Measurements of the molecular composition of this interstellar comet show an excess of heavy water molecules that is radically different from anything that has formed around our sun.
By Lee Billings & Joseph Howlett edited by Jeanne Brner
A deep image of interstellar comet 3I/ATLAS captured by the Gemini Multi-Object Spectrograph (GMOS) on Gemini South at Cerro Pachón in Chile, one half of the International Gemini Observatory, partly funded by the U.S. National Science Foundation (NSF) and operated by NSF NOIRLab.
International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist Image Processing: J. Miller and M. Rodriguez (International Gemini Observatory/NSF NOIRLab), Rector TA (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab)
Comet 3I/ATLAS, a object from another star which was found passing through our solar system at high speed last summernow disappears from telescopic view as it retreats towards interstellar space. But he continues to give lessons on his distant origins– and, therefore, demonstrate how special our solar system can be.
Astronomers spotted 3I/ATLAS just days after the icy comet came closest to the sun in late October 2025. With telescopes at the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, they used radio waves of light to study matter heated by starlight that the comet was shedding into space. Spectroscopic results showed water far more exotic and “heavy” than would be expected for a comet from our own solar system, a study suggests. published yesterday In Natural astronomy.
Like regular water, heavy water combines two hydrogen atoms with one oxygen atom to form each moisture molecule. But for the heavier version, at least one of those hydrogen atoms is a heavier isotope such as deuterium which, unlike a typical hydrogen atom, has one neutron.
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Whether or not deuterium is present in the water of a rocky body depends on the chemical processes that formed it. Specifically, cold temperatures greatly favor reactions that increase the amount of heavy water relative to running water. This ratio is therefore a sensitive probe of the thermal history of an aqueous reservoir.
The report “acts as a “thermometer” for formation environment of planetary systems” says Luis Salazar Manzano, a doctoral student at the University of Michigan and lead author of the study. Natural astronomy study. That’s why he and his co-authors were so shocked when ALMA data revealed that 3I/ATLAS contained a fraction of heavy water about 30 times that of typical comets in the solar system.
The discovery lands on top A stack of related results who reach an inescapable consensus: wherever this interstellar intruder came from, its origins must have been much colder and more alien than those of anything around our familiar sun. Previous studies suggest the comet is at least seven billion years old and possibly even more than 10 billion years old; either estimate far exceeds the age of the solar system, which formed about 4.5 billion years ago.
More than a month after Manzano and his colleagues used ALMA to monitor 3I/ATLAS in radio waves, picking out the subtle signatures of heavy water in its cloud-like gas “coma,” another team turned the infrared gaze of NASA’s James Webb Space Telescope (JWST) toward the comet. This team also found signs of deuterium. These JWST results have not yet been peer-reviewed, but they have been published in several preprints posted online.
“Our observations were the first evidence of such an improvement, and the JWST data reconfirmed what we had discovered with ALMA,” says Manzano co-author Teresa Paneque-Carreño, an assistant professor at the University of Michigan who pushed for valuable observation time on ALMA.
These spectroscopic studies of comets constitute a recent advance in astronomy. “It’s a very, very difficult measurement to make,” says Darryl Seligman, an astronomer at Michigan State University who was not directly involved in either the ALMA or JWST work. “This is almost unprecedented for comets in the solar system, and now they’ve done it for an interstellar comet,” says Seligman. “The fact that they were able to do it is just remarkable.”
There are two general, possibly overlapping, explanations for the extraordinary deuterium enrichment of 3I/ATLAS, Manzano says. The comet may have inherited its abundant deuterium from a “primordial prestellar environment” – the gas cloud from which its star formed – that was much colder than the one that produced our sun. But in principle, the deuterium level of 3I/ATLAS could also have been increased later due to the complex thermal processes it underwent during its formation and drift through the protoplanetary disk of its host system. However, these processes in a disk can also warm comets enough to reduce their deuterium levels. “Therefore, our interpretation is not only that the 3I/ATLAS host system was extremely cold, but also that the 3I/ATLAS material likely underwent relatively limited heat treatment.”
Regardless, Manzano says, the comet overabundance “still indicates a remarkable difference between the 3I/ATLAS host system and our own solar system.” The difference perhaps lay in the environment in which the system was born, which was perhaps more isolated and quieter than that of our sun; perhaps the anomalous deuterium results from the way 3I/ATLAS formed and migrated through the system’s disk, the size and shape of which may have kept the comet away from stellar radiation; perhaps it was a mixture of the two.
Comet 3I/ATLAS is just the latest interstellar novelty: 1I/ʻOumuamua, the very first object from another star to be seen by our sun, was also profoundly strange. It behaved so strangely when astronomers discovered it in 2017 that Seligman and others postulated it might be a frozen nitrogen iceberg from very frigid environments rather than an ordinary comet. The second such visitor, 2I/Borisov, was found in 2019. And although it had cold-related oddities, it more closely resembled solar comets than its predecessor.
What’s most exciting, Paneque-Carreño says, is the potential for future discoveries with ALMA and other cutting-edge telescopes. Thanks to new facilities such as the Vera C. Rubin Observatory in Chile, she adds, “the detection and analysis of interstellar objects will be more common, leading to a direct comparison between the chemical conditions of our solar system and others.” »
“Either the solar system is strange and unique, or the formation of planets in other stars is not well understood,” says Seligman. “They’re really two different ways of saying the same thing.”
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