objects by NASA’s WISE mission. Yet GJ 3483B remains the coldest brown dwarf found so far, and studying its dim atmosphere will require the next generation of large ground-based and space-based telescopes.
This result was published in in the Astrophysical Journal Letters. The authors are Kevin Luhman (Penn State), Adam J. Burgasser (UCSD), and John Bochanski (Penn State). Also see the Science News & Analysis article describing the discovery and this article from Sol Station.