When giant planets are forming, they often gather swirling disks of gas and dust around them, called circumplanetary disks. These disks help feed the growing planet and may also contain the raw materials to build moons. But studying them has been tough because they’re faint and sit very close to their host stars.
Now, thanks to NASA’s James Webb Space Telescope, scientists have made the first direct measurements of one such disk around a distant planet named CT Cha b, located 625 light-years away. This disk is rich in carbon and could be a moon-making zone, though no moons have been spotted yet in the data.
The young star near CT Cha b is just 2 million years old and still gathering material from its surroundings. But the moon-forming disk spotted by the James Webb Space Telescope isn’t part of the star’s main disk; it’s orbiting the planet itself, and the two are a whopping 46 billion miles apart.
Why it matters: Watching planets and moons form in real time helps scientists understand how planetary systems evolve across the galaxy. Since moons may outnumber planets, and some could even host life, this marks a significant step forward. Thanks to Webb’s sharp vision, we’re finally entering an era where we can see moons being born, and compare these cosmic nurseries to how our own solar system formed over 4 billion years ago.
Astronomers make first clear detection of a moon-forming disc around an exoplanet
“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 Zürich and member of the National Center of Competence in Research PlanetS.
Using the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope, scientists captured infrared signals from the distant planet CT Cha b. At first glance, Webb’s archived data suggested the presence of molecules in the planet’s surrounding disk, which could be potential building blocks for moons. That exciting clue led researchers to take a deeper dive.
But spotting the planet wasn’t easy. Its faint glow was hidden in the bright glare of its host star. To reveal the planet’s signal, scientists used high-contrast techniques to carefully separate the star’s light from the planet’s, like tuning out a spotlight to see a candle flickering beside it.
“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.
Scientists have discovered seven carbon-based molecules, including acetylene and benzene, in the disk surrounding the planet CT Cha b. This rich carbon chemistry stands in sharp contrast to the disk around the nearby star, which contains water but no carbon. The difference between the two suggests that these disks evolve chemically and very quickly, within just 2 million years.
This discovery also echoes theories about how Jupiter’s four major moons, the Galilean satellites, formed. They likely emerged from a similar circumplanetary disk, billions of years ago. Their neat, co-planar orbits suggest a shared origin. The outer moons, Ganymede and Callisto, are composed of half water ice, but likely have rocky cores made of carbon or silicon, similar to the ingredients now observed swirling around CT Cha b.
“We want to learn more about how our solar system formed its 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 utilize Webb to conduct a comprehensive survey of similar objects, thereby gaining a deeper understanding of the diversity of physical and chemical properties in the disks surrounding young planets.
Journal Reference:
- Gabriele Cugno and Sierra L. Grant. A Carbon-rich Disk Surrounding a Planetary-mass Companion. The Astrophysical Journal Letters. DOI 10.3847/2041-8213/ae0290