2026AJ....171...60K

Studying cold brown dwarfs is key to understanding the diverse characteristics of cold giant exoplanets atmospheres. COCONUTS-2 is a wide binary system composed of a T9 brown dwarf and an M3 star, which presents a unique opportunity to characterize a cold benchmark brown dwarf. As part of a JWST program to study the range of physical and atmospheric properties of the coldest brown dwarfs, we obtained NIRSpec G395H spectra (R ∼ 2700, 2.87-5.13 μm) and MIRI F1000W, F1280W, and F1800W photometry for COCONUTS-2 b. In this work, we find a 99% probability of the system belonging to the Corona of Ursa Major moving group (414 ± 23 Myr) using BANYAN Σ and its full kinematics. We also reestimate the astrometry of COCONUTS-2b using the MIRI data. We support this membership with a comparison of the rotation period, metallicity, and C/O ratio of the group with those of the COCONUTS-2 system. We also calculate its bolometric luminosity, which, combined with our age estimation, allows us to derive its mass, effective temperature, surface gravity, and radius with high precision. As a result of our analysis, we support the conclusion that COCONUTS-2 b is a planetary-mass object (7.5 ± 0.4 M_Jup), which was likely formed via the same mechanism as stars. In addition we compare the JWST spectrum to another object in the sample, J082507.35+280548.5 (0825+2805), a Y0.5 brown dwarf, which is a candidate member of the same moving group, but has a lower mass (3.7 ± 0.2 M_Jup). We identify absorption feature differences, which could indicate that 0825+2805 has stronger vertical mixing.