Panchromatic view of the frigid Jovian exoplanet COCONUTS-2 b

Cold exo-Jovian planets are beginning to be imaged and characterized using the James Webb Space Telescope (JWST) instruments. These observations often reveal new molecular species (CO_2, NH_3, PH_3), challenge atmospheric models, and raise questions about the formation pathways and evolution of these objects. We revisited the atmosphere of the cold ( _ $=483^ +44 -53 $ K), mature (414 Myr), and large-separation ($>5000$ au) Jovian exoplanet COCONUTS-2 b (WISEPA J075108.79-763449.6), adding new spectral information beyond 5 μm and combining them with existing spectrophotometry to consolidate the constraints on the object properties and identify disagreements from self-consistent atmospheric models. We used a high signal-to-noise MIRI-LRS spectrum (5.45--11 μm, . We also investigated the use of Gaussian processes to account for correlated noise either caused by the spectrograph or by systematic departures of models in the inversion framework. sim100) of COCONUTS-2 b revealing prominent molecular features of H_2O, CH_4, and NH_3. This dataset is combined with spectra from Gemini/FLAMINGOS-2 and JWST/NIRSpec (G395H), as well as photometry from WISE and Spitzer, resulting in almost continuous wavelength coverage from 1 to 15 μm. We analyzed the data using five grids of self-consistent atmospheric models, spanning a wide range of log(g), and M/H All models manage to fit the overall combined observations, but predict fainter flux in Y- and N-bands. Classical model comparison suggests that the ATMO2020++ synthetic specra (with and without PH_3) are statistically preferred. However, when accounting for correlated noise using Gaussian processes, Sonora Elf Owl models are favored, although they still provide a comparatively poor fit to the data with bulk properties inconsistent with cooling model predictions. Fitting for the correlated noise of the three spectroscopic instruments, the ATMO2020++ model yields constraints consistent with previous studies and evolutionary model predictions: _ $=496^ +5 -3 $ K, log(g) $=4.30^ +0.04 _ -0.02 $ dex, M/H $=-0.02^ +0.03 _ -0.02 $ dex, and R $=1.03^ +0.01 _ -0.02 $ The extended wavelength coverage provided by MIRI (accounting for 41% of the bolometric flux) completes the SED, yielding a precise luminosity estimation of log(L/L_⊙) $=-6.166 dex. Combined with a previous estimate of the system age (414 Myr), cooling models predict a mass of M $=7.3 The preferred models suggest a metallicity consistent with that of the primary, potentially supporting a binary-like formation scenario. Remaining discrepancies across spectral bands and between model grids suggest incomplete chemistry modeling and highlight the need for improved treatments of alkali condensation and diabatic processes for models at these low effective temperatures.