Brown Dwarf with a Co-Moving Group of Stars

Brown dwarfs are objects with masses that span the mass regime between planets and stars - roughly 13 to 75 times the mass of Jupiter. Since brown dwarfs do not have sufficient mass to sustain hydrogen fusion, they slowly cool and dim from the moment they are born. W2319+7645 is a newly discovered brown dwarf located ~85 light years away and its mass could be in the planetary-mass regime. There is a 79 percent probability W2319+7645 is part of the Argus co-moving group of stars and a 21 percent probability it is just a field object. Since a more massive brown dwarf takes a longer time to cool (i.e. becomes less luminous) than a less massive brown dwarf, knowing a brown dwarf’s age and luminosity can allow its mass to be estimated.

Young brown dwarfs associated with co-moving groups of stars can have their ages estimated more accurately than young brown dwarfs in the field. Assuming W2319+7645 is part of the Argus co-moving group of stars, then its age is somewhere between 30 to 50 million years. Furthermore, the distance and observed brightness of W2319+7645 allow its luminosity to be determined and the effective temperature of W2319+7645 is estimated to be 1800 ± 200 K. Based on its age and luminosity, the mass of W2319+7645 is estimated to be 12.1 ± 0.4 times the mass of Jupiter, placing it in the planetary-mass regime.

Reference:
Castro & Gizis (2015), “Discovery of an L4β Candidate Member of Argus in the Planetary Mass Regime: WISE J231921.92+764544.4”, arXiv:1512.06524 [astro-ph.SR]