Author ORCID Identifier

Aneesh Baburaj: 0000-0003-3708-241X

Jean-Baptiste Ruffio: 0000-0003-2233-4821

Marshall Perrin: 0000-0002-3191-8151

Jerry W. Xuan: 0000-0002-6618-1137

William O. Balmer: 0000-0001-6396-8439

Yayaati Chachan: 0000-0003-1728-8269

Quinn M. Konopacky: 0000-0002-9936-6285

Travis S. Barman: 0000-0002-7129-3002

Mathilde Mâlin: 0000-0002-2918-8479

Kielan K.W. Hoch: 0000-0002-9803-8255

Emily Rickman: 0000-0003-4203-9715

Kimberly Ward-Duong: 0000-0002-4479-8291

Julien H. Girard: 0000-0001-8627-0404

Isabel Rebollido: 0000-0002-4388-6417

Nikole K. Lewis: 0000-0002-8507-1304

Sara Seager: 0000-0002-6892-6948

Rémi Soummer: 0000-0003-2753-2819

Jay Anderson: 0000-0003-2861-3995

Document Type

Article

Publication Date

7-2026

Publication Title

The Astronomical Journal

Abstract

Characterizing the coldest directly imaged companions through direct spectroscopy has only recently become possible with the James Webb Space Telescope. We present moderate-resolution (R ∼ 2700) spectroscopic observations of the directly imaged planetary-mass companion (PMC), GJ 504 b, using the JWST/NIRSpec. The coldest imaged PMC of the pre-JWST era GJ 504 b is too faint for ground-based spectroscopy, with only photometric observations possible. Leveraging advanced postprocessing techniques with a forward-modeling framework, we detect the companion at high signal-to-noise (S/N > 300). We also present the first successful point-spread function (PSF) subtraction with angular differential imaging (ADI) in the NIRSpec point cloud, detecting GJ 504 b at S/N > 10 and reaching contrast limits < 10−4. The extracted 2.9–5.3 μm spectra show strong signatures of several molecular species, including H2O, 12C16O, CH4, CO2, NH3, H2S, 13C16O, and 12C18O. Atmospheric modeling of the spectra using petitRADTRANS yields an effective temperature = 564 ± 4 K, surface gravity  logg= 4.87-0.12'+0.13, metallicity [M/H] = 0.67-0.12'+0.13, C/O ratio = 0.64-0.02'+0.02, interstellar 12C/13C and 16O/18O isotopologue ratios, and strong evidence of disequilibrium chemistry and salt clouds. The retrieved parameters indicate a mass 25.2+8.4-6.0MJup, which is in agreement with the mass range (19–27 MJup) obtained from ATMO evolutionary models, implying an age of 2.5–4.0 Gyr. Lastly, we compare the abundances of GJ 504 b to its primary, obtaining a stellar abundance of sulfur (S), superstellar carbon (C), and, possibly, oxygen (O). The observed metal enrichment tentatively supports a planetlike formation, but does not entirely exclude stellar abundances for GJ 504 b.

Volume

172

Issue

28

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Rights

©2026. The Author(s). Published by the American Astronomical Society.

Version

Version of Record

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