Panchromatic view of the frigid Jovian exoplanet COCONUTS-2 b

¹ Laboratoire J.-L. Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, 06304 Nice, France
² IPAG, Université Grenoble-Alpes, CNRS, 38000 Grenoble, France
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Department of Physics & Astronomy, University of Rochester, Rochester, NY 14627, USA
⁵ Department of Astrophysics, American Museum of Natural History, New York, NY 10024, USA
⁶ Université Paris Cité, Université Paris-Saclay, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁷ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁸ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield, UK
¹⁰ Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA
¹¹ Institute of Particle Physics and Astrophysics, ETH Zürich, Wolfgang-Pauli-Str 27, 8049 Zürich Switzerland
¹² LIRA, Observatoire de Paris, Univ. PSL, CNRS, Sorbonne Universit´e, Univ. Paris Diderot, Sorbonne Paris Cit´e, 5 place Jules Janssen, 92195 Meudon, France
¹³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
¹⁴ NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁵ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹⁶ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Santiago, Chile
¹⁷ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
¹⁸ Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
¹⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁰ Fakultät für Physik, Universität Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
²¹ Physikalisches Institut, Universität Bern, Gesellschaftsstr. 6, CH-3012 Bern, Switzerland
²² AURA for the European Space Agency (ESA), ESA Ofcice, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile

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Received: 12 January 2026
Accepted: 7 April 2026

Abstract

Context. 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 (CO₂, NH₃, PH₃), challenge atmospheric models, and raise questions about the formation pathways and evolution of these objects.

Aims. We revisited the atmosphere of the cold (T_eff = 483₋₅₃⁺⁴⁴ K), mature (414 ± 23 Myr), and large-separation (>5000 au) Jovian exoplanet COCONUTS-2 b (WlSEPA 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.

Methods. We used a high signal-to-noise MIRI-LRS spectrum (5.45-11 μm, R_λ ∼ 100) of COCONUTS-2 b revealing prominent molecular features of H₂O, CH₄, and NH₃. 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 T_eff, log(g), and [M/H]. 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.

Results. 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 spectra (with and without PH₃) 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: T_eff = 496₋₃⁺⁵ K, log(g) = 4.30_−0.02^+0.04 dex, [M/H] = −0.02_−0.02^+0.03 dex, and R = 1.03_−0.02^+0.01 R_Jup. 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 ± 0.002 dex. Combined with a previous estimate of the system age (414 ± 23 Myr), cooling models predict a mass of M = 7.3 ± 0.3 M_Jup.

Conclusions. 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.