This paper reports the first moderate-resolution direct spectroscopy of GJ 504 b, a faint planetary-mass companion, using the James Webb Space Telescope (JWST). The team used the Near Infrared Spectrograph (NIRSpec) to record light from 2.9 to 5.3 micrometers at a resolving power of about 2,700. Before JWST, this object was too faint for ground-based spectroscopy and only photometric measurements were possible.

To extract the companion’s light they applied advanced post-processing and a forward-modeling framework. That approach let them detect the object at very high signal-to-noise (S/N > 300). They also demonstrate the first successful point-spread-function subtraction with angular differential imaging (ADI) applied to the NIRSpec point cloud. ADI is a method that uses the telescope’s changing orientation to separate the companion’s signal from the star’s glare. With ADI they detect GJ 504 b at S/N > 10 and reach contrast limits better than 10^-4.

The 2.9–5.3 μm spectrum shows clear signatures of many molecules. The authors report detections of water (H2O), carbon monoxide (12C16O), methane (CH4), carbon dioxide (CO2), ammonia (NH3), hydrogen sulfide (H2S), and less common isotopologues such as 13C16O and 12C18O. These molecular fingerprints come from absorption features in the recorded infrared light.

The team fit the spectrum with an atmospheric model called petitRADTRANS. The best-fit parameters are an effective temperature of 564 ± 4 K and a surface gravity log g = 4.87 (+0.13, −0.12). They retrieve a metallicity [M/H] = +0.67 (+0.13, −0.12), meaning the atmosphere appears richer in elements heavier than hydrogen and helium than the Sun. The carbon-to-oxygen ratio (C/O) is 0.64 ± 0.02. The fits also indicate disequilibrium chemistry (chemistry out of simple balance) and the presence of salt clouds. From these atmospheric properties they infer a mass of 25.2 (+8.4, −6.0) Jupiter masses, consistent with evolutionary-model estimates of 19–27 Jupiter masses and an age of about 2.5–4.0 billion years.