CaltechAUTHORS
Published February 2026 | Version Published
Journal Article Open

The Diversity of Cold Worlds: Age and Characterization of the Exoplanet COCONUTS-2 b

Creators

  • 1. ROR icon California Institute of Technology
  • 2. ROR icon Infrared Processing and Analysis Center
  • 3. ROR icon American Museum of Natural History
  • 4. ROR icon University of California, Santa Cruz
  • 5. ROR icon Espace pour la vie
  • 6. ROR icon University of Montreal
  • 7. ROR icon University of Hertfordshire
  • 8. ROR icon The University of Texas at Austin
  • 9. ROR icon Amherst College
  • 10. ROR icon Trinity College Dublin
  • 11. ROR icon United States Naval Observatory
  • 12. ROR icon San Francisco State University
  • 13. ROR icon The Graduate Center, CUNY
  • 14. ROR icon Hunter College
  • 15. Backyard Worlds: Planet 9, USA
  • 16. ROR icon Osservatorio Astrofisico di Torino
  • 17. ROR icon University of Chile

Abstract

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 MJup), 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 MJup). We identify absorption feature differences, which could indicate that 0825+2805 has stronger vertical mixing.

Copyright and License

© 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

R.K. would like to thank Peter R Eisenhardt and Jason Curtis for helpful discussions during the preparation of this manuscript. Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Program PID#2124 is supported through contract JWST GO-02124.002-A. This research made use of the Montreal Open Clusters and Associations (MOCA) database, operated at the Montréal Planétarium (J. Gagné 2024; J. Gagné et al. 2025, in preparation). This publication makes use of The Data & Analysis Center for Exoplanets (DACE), which is a facility based at the University of Geneva (CH) dedicated to extrasolar planet data visualization, exchange, and analysis. DACE is a platform of the Swiss National Centre of Competence in Research (NCCR) Planets, federating Swiss expertise in Exoplanet research. The DACE platform is available at https://dace.unige.ch. E.C. and R.A.M. acknowledge support from Chile FONDECYT/ANID #124 0049. R.A.M. acknowledges support from Fondo GEMINI, Astrónomo de Soporte GEMINI-ANID grant #3223 AS0002. J.M.V. acknowledges support from a Royal Society—Research Ireland University Research Fellowship (URF/1/221932, RF/ERE/221108). B.B. acknowledges support from the UK Research and Innovation Science and Technology Facilities Council [ST/X001091/1]. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute. The specific observations analyzed can be accessed via DOI: 10.17909/rxm9-qd05. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support to MAST for these data is provided by the NASA Office of Space Science via grant NAG5-7584 and by other grants and contracts.

Contributions

R.K. performed most of the analysis for this work, coordinated the collaboration with coauthors, and was responsible for writing and submitting the manuscript.

C.A.B. provided guidance and regular feedback during the development of the project, as well as funding for this project.

A.R.D. together with R.K. were responsible for the forward-modeling analysis and writing that section in the manuscript.

J.K.F. provided regular feedback during the development of the project, and helped to coordinate the collaboration between coauthors. J.K.F. is also the PI of the JWST GO program that obtained the spectrum and photometry for the objects studied in this work.

B.L. and G.S. provided valuable guidance with the forward-modeling analysis, model comparison, and analysis of molecule abundances. Both coauthors contributed to refining the manuscript through detailed reviews.

J.D.K. and F.M. estimated the new astrometry for COCONUTS-2 b, and F.M. was responsible for writing Section 2.3, describing this analysis.

J.G. provided valuable feedback on the membership analysis using Banyan Σ, and the tests done to estimate the chance alignment.

J.C. and B.B. added significant discussion on the composition of the atmosphere of COCONUTS-2 b, by comparing the results from this work with the retrieval analysis they are doing.

N.W. helped with the forward-modeling analysis by mentoring A.R.D. at the beginning of the project.

M.J.R. improved significantly the comparison to the 0825+2805 object by sharing the results from the retrieval analysis, and discussing the results from the forward modeling of COCONUTS-2 b.

D.C.B.G., J.M.V., A.C.S., and E.C.G. improved the manuscript significantly with a thorough review of the text, and provided insightful comments which improved the work.

S.A.M. provided valuable feedback on the estimation of bolometric luminosity of the object and development of SEDkit.

A.R. improved the manuscript significantly with a thorough review of the text.

R.S., E.C., and R.A.M. were responsible for obtaining the data we used in this work to estimate the astrometry of COCONUTS-2 b.

Software References

astropy (Astropy Collaboration et al. 2013, 2018, 2022); ChronoFlow (P. R. Van-Lane et al. 2025); dace-query; dynesty (J. S. Speagle 2020; S. Koposov et al. 2024); emcee (D. Foreman-Mackey et al. 2013); imcore; matplotlib (J. D. Hunter 2007); mpfit (C. B. Markwardt 2009); numpy (C. R. Harris et al. 2020); PypeIt (J. X. Prochaska et al. 2020a, c,2020b); SEDA (G. Suárez et al. 2021); SEDkit (J. C. Filippazzo et al. 2015; J. Filippazzo et al. 2025); scipy (P. Virtanen et al. 2020).

Files

Kiman_2026_AJ_171_60.pdf

Files (8.0 MB)

Name Size Download all
md5:79773dee0d9cacfe0ac37e3f58abaf00
8.0 MB Preview Download

Additional details

Related works

Is new version of
Discussion Paper: arXiv:2511.20923 (arXiv)
Is supplemented by
Dataset: 10.17909/rxm9-qd05 (DOI)

Funding

Jet Propulsion Laboratory
National Aeronautics and Space Administration
80NM0018D0004
National Aeronautics and Space Administration
GO-02124.002-A
Fondo Nacional de Desarrollo Científico y Tecnológico
1240049
Fondo Nacional de Desarrollo Científico y Tecnológico
GEMINI-ANID 3223AS0002
Taighde Éireann - Research Ireland
URF/1/221932
Taighde Éireann - Research Ireland
RF/ERE/221108
Science and Technology Facilities Council
ST/X001091/1
National Aeronautics and Space Administration
NAS5-26555
National Aeronautics and Space Administration
NAG5-7584

Dates

Submitted
2025-09-08
Accepted
2025-11-22
Available
2026-01-02
Published

Caltech Custom Metadata

Caltech groups
Infrared Processing and Analysis Center (IPAC), Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published