A newly discovered planet-forming disk has 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 by an international team of researchers that includes Penn State astronomers using the James Webb Space Telescope (JWST), challenges long-standing assumptions about the chemistry of planetary birthplaces, the researchers said.
The researchers described the disk, named XUE10, in a paper appearing today (Aug. 29) in the journal Astronomy & Astrophysics.
"Planets typically form in the cloud of gas and dust that rotates around a newly formed star," said Konstantin Getman, research professor of astronomy and astrophysics in the Penn State Eberly College of Science and an author of the paper. "In the warm inner regions of many planet-forming disks, water turns to vapor and becomes one of the main ingredients, while in the colder outer regions it stays locked up as ice. But in the inner regions of XUE10, water was barely detectable."
In conventional models of planet formation, small chunks of ice drift from the cold outer disk toward the warmer inner regions, where the rising temperatures cause the ice to sublimate and turn into gas. This process usually results in strong water vapor signatures in the disk's inner zones, detectable with JWST's Mid-Infrared Instrument (MIRI). However, in this case of XUE10, the JWST/MIRI spectrum showed a surprisingly strong carbon dioxide signature instead.
"This observation challenges current models of disk chemistry and evolution," said Bayron Portilla-Revelo, postdoctoral scholar in astronomy and astrophysics in the Penn State Eberly College of Science and an author of the paper. "These high carbon dioxide levels relative to water cannot be easily explained by standard disk models. We believe strong photochemical processes may be reshaping the chemistry of the disk, for example intense ultraviolet radiation from the host star or neighbouring massive stars could be breaking down the water molecules."
The researchers noted that stars and planets tend to form in clusters, and that objects in particularly dense regions are subject to large amounts of ultraviolet radiation from young stars in the area.
"JWST has allowed us to observe more distant protoplanetary disks than ever before, and we may see some patterns that will change our understanding of how planets form and evolve," Getman said. "We should continue to observe disks around young intermediate-mass stars like XUE10 to confirm if this unusual ratio of carbon dioxide to water is a trend or if XUE10 is particularly uncommon."
The researchers also detected rare isotopic variants of carbon dioxide - carbon dioxide molecules that contain a different number of neutrons, which may impact the physical properties like mass and density of the material, though the chemical properties are similar. The team detected carbon dioxide variants enriched in either carbon-13 or the oxygen isotopes oxygen-17 and oxygen-18.
"These isotopic variants of carbon dioxide 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," said Eric Feigelson, distinguished senior scholar and professor of astronomy and astrophysics in the Penn State Eberly College of Science and an author of the paper.
XUE10 lies in the massive star-forming region NGC 6357, about 1.7 kiloparsecs, or 33 quadrillion miles, from Earth. This stellar nursery contains many massive stars whose intense ultraviolet radiation floods the region and may influence the protoplanetary disks of nearby lower-mass stars. The unusual chemistry found in the protoplanetary disk of XUE10 was discovered by the international eXtreme Ultraviolet Environments (XUE) collaboration. The team is currently conducting a JWST survey of several rich star-forming regions, including NGC 6357, that span a range of ultraviolet radiation environments.
"This work suggests how the extreme radiation environments of dense areas of massive star-forming regions can alter the building blocks of planets," Feigelson said. "Comparing these environments with quieter, more isolated regions will be enlightening. Most stars and planets are born in star-forming regions, so understanding the processes that shape the physical and chemical properties of protoplanetary disks also helps us understand the diversity of planets that can form within them and their potential to host habitable life."
In addition to Getman, Portilla-Revelo and Feigelson, the team includes researchers from the Space Telescope Science Institute in Maryland, Stockholm University in Sweden, the Max Planck Institute for Astronomy in Germany, Radboud University in the Netherlands, the University of Paris-Saclay in France, Queen Mary University of London in the United Kingdom, the University of Antioquia in Colombia, the University of Hertfordshire in the United Kingdom, the Ludwig Maximilian University of Munich in Germany, the University of Bologna in Italy and the Austrian Academy of Sciences.
Funding from NASA supported the researchers at Penn State.
