Moon-forming disk found around massive planet

• Webb has spotted a moon-forming disk around a massive planet. That planet circles a star some 625 light-years distant from us.
• The disk around the exoplanet is rich in carbon. The host star also has a disk around it, but that disk does not have any carbon.
• The discovery provides insight into how the many moons in our own solar system might have formed.

ESA published this original story on September 29, 2025. Edits by EarthSky.

Moon-forming disk found around massive planet

Our solar system contains eight major planets and more than 400 known moons orbiting six of these planets. Where did they all come from? There are multiple formation mechanisms. The case for large moons, like the four Galilean satellites around Jupiter, is that they condensed out of a dust and gas disk encircling the planet when it formed. But that would have happened over 4 billion years ago, and there is scant forensic evidence today.

On September 29, 2025, ESA said that Webb has now provided the first direct view of material in a disk around a large exoplanet. An international team of astronomers have uncovered a carbon-rich disk encircling the world called CT Cha b, which is 625 light-years away from Earth.

These researchers published their results on September 29, 2025, in peer-reviewed The Astrophysical Journal Letters.

An exoplanet with a moon-forming disk

The young star the planet orbits is only 2 million years old and still accreting circumstellar material. However, the circumplanetary disk Webb discovered is not part of the larger accretion disk around the central star. The two objects are 46 billion miles (74 billion km) 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 now only entering an era where we can witness their formation.

This discovery fosters a better understanding of planet and moon formation, said the researchers. Webb’s data are invaluable for making comparisons to our solar system’s birth over 4 billion years ago. Co-author Sierra Grant of the Carnegie Institution for Science in Washington, D.C., said:

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.

Lead author Gabriele Cugno of the University of Zurich in Switzerland and member of the National Centre of Competence in Research PlanetS added:

We are seeing what material is accreting to build the planet and moons.

Dissecting starlight

Astronomers made infrared observations of CT Cha b 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. Grant said:

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.

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.

Genesis of moons

A circumplanetary disk of debris 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 made of carbon or silicon.

Cugno said:

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. 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.

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

Bottom line: Data from the Webb space telescope reveal that a massive planet around a distant star is encircled by a moon-forming disk. This discovery can help us understand how moons formed in our own solar system.

Source: A Carbon-rich Disk Surrounding a Planetary-mass Companion

Via ESA

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