James Webb Telescope Discovers Moon-Forming Disk Around Exoplanet CT Cha b

Updated on: September 29, 2025 | By: Jameswebb Discovery Editorial Team

Carbon-Rich Disk Around CT Cha b Hints at Moon Formation

NASA’s James Webb Space Telescope (JWST) has made a groundbreaking discovery: the first direct measurements of a potential moon-forming disk surrounding the massive exoplanet CT Cha b, located 625 light-years away in the constellation Chamaeleon. Using Webb’s Mid-Infrared Instrument (MIRI), researchers have uncovered a carbon-rich disk of dust and gas, teeming with molecules like acetylene and benzene, which could be the birthplace of moons. This finding, published in The Astrophysical Journal Letters on September 29, 2025, offers unprecedented insights into the formation of planets and their moons, drawing parallels to our own solar system’s origins over 4 billion years ago. Dive into this cosmic revelation and explore how it’s reshaping our understanding of planetary systems.

A Cosmic Construction Yard: CT Cha b’s Circumplanetary Disk

The young exoplanet CT Cha b, orbiting a 2-million-year-old star, is surrounded by a circumplanetary disk—a flattened ring of dust and gas distinct from the star’s larger accretion disk, separated by 46 billion miles. This disk, captured in infrared by Webb’s MIRI, is a potential “construction yard” for moons, though no moons were detected in the data. The discovery marks a significant milestone, as it’s the first time scientists have directly measured the chemical and physical properties of such a disk around an exoplanet.“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. The disk’s composition, rich in carbon-based molecules, contrasts sharply with the water-dominated disk around the host star, highlighting rapid chemical evolution in just 2 million years.

Unpacking the Chemistry of Moon Formation

Webb’s MIRI spectrograph revealed seven carbon-bearing molecules in CT Cha b’s disk, including:

• Diacetylene (C4H2)

• Hydrogen cyanide (HCN)

• Propyne (C3H4)

• Acetylene (C2H2)

• Ethane (C2H6)

• Carbon dioxide (CO2)

• Benzene (C6H6)

These molecules, detected through high-contrast methods to separate the planet’s faint signal from the star’s glare, suggest a carbon-rich environment conducive to forming rocky cores or gaseous moons. “We saw molecules at the location of the planet, and so we knew there was stuff in there worth digging for,” Grant explained. This chemical profile differs from the star’s water-rich disk, offering clues about how planetary systems diversify early in their evolution.The findings echo the formation of Jupiter’s Galilean moons—Io, Europa, Ganymede, and Callisto—which likely condensed from a similar disk billions of years ago. Ganymede and Callisto, with their water-ice surfaces and possible rocky cores, provide a reference for what CT Cha b’s disk might produce. “We’re trying to understand how moons come to be, their ingredients, and the physical processes at play,” said lead author Gabriele Cugno of the University of Zürich.

Why This Discovery Matters

Observing moon formation is key to understanding planetary system evolution. Moons likely outnumber planets in our galaxy, and some, like Europa, are prime candidates for hosting life. CT Cha b’s disk offers a rare glimpse into this process, allowing comparisons to our solar system’s birth. Key insights include:

• Planet and Moon Formation: The disk’s materials are actively building CT Cha b and potential moons, mirroring processes from 4 billion years ago.

• Chemical Evolution: The carbon-rich disk’s contrast with the star’s water-rich disk shows rapid chemical divergence, informing models of planetary atmospheres.

• Timescales and Processes: Webb’s data helps pinpoint how quickly disks form moons and what physical conditions drive these events.

The team plans a broader survey of similar systems using Webb to explore the diversity of circumplanetary disks, building on this discovery to map the prevalence of moon-forming environments.

Webb’s Infrared Prowess: Decoding Starlight

Webb’s MIRI instrument, operating in the mid-infrared (5–28 μm), was crucial for this discovery. Its medium-resolution spectrograph disentangled the planet’s faint signal from the host star’s glare, a feat requiring a year of data analysis. The detection of molecules like benzene and acetylene confirms the disk’s potential to form moons, while the absence of water highlights a unique chemical pathway compared to the star’s disk.This observation builds on Webb’s legacy of probing distant worlds, from exoplanet atmospheres to star-forming regions like Sagittarius B2. Positioned at the L2 Lagrange point, JWST’s infrared capabilities reveal hidden details, making it the world’s premier space observatory.

The Artistic Vision: CT Cha b’s Disk

While no direct image of CT Cha b’s disk exists due to its faintness, an artist’s rendering illustrates the scene: a dark red exoplanet encircled by a clumpy, reddish-orange disk of dust and gas, with its yellow host star and debris disk in the background. The illustration lists the detected molecules, emphasizing the disk’s moon-forming potential.Image Credit: NASA, ESA, CSA, STScI, Gabriele Cugno (University of Zürich, NCCR PlanetS), Sierra Grant (Carnegie Institution for Science), Joseph Olmsted (STScI), Leah Hustak (STScI). Alt text: Artist’s concept of a moon-forming disk around exoplanet CT Cha b, showing a reddish-orange dust and gas disk, a dark red planet, and a yellow host star with a debris disk.

Looking Ahead: A New Era of Moon Formation Studies

This discovery opens a new chapter in exoplanet research, with Webb poised to survey more circumplanetary disks. By studying their diversity, astronomers hope to answer: How common are moon-forming disks? Do they favor rocky or icy moons? Could they host life? These questions tie directly to our solar system’s history and the search for habitable worlds.For the latest JWST discoveries, including CT Cha b’s moon-forming disk, visit www.jameswebbdiscovery.com. Explore related findings like Sagittarius B2’s star-forming cloud or the Alpha Centauri planet candidate to stay connected with the cosmos.