2025ApJ...991L..46C

During the final assembly of gas giant planets, circumplanetary disks (CPDs) of gas and dust form due to the conservation of angular momentum, providing material to be accreted onto the planet and the ingredients for moons. The composition of these disks has remained elusive, as their faint nature and short separations from their host stars have limited our ability to access them. Now, with the spatial and spectral resolution of the JWST/MIRI Medium-Resolution Spectrograph, we can observe and characterize this reservoir for wide-orbit planetary-mass companions for the first time. We present the mid-infrared spectrum from the CPD surrounding the young companion CT Cha b. The data show a carbon-rich chemistry with seven carbon-bearing molecules (up to C₆H₆) and one isotopolog detected and indicate a high gaseous C/O > 1 that is in contrast with the elemental abundance ratios typically measured in directly imaged gas giant atmospheres. This carbon-rich chemistry is also in stark contrast to the spectrum of the disk surrounding the host star, CT Cha A, which shows no carbon-bearing molecules. This difference in disk chemistry between the host disk and its companion indicates rapid, divergent chemical evolution on ∼million-year timescales. Nonetheless, the chemical properties of the CPD follow trends observed in isolated objects, where disks transition from an oxygen-rich to carbon-rich composition with decreasing host mass. Our results provide the first direct insight into the chemical and physical properties of material being accreted onto a gas giant analog and into its potential moon system.