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WISE J053516.80−750024.9 (designation abbreviated to WISE 0535−7500) is either a brown dwarf or a free planet. It has spectral class ≥Y1 and is located in constellation Mensa. It is estimated to be 13 light-years from Earth. As of October 2015, that makes it the third-nearest Y dwarf, after WISE 0855−0714 and WISE 0350−5658. It is nearer than all but four naked eye stars, and the 31st-nearest star system to the Solar System. If a free planet, it is the second-nearest, the nearest being WISE 0855−0714. If a brown dwarf, then it is the ninth-nearest brown dwarf.
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Discovery
WISE 0535−7500 was discovered in 2012 by J. Davy Kirkpatrick et al. from data, collected by Wide-field Infrared Survey Explorer (WISE) Earth-orbiting satellite — NASA infrared-wavelength 40-centimetre (16 in) space telescope, which mission lasted from December 2009 to February 2011. In 2012 Kirkpatrick et al. published a paper in The Astrophysical Journal, where they presented the discovery of seven new found by WISE brown dwarfs of spectral type Y, among which also was WISE 0535−7500.
Distance
Trigonometric parallax of WISE 0535−7500 is 0.250 ± 0.079 arcsec, corresponding to a distance of 4.0+1.9
−-0.9 pc and 13.0+6.1
−3.1 ly.
WISE 0535−7500 distance estimates
Non-trigonometric distance estimates are marked in italic. The best estimate is marked in bold.
Y dwarf
Brown dwarfs are defined as substellar objects that have at some time in their lives burnt deuterium in their interior. The borderline between a brown dwarf and a planet is conventionally taken to be 13 times the mass of Jupiter. All brown dwarfs are either L dwarfs, T dwarfs or Y dwarfs, in order of decreasing temperature. An increasing number after the letter in the spectral type also means decreasing temperature, a Y2 dwarf is cooler than a Y1 dwarf is cooler than a Y0 dwarf. Planets can also be L dwarfs, T dwarfs or Y dwarfs.
If planets are L dwarfs or T dwarfs then they must be both young (younger than 350 million years) and heavy. The cooler a Y dwarf, the higher the likelihood that it is a planet. WISE 0535-7500 is the second-nearest Y dwarf that could be of a type cooler than Y1, so could easily be the second-nearest free planet to Earth. Of the two nearer Y dwarfs, WISE 0855−0714 has an effective temperature below the freezing point of water, and WISE 0350−5658 has an effective temperature above the boiling point of water. WISE 0535−7500 is of a spectral type between that of WISE 0855−0714 and WISE 0350−5658 so the possibility exists that it may in the correct temperature range for liquid water to exist on its surface.