WISE J0146+4234AB is a substellar binary consisting of a T9 brown dwarf and a slightly cooler Y0 brown dwarf in a tight orbit around one another. Brown dwarfs cool with time and more massive brown dwarfs cool more slowly. Observations show both components of WISE J0146+4234AB are less luminous than any other substellar binaries currently known. This indicates both components must be around planetary mass (~12 to 21 times Jupiter’s mass) and probably very old as well.
The low luminosity also implies that the T9 brown dwarf is unusually cold. Assuming an age ~10 billion years, its temperature is only 345 ± 45 K. Its companion Y0 brown dwarf is slightly cooler at 330 ± 45 K. Up close; these substellar objects will appear utterly black. Only in the infrared do they shine.
Both components of WISE J0146+4234AB have a projected separation ~ 0.9 AU and an estimated orbital period less than ~10 years. This makes WISE J0146+4234AB one of the tightest known ultra-cool substellar binary. Its relatively short orbital period will allow better mass measurements in the coming few years from observing the motion of both components around one another. Other such binaries have orbital periods of several tens to hundreds of years.
Trent J. Dupuy, Michael C. Liu, S. K. Leggett, “Discovery of a Low-Luminosity, Tight Substellar Binary at the T/Y Transition”, arXiv:1502.04707 [astro-ph.SR]