We have used multi-epoch images from the Infrared Array Camera on board the Spitzer Space Telescope to search for substellar companions to stars in the solar neighborhood based on common proper motions .
Through this work , we have discovered a faint companion to the white dwarf WD 0806-661 .
The comoving source has a projected separation of 130 \arcsec , corresponding to 2500 AU at the distance of the primary ( 19.2 pc ) .
If it is physically associated , then its absolute magnitude at 4.5 µm is \sim 1 mag fainter than the faintest known T dwarfs , making it a strong candidate for the coolest known brown dwarf .
The combination of M _ { 4.5 } and the age of the primary ( 1.5 Gyr ) implies an effective temperature of \sim 300 K and a mass of \sim 7 M _ { Jup } according to theoretical evolutionary models .
The white dwarf ’ s progenitor likely had a mass of \sim 2 M _ { \odot } , and thus could have been born with a circumstellar disk that was sufficiently massive to produce a companion with this mass .
Therefore , the companion could be either a brown dwarf that formed like a binary star or a giant planet that was born within a disk and has been dynamically scattered to a larger orbit .