Astronomers puzzled by carbon-loaded protoplanetary disk

Astronomers puzzled by carbon-loaded protoplanetary disk
by Robert Schreiber
Berlin, Germany (SPX) Sep 01, 2025
A research team led by Stockholm University has identified a planet-forming disk with an unexpectedly high concentration of carbon dioxide, detected in the terrestrial planet-forming region around a young star. The findings, made with the James Webb Space Telescope (JWST), challenge long-standing assumptions that water vapor dominates these zones.

"Unlike most nearby planet-forming disks, where water vapor dominates the inner regions, this disk is surprisingly rich in carbon dioxide," said lead author Jenny Frediani, a PhD student at the Department of Astronomy. "In fact, water is so scarce in this system that it's barely detectable - a dramatic contrast to what we typically observe."

Conventional planet formation models predict that icy pebbles drift inward from the outer disk, sublimating into water vapor as temperatures rise. This process normally results in strong water signatures. However, JWST's Mid-Infrared Instrument (MIRI) instead revealed an unusually intense carbon dioxide spectrum.

"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," Frediani explained.

Researcher Arjan Bik of Stockholm University added, "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 team also identified rare isotopologues of carbon dioxide enriched with carbon-13 and oxygen-17 or oxygen-18. These isotopic variants may help explain puzzling isotopic signatures in meteorites and comets from the early Solar System.

The CO2-rich disk lies in the massive star-forming region NGC 6357, about 1.7 kiloparsecs (53 quadrillion kilometers) from Earth. The discovery emerged from the eXtreme Ultraviolet Environments (XUE) collaboration, which investigates how strong radiation fields alter disk chemistry.

"This is an exciting discovery," said Maria-Claudia Ramirez-Tannus of the Max Planck Institute for Astronomy and leader of the XUE collaboration. "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."

JWST's MIRI instrument enables astronomers to probe distant, dust-enshrouded disks at mid-infrared wavelengths, providing unprecedented insight into the conditions that drive planetary birth. Developed with contributions from Stockholm University and Chalmers, MIRI combines camera, spectrograph, and coronagraph capabilities, optimized for exoplanet and disk studies.

Research Report:The CO_2-rich terrestrial planet-forming region of an externally irradiated Herbig disk