A unique chemical composition was identified in a comet from another star system, presenting characteristics distinct from any other similar object observed in the vicinity of the Sun to date.
Throughout history, only three celestial bodies outside our solar system have been detected in transit. The first, 1I/Oumuamua, appeared in 2017 with a shape unknown to astronomers, leaving many questions. The second, 2I/Borisov, demonstrated a familiar chemical structure, behaving like a local comet. Now, the third, 3I/ATLAS, is proving to be even more peculiar.
New analyzes carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) indicate that comet 3I/ATLAS contains an unusual amount of methanol, compared to other molecules. The proportions recorded place this comet among the richest in methanol ever investigated, including those that formed in our own solar system. This suggests that the environment where this object was generated in its home star system had conditions that imprinted a unique chemical signature on its ice.
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“Studying 3I/ATLAS is like obtaining a fingerprint of a distinct solar system,” said Nathan Roth, lead author of the study published in The Astrophysical Journal Letters and professor at American University. “The details reveal its makeup, and it is filled with methanol in a way we rarely observe in comets in our own solar system.”
Roth’s team used ALMA’s Atacama Compact Array, a cluster of smaller antennas within the large observatory in the Atacama Desert, Chile, to track comet 3I/ATLAS at different times in late 2025. As the comet approached the Sun, solar heat caused the ice on its surface to sublimate, turning it directly into gas and forming a glowing haze around its nucleus known as a coma.
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This coma became the focus of measurements. Analyzing the faint submillimetre signals it emitted, the team identified the chemical signatures of two organic substances: methanol (CH₃OH) and hydrogen cyanide (HCN), a nitrogenous compound often found in comets in the solar system.
The data collected on two different observation dates was remarkable. The team measured methanol production rates in relation to HCN between 70 and 120. For comets that originated around the Sun, these rates are significantly lower, and the high methanol content alone has placed 3I/ATLAS in a completely different category from previously studied objects.
The ALMA information showed more than simple abundance: methanol and hydrogen cyanide do not emerge from the comet in the same way. HCN appears to have its origin almost entirely in the solid nucleus, a usual behavior for comets in our system.
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On the contrary, methanol comes simultaneously from two sources: the main nucleus and the icy dust grains that float in the coma itself. These grains function as secondary emitters; When heated by sunlight, the methanol ice they carry turns into gas, directly feeding the gas cloud around the comet.
This process is called prolonged degassing. Although it has already been recorded in some comets in our solar system, the ALMA Observatory highlights that this is the first time it has been spatially mapped in an interstellar object. This distinction is crucial, as it allowed the team to determine the origin of the methanol, not just its presence.
The high concentration of methanol is not the only chemical peculiarity associated with comet 3I/ATLAS. Previous assessments carried out with the James Webb Space Telescope had already revealed that, when the comet was still far from the Sun, its coma was composed mainly of carbon dioxide, instead of water, which is common in comets in the solar system at similar distances.
ALMA data now adds methanol to this unusual inventory, drawing a chemical profile that deviates from known standards in multiple respects. Two different instruments, observing the same object under varying conditions, presented results outside what is considered typical for local comets, expanding understanding of the diversity of materials in space.
Methanol forms on the surface of cold interstellar dust grains through a mechanism that adds hydrogen atoms to carbon monoxide at extremely low temperatures. The effectiveness of this transformation is influenced by temperature, radiation exposure and the elemental composition of the initial grain condensation environment. A disproportionately large ratio of methanol to molecules like HCN may indicate that the material remained in an area of a protoplanetary disk that was exceptionally cold or chemically unique, although the study did not specify a particular origin.
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With only three confirmed interstellar objects, each of them carries immense scientific importance. Oumuamua did not develop a detectable coma and accelerated in a way that water ice alone could not explain. Borisov presented chemistry so similar to that of comets in the solar system that he suggested that planetary formation produces similar results in many places in the galaxy.
Comet 3I/ATLAS, however, complicates this perspective on the formation of planets. It arrives with a composition that surpasses anything observed in our neighborhood, showing that interstellar comets can, in fact, have their chemistry shaped by conditions intrinsically different from other stellar systems, defying the expected uniformity.