Planets and moons are inseparable companions and while astronomers are unravelling the complexity of planet formation, moon formation remains mysterious. Our Solar System is fully formed, so to observe exoplanet and exomoon formation, we have to look to other stars. But while we’re getting better at detecting exoplanets, detecting exomoons is much more challenging. Their small sizes relative to the exoplanets they orbit renders them practically invisible.
But as with many issues in astronomy, the JWST is expanding our horizons. A pair of astronomers from Switzerland and the USA used the space telescope to examine the moon-forming circumplanetary disk (CPD) around a massive gas giant about 625 light-years away. This is the first time scientists have characterized the chemical components in this type of disk.
The research letter is titled “A Carbon-rich Disk Surrounding a Planetary-mass Companion,” and it’s published in The Astrophysical Journal Letters. The authors are Gabriele Cugno and Sierra L. Grant. Cugno is from the Department of Astrophysics at the University of Zurich, and Grant is from the Earth and Planets Laboratory at the Carnegie Institution for Science.
“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 authors write. These disks are very difficult to observe, and determining their chemistry is challenging. But the JWST’s Mid-Infrared Instrument (MIRI) is bringing these disks and their chemistry within reach. “We present the mid-infrared spectrum from the CPD surrounding the young companion CT Cha b,” the researchers write.
CT Chamaeleontis (CT Cha) is a very young T-Tauri star only about 2 million years old. CT Cha b is either a brown dwarf or a massive gas giant with about 17 Jupiter masses. CT Cha b is about 46 billion miles, or almost 500 astronomical units, from the star. That wide separation between the two made it clear to the astronomers that they were seeing a CPD and not the disk around the star, which is still growing by accreting material from its disk.
“We can see evidence of the disc around the companion, and we can study the chemistry for the first time,” said co-author Cugno in a press release. “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 co-author Cugno.
The JWST found multiple carbon-bearing molecules in CT Cha b’s circumplanetary disk. This carbon-rich chemistry contrasts with the observed atmospheric chemistry of exo-gas giants. “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,” the authors write.
This figure illustrates some of the carbon-bearing molecules found in the disk around CT Cha b. Image Credit: Cugno and Grant, 2025. ApJL
The disparity between the carbon-rich chemistry in the CPD and the lack of carbon around the star is evidence of how rapidly disks can evolve. “This difference in disk chemistry between the host disk and its companion indicates rapid, divergent chemical evolution on ∼million-year timescales,” the authors write. Understanding how CPDs and stellar disks can be so different will go a long way to explaining the moons we see in our Solar System.
Our Solar System has eight planets and more than 400 moons orbit six of them. (Mercury and Venus are moonless.) The hundreds of moons form a menagerie of satellites. Jupiter’s Ganymede is the largest, while Saturn’s Mimas is the smallest, or at least the smallest one that’s round. Jupiter’s Io is extremely volcanic, while Europa and Saturn’s Enceladus are two of the Solar System’s frozen ocean moons. Then there’s Earth’s Moon, which is the largest Moon in the Solar System relative to its planet.
Understanding Solar Systems, planets, and even habitability requires understanding moons.
“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,” said Cugno. “How do these moons come to be? What are their 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.”
Uncovering the chemistry of a moon-forming disk is a big step in understanding moons. But the chemistry didn’t jump out of the data. Since the light from the star nearly overwhelms that from the planet and its disk, the astronomers had to find a way to sort through it all.
“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,” said Grant.
The researchers found seven separate carbon-bearing molecules in the planet’s disk: acetylene, benzene, carbon dioxide, ethane, hydrogen cyanide, diacetylene, and propyne. This contrasts with the disk around the star, which contains water but no carbon. Since the star is only two million years old, this discrepancy is solid evidence of how quickly chemistry can change.
This figure shows molecular spectral cross correlation (SCC) maps of the CT Cha system. SCCs help astronomers detect and map molecules in protoplanetary and circumplanetary disks. While the star CT Cha A shows only H20, carbon-bearing molecules are found around its companion, CT Cha b. Image Credit: Cugno and Grant, 2025. ApJL
The same chemical trends are observed in other, isolated objects. Disks trend from oxygen-rich to carbon-rich with increasing host mass, according to the researchers. Other T Tauri stars have the same chemistry as the star CT Cha A.
But that doesn’t mean that scientists know why this is the case, and there are several hypotheses. “Efficient radial drift can carry oxygen-bearing ices rapidly inward, such that the oxygen-rich gas is accreted onto the central object very early, leaving a carbon-rich reservoir behind,” the researchers write. The carbon content could also be because there’s low dust opacity and the JWST is seeing deeper into the disk where more carbon resides.
This is one of the many unanswered questions in planet and moon formation that astronomers hope the JWST and future telescopes will answer. The authors describe these observations as a “new frontier in our understanding of planet formation and the first opportunity to observationally characterize moon-forming environments.” Understanding how chemistry evolves in these disks sheds light on the variety of moons we see in our own Solar System.
The JWST’s observational power means that more of these disks will be discovered and analyzed chemically. Currently, there are nine CPDs accessible to the JWST, and the researchers intend to survey them in the coming year. That large sample size will be very instructive.
“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,” said Cugno. “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.”