Witnessing the Formation of Moons

NASA’s James Webb Space Telescope has provided the first direct measurements of the chemical and physical properties of a potential moon-forming disk encircling a large exoplanet. The carbon-rich disk surrounding the world called “CT Cha b”, which is located 625 light-years away from Earth, is a possible construction yard for moons, although no moons are detected in the Webb data. Theresults published today in The Astrophysical Journal Letters.

The young star the planet orbits 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. The two objects are 46 billion miles apart.

Observing planet and moon formation is fundamental to understanding the evolution of planetary systems across our galaxy. Moons likely outnumber planets, and some might be habitats for life as we know it. But we are only now entering an era where we can witness their formation.

According to the researchers, this discovery fosters a better understanding of planet and moon formation. Webb’s data is invaluable for making comparisons to our solar system's birth over 4 billion years ago.

“We can see evidence of the disk around the companion, and we can study the chemistry for the first time. We're not just witnessing moon formation — we're also witnessing this planet’s formation,” said co-lead author Sierra Grant of the Carnegie Institution for Science in Washington. 

“We are seeing what material is accreting to build the planet and moons,” added main lead author Gabriele Cugno of the University of Zurich (UZH) and member of the National Center of Competence in Research PlanetS of the Swiss National Science Foundation.

Infrared observations of CT Cha b were made with Webb’s MIRI (Mid-Infrared Instrument) using its medium resolution spectrograph. An initial look into Webb’s 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 researchers 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,” said Grant.

Ultimately, the team 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.

A circumplanetary disk has long been hypothesized as the birthplace of Jupiter’s four major moons. These Galilean satellites must have condensed out of such a flattened disk billions of years ago, as evident in their co-planar orbits about Jupiter. The two outermost Galilean moons, Ganymede and Callisto, are 50% water ice. But they presumably have rocky cores, perhaps either of carbon or silicon.

“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.”

In the coming year, the team will use Webb to perform acomprehensive survey of similar objects, to better understand the diversity of physical and chemical properties in the disks around young planets.

Literature
Gabriele Cugno, Sierra Grant. A carbon-rich disk feeding a planetary-mass companion. Astrophysical Journal Letters. September 29, 2025. DOI: https://doi.org/10.3847/2041-8213/ae0290