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Object from outside the Solar System caught scientists’ attention for combining extreme age, water with rare composition, and chemical signs of a very cold formation environment, preserving clues about ancient planetary systems that emerged billions of years before the Sun.
The interstellar comet 3I/ATLAS may be among the oldest objects ever observed passing through the Solar System, with an estimated origin of up to 12 billion years, according to a study published on June 22, 2026, in the journal Nature.
According to the researchers’ analysis, its chemical composition does not match that of known comets from the Solar System, indicating formation in an extremely cold, ancient, and metal-poor environment, very different from the one that gave rise to the Sun and the planets.
The measurements were made with the James Webb Space Telescope, using the NIRSpec instrument, which mapped the chemical components of 3I/ATLAS as the comet moved away from the Sun in December 2025.
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For NASA, the data offers a rare opportunity to directly study material preserved from a planetary system formed long before the Sun, in a remote stage of the galaxy’s history.
The main clue appeared in the water present in the comet, whose composition revealed a chemical signature considered unusual by the scientists responsible for the analysis.
With Webb, the team detected exceptionally high levels of deuterium, known as heavy hydrogen, in a proportion about 30 times greater than that observed in comets originating in the Solar System.
This signature suggests that the material of 3I/ATLAS formed at temperatures of up to about 30 kelvin, equivalent to approximately -243°C, in a region much colder than the environments associated with local comets.
In Nature, the researchers report that the water of 3I/ATLAS has a D/H ratio of 0.98%, with a margin of error of 0.06%, a value more than an order of magnitude higher than found in known comets.
Unusual proportions between carbon-12 and carbon-13 were also identified, outside the typical patterns of the Solar System, nearby interstellar clouds, and already analyzed protoplanetary disks.
Interstellar comet 3I/ATLAS has rare composition
Classified as an interstellar object, 3I/ATLAS formed outside the Solar System and crossed the region dominated by the Sun’s gravity during its passage through our system.
According to NASA, the name combines the condition of being the third confirmed interstellar object and the identification of the ATLAS system, acronym for Asteroid Terrestrial-impact Last Alert System, responsible for the detection.
The composition measured by Webb led researchers to relate the comet to a very ancient phase of galactic history, marked by planetary formation environments different from those observed near the Solar System.
According to the Nature article, the isotopic carbon data, interpreted with models of the Milky Way’s chemical evolution, indicate that the body may have aggregated up to 12 billion years ago, after an initial period of intense star formation.
Martin Cordiner, astrochemist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, leads the study and stated that the observation opens a window to a distant time and place.
“This was a unique opportunity to study an ancient object from the distant galaxy, probably predating our Sun and the Solar System,” he said, in a statement from the space agency.
The comparison with the Solar System helps to gauge the difference between 3I/ATLAS and objects formed in Earth’s vicinity, especially when observing the estimated age and isotopic composition.
While the Sun and planets formed about 4.5 billion years ago, 3I/ATLAS may have emerged in an earlier phase, when the galaxy was still undergoing intense star formation.
This difference is recorded in the comet’s own chemistry, especially in the relationship between hydrogen and carbon isotopes, which acts as a kind of archive of the environmental conditions where the body formed.
Heavy water reveals extremely cold origin
Among the clues gathered by scientists, the excess of deuterium stood out because this isotope tends to accumulate more easily in molecules formed in very cold environments.
For the researchers, the material incorporated into 3I/ATLAS was likely exposed to a lot of radiation but did not undergo prolonged heating capable of transforming its heavy water into a composition similar to that found on Earth.
NASA states that this combination points to a cold, dense, and very ancient origin system, with chemical characteristics preserved over billions of years of travel through interstellar space.
In the Nature article, the extreme isotopic signatures are also associated with a relatively metal-poor environment, an expression used in astronomy to describe regions with a lower presence of elements heavier than hydrogen and helium.
Besides water, Webb measured chemical relationships linked to carbon in molecules such as monoxide and carbon dioxide, expanding the base used by researchers to reconstruct the object’s origin.
With this data, the team managed to recover part of the conditions of the planetary system in which the comet was born, before being launched into interstellar space and following an independent trajectory among the stars.
James Webb analyzed material from another planetary system
The interpretation presented by the researchers treats the 3I/ATLAS as a preserved fragment of an ancient planetary system, formed around a star different from the Sun.
This type of object is of interest to astronomy because it functions as a natural sample of material coming from another system, something that telescopes can only analyze when interstellar bodies come within the reach of terrestrial or space observatories.
Cordiner stated that the comet offers information about how the Solar System may be unusual compared to other systems, especially when its chemistry is compared with that of objects formed in older regions.
NASA also highlighted that the analysis of these bodies helps to assess the diversity of chemical environments in which planets and small icy objects form throughout the galaxy’s history.
Another point observed by the team was the presence of molecules associated with prebiotic chemistry, a group of compounds that helps scientists investigate how life-related ingredients may be distributed in distant environments.
Stefanie Milam, co-author of the study and researcher at NASA Goddard, stated that the search for rare isotopes is relevant not only for the chemical data but for what it reveals about the origin of these ingredients in the galaxy.
NASA treats 3I/ATLAS as a natural object
Although unproven theories have associated the 3I/ATLAS with artificial hypotheses, the available observations support the interpretation that it is a natural body with cometary behavior.
NASA describes the object as an interstellar comet and reports that heating by the Sun transformed part of its ancient ices into a bright coma of gas, suitable for spectroscopic analysis.
Besides the composition, the classification relies on the observed trajectory and the data collected by James Webb, which reinforce the reading of an icy object formed outside the Solar System.
The set of information presented in Nature points to an ancient planetesimal, preserved for billions of years since its formation in another system until its passage through the observable region of the Solar System.
The study is still undergoing final editorial stages at Nature, which reported having made an unedited version of the manuscript available for early access to the discoveries.
Even at this stage, the results already form one of the most detailed analyses of an interstellar object and help compare the chemistry of the Solar System with that of much older planetary environments.
