Brown dwarfs are objects that are too low in mass to sustain hydrogen fusion in their cores and they occupy the mass range between gas giant planets and the lowest mass stars. The upper limit for the mass of a brown dwarf is around 80 times the mass of Jupiter while the lower limit for the mass of a brown dwarf is undefined as it overlaps with the masses of gas giant planets. Methane-bearing spectral class T brown dwarfs are the coolest known class of brown dwarfs. Although a large number of brown dwarfs are know, there remains a large gap between the temperature of the coolest known brown dwarfs and the gas giant planets in our solar system. The coolest known brown dwarfs have temperatures of around 500 degrees Kelvin while the gas giant planets in our solar system have temperatures of around 150 degrees Kelvin. Theoretical studies have shown that brown dwarfs in this temperature range exhibit spectroscopic characteristics that are distinct from the spectral class T brown dwarfs, such as ammonia absorption lines and scattering from clouds of water ice. Any brown dwarfs in this temperature range can be categorized into a new and cooler spectral class known as spectral class Y.
A paper by Kevin Luhman, et al. (2011) entitled “Discovery of a Candidate for the Coolest Known Brown Dwarf” describes the discovery of what might be the coolest known brown dwarf and a likely prototype for the spectral class Y. With an estimated temperature of 300 degrees Kelvin, WD 0806-661 B is a candidate for the coolest known brown dwarf and also cool enough for its atmosphere to contain clouds of water ice. WD 0806-661 B is in a wide orbit around a white dwarf star and if a similar age to its host star is assumed, WD 0806-661 B will be around 1.5 billion years old. Furthermore, based on evolutionary models of cooling brown dwarfs, WD 0806-661 B is estimated to have a mass of around 7 times the mass of Jupiter and this falls well within the range of masses for the more massive extrasolar planets. There are two mechanisms in which an object like WD 0806-661 B could have formed. Firstly, WD 0806-661 B could have formed from the coalescence of a fragmented cloud of gas at its current large distance from its host star. Secondly, WD 0806-661 B could be a gas giant planet that had been dynamically scattered into a much more distant orbit around its host star. If subsequent observations confirm WD 0806-661 B to be the coolest known brown dwarf, it will become a valuable target for studying atmospheres in an entirely new temperature regime that will consequently aid searches for the coldest brown dwarfs with facilities such as the Wide-field Infrared Survey Explorer (WISE) and the James Webb Space Telescope (JWST).