April 24, 2026

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Alien comet reveals our solar system is the oddball

Measurements of this interstellar comet’s molecular makeup show an excess of heavy water molecules that is dramatically different from anything known to have ever formed around our sun

By Lee Billings & Joseph Howlett edited by Jeanna Bryner

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 & M. Rodriguez (International Gemini Observatory/NSF NOIRLab), T.A. Rector (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab)

Comet 3I/ATLAS, an object from another star that was found speeding through our solar system last summer, is now fading from telescopic view as it retreats back to interstellar space. But it continues to offer lessons about its faraway origins—and, consequently, to demonstrate how special our solar system may be.

Astronomers caught a glimpse of 3I/ATLAS just days after the icy comet made its closest approach to the sun in late October 2025. With the telescopes of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, they used radio waves of light to study the starlight-warmed material the comet was venting into space. The spectroscopic results showed way more exotic, “heavy” water than would be expected for a comet from our own solar system, according to research published yesterday in Nature Astronomy.

Like ordinary water, heavy water pairs two hydrogen atoms with one oxygen atom to make each molecule of moisture. But for the weightier 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 water in a rocky body contains deuterium depends on the chemical processes that formed it. Specifically, cold temperatures greatly favor reactions that pump up the amount of heavy water relative to everyday water. So that ratio is a sensitive probe of a watery reservoir’s thermal history.

The ratio “acts as a ‘thermometer’ for the formation environment of planetary systems,” says Luis Salazar Manzano, a doctoral student at the University of Michigan and lead author of the Nature Astronomy study. That is why he and his co-authors were so shocked when ALMA’s data revealed 3I/ATLAS had a heavy water fraction that was about 30 times greater than that of typical solar system comets.

The finding lands atop a stack of related results that reach an inescapable consensus: wherever this interstellar interloper hails from, its origins must’ve been much colder and more alien than that of anything found around our familiar sun. Previous studies have suggested the comet is at least seven billion years old and perhaps even older than 10 billion years; either estimate greatly 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, teasing apart the subtle signatures of heavy water in its cloudlike “coma” of gas, a different team turned the infrared gaze of NASA’s James Webb Space Telescope (JWST) to the comet. That team also found signs of deuterium. Those JWST results have yet to be peer-reviewed, but they were publicized in multiple preprints posted online.

“Our observations were the first evidence of such an enhancement, and the JWST data came to reconfirm what we had discovered with ALMA,” says Manzano’s co-author Teresa Paneque-Carreño, an assistant professor at the University of Michigan, who lobbied for precious observing time on ALMA.

These spectroscopic studies of comets are a recent breakthrough in astronomy. “That is a very, very difficult measurement to make,” says Darryl Seligman, an astronomer at Michigan State University, who wasn’t directly involved in either the ALMA or the JWST work. “It’s almost unprecedented for solar system comets, and now they’ve done it for an interstellar comet,” Seligman says. “The fact they were able to do it is just remarkable.”

There are two broad and potentially overlapping explanations for 3I/ATLAS’s extraordinary deuterium enrichment, Manzano says. The comet could have inherited its abundant deuterium from a “primordial prestellar envir