The first detection of molecules in a circumplanetary disk

Artist’s concept of the CT Cha system, along with the molecules that we found in the disk around the planetary-mass companion. Credit: NASA, ESA, CSA, STScI, Gabriele Cugno (University of Zurich, NCCR PlanetS), Sierra Grant (Carnegie Institution for Science), Joseph Olmsted (STScI), Leah Hustak (STScI)

Circumplanetary disks are an inevitable by-product of giant-planet formation. They constitute the final reservoir of material available to forming giant planets and are the birthplaces of moons and satellites. Due to technical limitations, the composition of these disks has been inaccessible, but now with the James Webb Space Telescope we are finally able to detect and study the emission coming from circumplanetary disks for the first time. 

In Cugno & Grant (2025), we used the angular resolution and spectral resolution of JWST/MIRI MRS to tease out the composition of a circumplanetary disk for the first time. We analyzed observations of the CT Cha system, which consists of a Sun-like star (0.9 solar masses), CT Cha A, and a planetary-mass companion (14-24 M_Jup), CT Cha b.

We found that the disk around the companion is extremely carbon-rich, with emission from seven carbon-bearing molecules, including species as complex as benzene (C₆H₆), and the isotopologue ¹³CCH₂. This carbon-rich chemistry is in stark contrast with the spectrum of the circumstellar disk around the host star, which we show has no emission from carbon-bearing molecules and instead has only oxygen-bearing species. This coeval chemical dichotomy within a single star-plus-companion system provides direct evidence that rapid, divergent chemical evolution occurs on ~Myr timescales. We put the system into context by comparing CT Cha A and b to the largest sample of JWST data on protoplanetary disks compiled thus far, finding that CT Cha A is chemically similar to other Sun-like systems. CT Cha b instead extends previously found chemical correlations down to the planetary-mass regime for the first time.

This result was presented in a number of press releases, including one from NASA. 

Top left: The calibrated JWST/MIRI MRS image of the CT Cha system. Bottom left: A spectral cross-correlation map revealing the faint planetary-mass companion, CT Cha b. Right: The JWST/MIRI MRS spectrum of CT Cha b (black) compared to the total model (red). The molecular components that make up the spectrum are shown in the colors below. Insets on top show zoom-ins of the main molecular features. Figure from Cugno & Grant (2025).