Astronomers have obtained the first detailed spectrum of the cold planetary companion GJ 504 b using the James Webb Space Telescope. While the object was previously limited to photometry, researchers have now successfully decomposed its radiation into specific components within the 2.9–5.3 micrometer range.
A team led by Anish Baburaj from the University of Arizona utilized the NIRSpec instrument on the JWST for the study. The observations were conducted as part of the High Contrast program, employing advanced data processing techniques and angular differential imaging. The spectrum was captured with a signal-to-noise ratio exceeding 300, enabling the confident identification of water, methane, carbon dioxide, ammonia, and hydrogen sulfide molecules, as well as carbon and oxygen isotopes.
Atmospheric modeling revealed an effective temperature of 564 ± 4 Kelvin, a surface gravity of log g ≈ 4.87, and a metallicity of [M/H] = 0.67. The data also showed evidence of non-equilibrium chemistry and the presence of salt clouds. The object's mass is estimated at 25.2 Jupiter masses, with an age ranging from 2.5 to 4 billion years.
Comparing the planet's chemical composition to its host star revealed elevated levels of carbon and potentially oxygen. This suggests a planet-like formation scenario, though a stellar origin cannot be entirely ruled out at this stage.
The findings were published in a preprint on arXiv on June 17, 2026. These results pave the way for studying the atmospheres of the coldest directly imaged objects, which were previously beyond the reach of spectroscopy.
