2025AJ....169....9Z

Located 10.888 pc from Earth, COCONUTS-2b is a planetary-mass companion to a young (150-800 Myr) M3 star, with a wide orbital separation (6471 au) and a low companion-to-host mass ratio (0.021 ± 0.005). We have studied the atmospheric properties of COCONUTS-2b using newly acquired 1.0-2.5 μm spectroscopy from Gemini/Flamingos-2. The spectral type of COCONUTS-2b is refined to T9.5 ± 0.5 based on comparisons with T/Y dwarf spectral templates. We have conducted an extensive forward-modeling analysis, comparing the near-infrared spectrum and mid-infrared broadband photometry of COCONUTS-2b with 16 state-of-the-art atmospheric model grids developed for brown dwarfs and self-luminous exoplanets near the T/Y transition. The PH₃-free ATMO2020++, ATMO2020++, and Exo-REM models best match the specific observations of COCONUTS-2b, regardless of variations in the input spectrophotometry. This analysis suggests the presence of disequilibrium chemistry, along with a diabatic thermal structure and/or clouds, in the atmosphere of COCONUTS-2b. All models predict fainter Y-band fluxes than observed, highlighting uncertainties in the alkali chemistry models and opacities. We determine a bolometric luminosity of $\mathrm{log}({L}_{\mathrm{bol}}/{L}_{\odot })=-6.18$ dex, with a 0.5 dex wide range of [-6.43, -5.93] dex that accounts for various assumptions of atmospheric models. Using several thermal evolution models, we derive an effective temperature of ${T}_{\mathrm{eff}}={483}_{-53}^{+44}$ K, a surface gravity of $\mathrm{log}(g)={4.19}_{-0.13}^{+0.18}$ dex, a radius of $R={1.11}_{-0.04}^{+0.03}$R_Jup, and a mass of M = 8 ± 2 M_Jup. Various atmospheric model grids consistently indicate that COCONUTS-2b's atmosphere likely has subsolar or near-solar metallicity and C/O. These findings provide valuable insights into COCONUTS-2b's formation history and the potential outward migration to its current wide orbit.