A study in Nature Astronomy found that the water in the interstellar comet 3I/ATLAS is dozens of times richer in deuterium than comets in the solar system. The finding suggests that the comet formed in a cold environment very different from that in which the solar system formed.
Interstellar comet 3I/ATLAS, discovered less than a year ago as it passed through the solar system, is providing astronomers with a rare glimpse into the conditions under which comets and planetary systems form outside our cosmic environment. A new study led by the University of Michigan, published in Nature Astronomy, has found that the comet’s water contains an unusually high amount of deuterium, a heavy isotope of hydrogen.
Deuterium is a heavier version of hydrogen. While a regular hydrogen atom has only one proton, deuterium has both a proton and a neutron. When deuterium combines with water molecules, it forms deuterium-rich water, sometimes called “heavy water.” Such water exists on Earth and in comets in the solar system, but according to the new study, the rate measured by 3I/ATLAS is much higher than anything measured so far in local comets.
According to the researchers, the ratio of deuterium to regular hydrogen in the comet's water is about 30 times higher than that measured in any comet in the solar system, and about 40 times higher than that found in Earth's oceans. Luis Salazar Manzano, a doctoral student in astronomy at the University of Michigan and the study's lead author, said the findings show that the conditions in which our solar system formed are not necessarily typical of other planetary systems in the galaxy.
The ratio of deuterium to hydrogen – a fingerprint of the conditions of formation
The chemical ratio of deuterium to hydrogen is used by scientists as a kind of “fingerprint” of the conditions of formation. High levels of deuterium in water may indicate formation in an extremely cold environment, where various chemical processes enrich the water with deuterium. In the case of 3I/ATLAS, the researchers conclude that the comet likely formed in a colder region with lower radiation levels than those typical of parts of the solar system where comets formed.
According to Teresa Panca-Carneau, co-lead author of the study and professor of astronomy at the University of Michigan, the finding provides evidence that the conditions that led to the formation of the solar system are not necessarily common everywhere in the galaxy. She says this may sound obvious, but to establish this scientifically, direct measurements of objects formed in other systems are needed.
The study was made possible by a combination of timing and advanced instrumentation. After the comet was discovered, the research team was able to obtain observation time at the MDM Observatory in Arizona, where early signs of gas being released from the comet were detected. Later, observations were combined with the ALMA array in Chile, one of the world’s most sensitive instruments in the millimeter and submillimeter wavelength range. ALMA made it possible to distinguish between regular water and water containing deuterium, and to calculate the ratio between the two forms.
A sample of three so far
According to the researchers, this is the first time this type of analysis has been successfully performed on an interstellar object. So far, only three interstellar objects have been discovered that have crossed the solar system, including 3I/ATLAS. Each such object represents a rare opportunity to study material formed around other stars, without sending a spacecraft to a distant planetary system.
The broader implication of the study is that astronomers can begin to use interstellar comets as natural test tubes for understanding the formation of planetary systems across the galaxy. If more such objects are discovered in the future, they could be compared to see how diverse the conditions under which water, ice, comets, and planets form in other systems are.
The researchers estimate that such detections may become more common as new survey observatories come into operation, which will scan the sky more frequently and with greater sensitivity. However, Panca-Carnio also stressed the importance of keeping the night sky dark. Interstellar objects are often small and faint, and light pollution and bright satellites can make timely detections difficult.
The study does not determine which star system 3I/ATLAS came from, but it shows that the comet carried with it a chemical signature of where it formed. In this case, the signature points to an environment very different from that which gave birth to comets in the Solar System. Thus, a small icy body, ejected from another planetary system and wandering among the stars, becomes a tool for studying how planetary systems form throughout the Milky Way.
for the scientific article DOI: 10.1038/s41550-026-02850-5
Quick FAQ
What is special about Comet 3I/ATLAS?
It is one of only three interstellar objects discovered so far in the solar system. It was likely formed in another planetary system and ejected from it before crossing our environment.
What did the researchers discover in the comet's water?
The researchers found an unusual ratio of deuterium to regular hydrogen in the comet's water. The ratio is much higher than that measured in comets in the solar system and in Earth's oceans.
why is it important?
The chemical composition of the water serves as a clue to the conditions under which the comet formed. In the case of 3I/ATLAS, it suggests a colder and different environment than that in which the solar system formed.
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