JWST-TST High Contrast: First Direct Spectroscopy of GJ 504 b Reveals Clouds and Possible Metal Enrichment

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 μ m spectra show strong signatures of several molecular species, including H 2 O, 12 C 16 O, CH 4 , CO 2 , NH 3 , H 2 S, 13 C 16 O, and 12 C 18 O. Atmospheric modeling of the spectra using petitRADTRANS yields an effective temperature = 564 ± 4 K, surface gravity log g = 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 12 C/ 13 C and 16 O/ 18 O isotopologue ratios, and strong evidence of disequilibrium chemistry and salt clouds. The retrieved parameters indicate a mass 25.2 − 6.0 + 8.4 M Jup , which is in agreement with the mass range (19–27 M Jup ) 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.