New Clues to Ancient, Distant Origin of Comet 3I/ATLAS | Webb Telescope

New Clues to Ancient, Distant Origin of Comet 3I/ATLAS | Webb Telescope

Webb’s Near-Infrared Spectrograph (NIRSpec) instrument can map specific chemical and molecular signatures, as seen here in its three images of comet 3I/ATLAS, each highlighting a part of the comet’s contents.

Researchers use NIRSpec’s Integral Field Unit. It provides a spectrum of every image pixel, to dive deeper into the details of cosmic objects than they can with the telescope’s imaging instruments alone. This is crucial for a rare object like 3I/ATLAS. It is only the third comet from outside the Solar System ever studied, and the first to be observed by an instrument capable of capturing as much detail as NIRSpec. With NIRSpec’s data, researchers can build a picture of where the comet may have come from and what its home system was like and then compare that to familiar conditions in the Solar System.

Image Description: Comparison of three telescope images side by side. They are roughly spherical but pixelated with more intense color saturation in the center. From left to right: smallest sphere is blue and labeled H2O, orange is larger and labeled CO2, and red is largest and labeled CO. A scale bar at the lower left is labeled 1300 km/1 arcsecond and is about one fourth of each of the three images. A compass at the lower right shows north pointing up to 12 o’clock, east pointing left to 9 o’clock, and a fainter arrow labeled to Sun pointing down to 8 o’clock.

Measurements of specific element varieties by Webb’s NIRSpec (Near-Infrared Spectrograph) instrument show how different the interstellar comet 3I/ATLAS is from comets originating in our own Solar System. Researchers used NIRSpec to measure carbon-13, which contains an extra neutron, relative to the more common carbon-12. They also measured the abundance of heavy hydrogen, which is a hydrogen atom with an added neutron.

Webb’s NIRSpec found a surprisingly large amount of heavy hydrogen, with a low abundance of carbon-13, indicating that 3I/ATLAS came from a place very different from our own Solar System. Researchers say early analysis of these results indicates that 3I/ATLAS was ejected from its origin system billions of years ago.

Image Description: Infographic showing the differences in measured ratios of heavy carbon and heavy hydrogen between Solar System comets and interstellar comet 3I/ATLAS. The top portion of the infographic has headline Heavy Carbon, plus a horizontal scale in increments of 50 ranging from zero to 250 measuring the ratio of Carbon-12 to Carbon-13. Three Solar System comets appear just below 100 on the scale, while 3I/ATLAS appears above 150 for carbon monoxide and about 170 for carbon dioxide. The bottom portion of the infographic has the headline Heavy Hydrogen, and a horizontal scale ranging from 10 to the negative fifth power on the left to approximately 10 to the negative first power on the right, though 10 to the first is not labeled. This scale is labeled Ratio of Heavy Hydrogen Measured in Water. Eleven Solar System comets appear on the graph, all falling to the right of 10 to the negative fourth power. Comet 3I/ATLAS appears at 10 to the negative second power.

The third identified interstellar comet in human history has a surprising chemical makeup, raising questions as to how common, or unusual, conditions in our own Solar System may be.

As interstellar comet 3I/ATLAS began moving away from the Sun in December 2025, astronomers used the NASA/European Space Agency/Canadian Space Agency James Webb Space Telescope to capture detailed measurements of its chemical components. The comet was freshly warmed from its closest pass by the Sun, and its ancient ice had been converted to a bright coma of gas ideal for observation.

Webb captured detailed data, including chemical ratios of carbon and deuterium, also known as heavy hydrogen, that are not found in Solar System comets. The results surprised researchers. Working backward, astronomers used the components that make up comet 3I/ATLAS to understand the environment where it formed.

A paper detailing the findings was published on June 22, 2026, in the journal Nature.

The comet’s name comes from its status as the third confirmed interstellar comet, meaning it originated outside the Solar System, and the telescope that first spotted it, the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS).

“This was a unique opportunity to study an ancient object from the distant Galaxy, probably pre-dating our Sun and Solar System,” said astro-chemist Martin Cordiner of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study. “On the one hand, we get direct insight into that distant time and place, and on the other, we learn something about how unusual our own Solar System may be.”

Cordiner and the research team joined astronomers from many sub-disciplines in taking the opportunity to get a look at 3I/ATLAS on its journey through the Solar System. They received approval to interrupt Webb’s planned schedule of observations to make use of its Near-Infrared Spectrograph (NIRSpec) instrument to study the comet.

NIRSpec revealed exceptionally high levels of deuterium, about 30 times more than seen in Solar System comets. This implies that 3I/ATLAS may have originated in a very cold system much earlier in the history of our galaxy. During its formation, the material that became incorporated into 3I/ATLAS was likely exposed to plenty of radiation, but not any long-term warmth that would have reprocessed its “heavy water” ice, with deuterium, into the type of H2O ice we are familiar with on Earth.

Additionally, NIRSpec showed only traces of carbon-13 compared to lighter-weight carbon-12. This also points to a very old origin for 3I/ATLAS, as stellar systems become enriched with carbon-13 over time as generations of stars are born and die in the galaxy. That is why there are higher levels of carbon-13 in our system, around our Sun, which formed relatively recently, 4.5 billion years ago.

The research team estimates that 3I/ATLAS could have formed as long as 10 to 12 billion years ago, during the Universe’s “cosmic noon,” when star formation was at its height. Its young origin system was likely ensconced in a relatively cold, dense cloud. The abundance of heavy water shows that 3I/ATLAS spent its formative years in a deeply frozen state.

A separate study using the European Southern Observatory's Very Large Telescope, led by astronomer Cyrielle Opitom of the University of Edinburgh, complements Webb’s findings with an analysis of 3I/ATLAS’s carbon and nitrogen varieties in the form of the chemical cyanide.

“For us as scientists, finding these rare isotopes is fascinating, but the bigger picture here is looking at the possibilities of prebiotic chemistry elsewhere in the galaxy,” said Stefanie Milam of NASA Goddard and co-author of the study with Cordiner. “So far, we know of only one place in the vast cosmos where chemical ingredients led to life—our Solar System, our Earth. Analysis of these interstellar objects is a major step towards learning how common, or uncommon, the conditions for the evolution of life are in the Universe.”

Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).

Image Credit: NASA, ESA, CSA, STScI, M.Cordiner (Catholic University of America, GSFC)
Release Date: June 22, 2026

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