Astronomers using the NASA/ESA/CSA James Webb Space Telescope have found the carbon-rich disk surrounding CT Cha b, a giant exoplanet around 620 light-years away from Earth in the constellation of Chamaeleon. Their results provide the first direct insight into the chemical and physical properties of material being accreted onto a gas giant and into its potential moon system.
An artistic rendering of a dust and gas disk encircling the young exoplanet CT Cha b. Image credit: NASA / ESA / CSA / STScI / G. Cugno, University of Zürich & NCCR PlanetS / S. Grant, Carnegie Institution for Science / J. Olmsted, STScI / L. Hustak, STScI.
CT Cha, also known as PDS 44 and TIC 454259409, is only 2 million years old and still accreting circumstellar material.
However, the circumplanetary disk discovered by Webb is not part of the larger accretion disk around the central star.
“We can see evidence of the disk around the companion, and we can study the chemistry for the first time,” said Dr. Sierra Grant, an astronomer at the Carnegie Institution for Science.
“We’re not just witnessing moon formation — we’re also witnessing this planet’s formation.”
“We are seeing what material is accreting to build the planet and moons,” added Dr. Gabriele Cugno, an astronomer at the University of Zurich and the National Centre of Competence in Research PlanetS.
Infrared observations of CT Cha b were made with Webb’s MIRI (Mid-Infrared Instrument) using its medium resolution spectrograph.
An initial look into the Webb archival data revealed signs of molecules within the circumplanetary disk, which motivated a deeper dive into the data.
Because the planet’s faint signal is buried in the glare of the host star, the astronomers had to disentangle the light of the star from the planet using high-contrast methods.
“We saw molecules at the location of the planet, and so we knew that there was stuff in there worth digging for and spending a year trying to tease out of the data. It really took a lot of perseverance,” Dr. Grant said.
Ultimately, the researchers discovered seven carbon-bearing molecules within the planet’s disk, including acetylene (C2H2) and benzene (C6H6).
This carbon-rich chemistry is in stark contrast to the chemistry seen in the disk around the host star, where the researchers found water but no carbon.
The difference between the two disks offers evidence for their rapid chemical evolution over only 2 million years.
“We want to learn more about how our Solar System formed moons. This means that we need to look at other systems that are still under construction. We’re trying to understand how it all works,” Dr. Cugno said.
“How do these moons come to be? What are the ingredients? What physical processes are at play, and over what timescales?”
“Webb allows us to witness the drama of moon formation and investigate these questions observationally for the first time.”
The findings were published today in the Astrophysical Journal Letters.
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Gabriele Cugno & Sierra L. Grant. 2025. A Carbon-rich Disk Surrounding a Planetary-mass Companion. ApJL 991, L46; doi: 10.3847/2041-8213/ae0290
