For centuries, humanity has looked at the stars and wondered: how do planets come to be? Thanks to the powerful eyes of the James Webb Space Telescope (JWST), we are now glimpsing answers in astonishing detail. Yet, with every new discovery, we also encounter fresh mysteries.
In a recent study led by Jenny Frediani, a Ph.D. student at Stockholm University, astronomers uncovered a planet-forming disk unlike any seen before. Instead of being filled with water vapor—the usual signature of newborn planetary systems—this disk is strikingly rich in carbon dioxide (CO₂). The discovery, published in Astronomy & Astrophysics, challenges long-standing ideas about the chemistry of planet birth and forces scientists to rethink how worlds like Earth might begin.
“This disk is dramatically different from what we usually observe,” Frediani explains. “The water is so scarce that it’s barely detectable. Instead, carbon dioxide dominates.”
The finding is more than just surprising—it could reshape our understanding of how planets form and whether they end up being habitable.
How Planets Are Born
To understand why this is so unusual, let’s step back to the beginning of a star’s life. When a star forms, it emerges from a dense cloud of gas and dust. Surrounding the newborn star is a rotating protoplanetary disk—a swirling structure of gas, dust, and ice that serves as raw material for planets.
In traditional models, the outer regions of the disk are rich in icy pebbles coated with water. Over time, these pebbles drift inward toward the star. As they approach warmer zones, the heat causes the ice to sublimate, releasing large amounts of water vapor. This process usually leads to strong water signatures in the disk’s inner regions—the very areas where Earth-like planets may eventually form.
But in this case, JWST’s MIRI instrument (Mid-Infrared Instrument) revealed something completely different: the expected water vapor was missing, replaced instead by an unusually strong carbon dioxide signal.
A Chemical Puzzle
This discovery throws a wrench into established models of disk chemistry. “High carbon dioxide levels relative to water cannot be easily explained by standard disk evolution processes,” Frediani notes.
So, what’s happening here? One possibility is that intense ultraviolet radiation is playing a key role. The young star itself—or perhaps nearby massive stars—may be flooding the disk with high-energy light, breaking apart water molecules and leaving behind carbon dioxide.
“This level of CO₂ abundance in the planet-forming zone is unexpected,” says Arjan Bik, researcher at Stockholm University. “It suggests that radiation is reshaping the chemistry in dramatic ways.”
If true, it would mean that the environments where planets form can vary far more than scientists previously thought. And that, in turn, could explain why exoplanets discovered across the galaxy show such remarkable diversity in their atmospheres and surface conditions.
The Fingerprints of Isotopes
The JWST data revealed another surprise: rare isotopic variants of carbon dioxide were clearly visible in the spectrum. These isotopologues contain slightly different versions of carbon and oxygen—such as carbon-13, oxygen-17, and oxygen-18.
Though subtle, these chemical fingerprints could help scientists solve puzzles closer to home. Meteorites and comets in our own solar system carry unusual isotopic ratios, remnants of the conditions under which our planets formed billions of years ago. By studying isotopes in distant disks like this one, astronomers may finally begin to connect the dots between those ancient relics and the processes still unfolding in young star systems today.
A Birthplace in a Harsh Environment
This CO₂-rich disk is not tucked away in a quiet corner of space. It lies within NGC 6357, a vast and chaotic star-forming region located about 1.7 kiloparsecs (roughly 53 trillion kilometers) away. This is no gentle nursery—it is an environment bathed in intense radiation from massive stars.
That context matters. According to Maria-Claudia Ramirez-Tannus of the Max Planck Institute for Astronomy and lead of the XUE collaboration (eXtreme Ultraviolet Environments), the discovery shows just how powerfully radiation can shape planet formation.
“It reveals how extreme radiation environments—common in massive star-forming regions—can alter the building blocks of planets,” she says. “Since most stars, and probably most planets, are born in such regions, this is crucial for understanding the diversity of planetary atmospheres and their habitability potential.”
In other words, Earth’s story—where water was abundant during the planet’s early history—may not be the universal rule. Other worlds might emerge in environments where carbon dioxide, not water, plays the starring role.
The Power of James Webb
This breakthrough was only possible because of the extraordinary capabilities of the James Webb Space Telescope. Its MIRI instrument, developed with contributions from Stockholm University and Chalmers, can peer into the dust-shrouded regions where ordinary telescopes see nothing. Operating in the mid- to long-infrared (5 to 28 microns), MIRI acts as both a camera and a spectrograph, capable of detecting the faint chemical signatures hidden in planet-forming disks.
What makes JWST revolutionary is not just its sharp vision but its ability to analyze light with exquisite sensitivity. In this case, it allowed astronomers to detect not only the unexpected dominance of carbon dioxide but also its isotopic variants—a level of detail never achieved before.
Rethinking Planetary Origins
The implications of this discovery ripple far beyond a single star system. If disks rich in carbon dioxide are common in harsh radiation environments, then the chemistry of planet formation across the galaxy may be far more diverse than previously imagined.
For exoplanet research, this could help explain why some planets are covered in thick CO₂ atmospheres like Venus, while others remain rich in water. It also raises questions about habitability. Worlds born in CO₂-rich environments may evolve very differently from Earth, with consequences for whether life could ever arise.
Science thrives on such surprises. Just when astronomers thought they understood the general blueprint of planet formation, JWST revealed a twist in the story. And as more disks are studied, more twists are likely to follow.
A Window Into Our Own Past
In some sense, every time we peer into a planet-forming disk, we are looking into a mirror. Billions of years ago, our own solar system began in just such a swirling cloud of dust and gas. By studying other disks—especially those in extreme environments—we gain perspective on our origins and why Earth turned out the way it did.
The CO₂-rich disk in NGC 6357 is a reminder that the universe is endlessly creative, weaving worlds from different chemical threads. Some will be watery like Earth, others dry and carbon-heavy, and many perhaps beyond our current imagination.
What unites them all is the same process of transformation: from stardust to planets, from chaos to the possibility of life.
The Beginning of a New Chapter
The James Webb Space Telescope has only just begun its mission, yet it is already rewriting the story of planet formation. With every observation, we learn that the cosmos is stranger, more diverse, and more dynamic than we ever assumed.
For Jenny Frediani and her colleagues, the work is just beginning. The CO₂-rich disk raises more questions than it answers: How common are such environments? Can we trace the exact mechanisms destroying water and boosting carbon dioxide? And what does this mean for the long-term prospects of planets born under such conditions?
The beauty of science is that the mystery is never fully solved. Each discovery is not an ending but an invitation—to look deeper, to ask better questions, to embrace the wonder of not yet knowing.
And so, as Webb continues to gaze into the nurseries of stars and planets, we stand at the edge of revelation. Somewhere out there, in the shimmering carbon-rich clouds of a young system, a planet may be forming. Its story is only beginning, but already it is reshaping ours.
More information: Jenny Frediani et al, XUE: The CO_2-rich terrestrial planet-forming region of an externally irradiated Herbig disk, Astronomy & Astrophysics (2025). DOI: 10.1051/0004-6361/202555718
