Unusual carbon dioxide-rich disk detected around young star challenges planet formation models

A study led by Jenny Frediani at Stockholm University has revealed a planet-forming disk with a strikingly unusual chemical composition: an unexpectedly high abundance of carbon dioxide (CO₂) in regions where Earth-like planets may one day form. The discovery, made using the James Webb Space Telescope (JWST), challenges long-standing assumptions about the chemistry of planetary birthplaces. The study is published in Astronomy & Astrophysics.

“Unlike most nearby planet-forming disks, where water vapor dominates the inner regions, this disk is surprisingly rich in carbon dioxide,” says Jenny Frediani, PhD student at the Department of Astronomy, Stockholm University.

“In fact, water is so scarce in this system that it’s barely detectable — a dramatic contrast to what we typically observe.”

A newly formed star is initially deeply embedded in the gas cloud from which it was formed and creates a disk around itself where planets in turn can be formed. In conventional models of planet formation, pebbles rich in water ice drift from the cold outer disk toward the warmer inner regions, where the rising temperatures cause the ices to sublimate. This process usually results in strong water vapor signatures in the disk's inner zones. However, in this case, the JWST/MIRI spectrum shows a puzzlingly strong carbon dioxide signature instead.

“This challenges current models of disk chemistry and evolution since the high carbon dioxide levels relative to water cannot be easily explained by standard disk evolution processes,” Jenny Frediani explains.

Arjan Bik, researcher at the Department of Astronomy, Stockholm University, adds, “Such a high abundance of carbon dioxide in the planet-forming zone is unexpected. It points to the possibility that intense ultraviolet radiation — either from the host star or neighbouring massive stars — is reshaping the chemistry of the disk.”

The researchers also detected rare isotopic variants of carbon dioxide, enriched in either carbon-13 or the oxygen isotopes ¹⁷O and ¹⁸O, clearly visible in the JWST data. These isotopologues could offer vital clues to long-standing questions about the unusual isotopic fingerprints found in meteorites and comets — relics of our own Solar System's formation.

This CO₂-rich disk was found in the massive star-forming region NGC 6357, located approximately 1.7 kiloparsecs (about 53 quadrillion kilometers) away. The discovery was made by the eXtreme Ultraviolet Environments (XUE) collaboration, which focuses on how intense radiation fields impact disk chemistry.

Maria-Claudia Ramirez-Tannus from the Max Planck Institute for Astronomy in Heidelberg and lead of the XUE collaboration says that it is an exciting discovery: “It reveals how extreme radiation environments — common in massive star-forming regions — can alter the building blocks of planets. Since most stars and likely most planets form in such regions, understanding these effects is essential for grasping the diversity of planetary atmospheres and their habitability potential.”

Thanks to JWST’s MIRI instrument, astronomers can now observe distant, dust-enshrouded disks with unprecedented detail at infrared wavelengths — providing critical insights into the physical and chemical conditions that govern planet formation. By comparing these intense environments with quieter, more isolated regions, researchers are uncovering the environmental diversity that shapes emerging planetary systems. Astronomers at Stockholm University and Chalmers have helped develop the MIRI instrument which is a camera and a spectrograph that observes mid- to long-wavelength infrared radiation from 5 microns to 28 microns. It also has coronagraphs, specifically designed to observe exoplanets.

The study “XUE: The CO_2-rich terrestrial planet-forming region of an externally irradiated Herbig disk” is published in Astronomy & Astrophysics.