Ross 19B: An Extremely Cold Companion Discovered via the Backyard Worlds: Planet 9 Citizen Science Project

Through the Backyard Worlds: Planet 9 citizen science project, we have identified a wide-separation (∼10', ∼9900 au projected) substellar companion to nearby (∼17.5 pc), mid-M dwarf Ross 19. We developed new formalism for determining chance alignment probabilities based on BANYAN Σ tool, and find 100% probability that this is physically associated pair. detailed examination of 19A, system metal-poor ([Fe/H] = −0.40 ± 0.12) with an age Gyr. Combining existing photometry astrometry, 19B one coldest known companions, spectral type T/Y boundary, effective temperature K, mass in range 15–40 MJup. This new, extremely cold benchmark compelling target characterization future spectroscopic observations using facilities such as Hubble Space Telescope or James Webb Telescope.