Interstellar comet 3I/ATLAS has 70 times more methanol than any comet from our Sun

Interstellar comet 3I/ATLAS has handed astronomers a chemical fingerprint nothing in our solar system can match. Only the third object ever confirmed to arrive from outside our solar system, it carries methanol in concentrations between 70 and 120 times its hydrogen cyanide content — a ratio so far outside the range of known comets that the team running the analysis had to double-check their instruments.

In the catalog of roughly 30,000 known comets from our solar system, the most methanol-rich objects peak at a ratio of around three. The number for 3I/ATLAS is not a measurement of degree. It is a signal of a different chemistry entirely.

Where the methanol comes from

The research team, led by Nathan Roth at American University, used the Atacama Large Millimeter/submillimeter Array’s Atacama Compact Array in Chile. They observed 3I/ATLAS on multiple dates in late 2025, as the comet moved closer to the Sun and its activity intensified.

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In standard comets, methanol — like most volatile molecules — outgasses primarily from the nucleus, the central solid body. But 3I/ATLAS does something else. In this comet, methanol releases from both the nucleus and from millions of tiny icy grains suspended in the surrounding coma, the cloud of gas and dust that forms as sunlight heats the comet. That double-source outgassing is the first time scientists have tracked it in any interstellar object.

The other molecule the team measured, hydrogen cyanide, behaves conventionally: it comes almost entirely from the nucleus, just as it does in solar system comets. The contrast makes the methanol anomaly sharper. HCN is the baseline. Everything above it is the fingerprint of another star system.

What the chemistry says about where it formed

Methanol forms in cold molecular clouds — the dense interstellar gas that collapses into new stars and planetary systems. Its concentration in comets depends on the temperature, density, and ultraviolet radiation environment of the protoplanetary disk where the comet was assembled. A methanol-to-HCN ratio of 70 to 120 implies that the disk around 3I/ATLAS’s parent star had substantially more methanol-producing chemistry than the disk that built our own solar system’s comets.

Whether that means more carbon-rich ice, a colder formation zone, or longer exposure to the molecular cloud environment before incorporation into a planetesimal remains an open question. What it settles is that the ratio is not noise. On two separate observing nights, the measurements landed at 70 and 120 — different values, but both so far above solar system ranges that the conclusion is the same.

What makes 3I/ATLAS different from the first two interstellar visitors

The first interstellar object ever detected, 1I/’Oumuamua, passed through in 2017 and left more questions than answers. It had no detectable coma, no visible gas, and an acceleration that couldn’t be fully explained by solar radiation pressure alone. Scientists still disagree about what it was. The second interstellar visitor, 2I/Borisov, arrived in 2019 and looked reassuringly familiar: a comet that outgassed normally and carried molecules consistent with solar system comets. Borisov was the encouraging message that planetary systems elsewhere build familiar chemistry.

3I/ATLAS is the counter-argument. It looks like a comet in structure — it has a nucleus, a coma, and jets — but its chemistry doesn’t match. It was discovered on July 1, 2025, by the ATLAS survey, and within weeks the astronomical community mobilized dozens of instruments to observe it before it left the inner solar system. The ALMA observations in late 2025 are among the most chemically detailed of the campaign.

What 3I/ATLAS does not settle

The paper is careful about what the methanol excess explains and what it doesn’t. Roth and colleagues do not claim to identify the parent star system or to reconstruct the disk conditions precisely. The observations cover the outgassing behavior near perihelion — the comet’s closest approach to the Sun — but the same comet at a different stage of its journey, or from a different vantage point, might show different ratios as different volatiles become active.

A peer-reviewed study from a single instrument campaign also carries methodological limits. The team’s methanol-to-HCN ratios depend on spectral line measurements at submillimeter wavelengths, where atmospheric absorption and beam dilution introduce uncertainties. The values of 70 and 120 are not identical across the two nights, which reflects the natural variability of cometary outgassing as the nucleus rotates and the Sun’s heating shifts. Neither observation is consistent with any solar system comet — but the variability between them is itself a reminder that 3I/ATLAS is a dynamic object being measured at one point in its trajectory.

The team also notes that the ratio is not a complete chemical portrait. Dozens of other molecules have been detected or upper-bounded in solar system comets, and the 3I/ATLAS campaign captured only methanol and HCN at radio wavelengths. Ultraviolet, infrared, and optical spectroscopy from other instruments observing the same comet will build a fuller picture over the coming months as the combined dataset from the 2025 campaign is published.

Common questions about 3I/ATLAS

What is 3I/ATLAS?

3I/ATLAS is the third interstellar object ever confirmed to enter our solar system from outside it. It was discovered on July 1, 2025 by the ATLAS automated survey telescope. The first was 1I/’Oumuamua (2017) and the second was 2I/Borisov (2019). The designation 3I marks it as the third confirmed interstellar interloper.

Why does methanol matter in comets?

Methanol (CH₃OH) is a simple organic molecule that forms at low temperatures in the molecular clouds where stars and planets develop. Its abundance relative to other molecules — especially hydrogen cyanide — reflects the temperature and chemistry of the protoplanetary disk where the comet was assembled. A very high methanol ratio suggests formation conditions distinct from those that built our solar system’s comets.

How do scientists measure molecules in a comet?

Radio telescopes like ALMA detect specific molecules by measuring the microwave and millimeter-wave radiation they emit when their molecular bonds vibrate. Each molecule emits at characteristic frequencies — a spectral fingerprint. By measuring the intensity of these emission lines in the comet’s coma, astronomers calculate how much of each molecule is present relative to others.

Could 3I/ATLAS carry organic molecules from another solar system to Earth?

No. 3I/ATLAS will pass through the solar system and leave permanently — it is on a hyperbolic trajectory that will carry it back into interstellar space. The comet itself poses no collision risk with Earth. But its chemistry does provide direct evidence that organic molecules common in our solar system also form in other planetary systems, in different proportions.

What happens to 3I/ATLAS next?

3I/ATLAS has already passed its closest approach to the Sun and is receding. Additional observations from the 2025 campaign — covering ultraviolet, infrared, and visible wavelengths — are being analyzed and will be published over the coming months. The ALMA data represents one chemical measurement from one instrument; combining it with data from Hubble, JWST, SPHEREx, and planetary missions that observed the comet in 2025 will build the most complete chemical portrait of any interstellar object ever measured.

The ALMA methanol paper draws on observations from late 2025, when 3I/ATLAS was near its closest approach to the Sun and at its most chemically active. It was published September 9, 2026. Ultraviolet, infrared, and optical data from the same campaign — from Hubble, JWST, SPHEREx, and multiple planetary missions — are still being analyzed. Every one of those datasets will land against the same question: whether the methanol excess is the whole story, or just the molecule that was loudest.

Reference: Roth et al., “Methanol and HCN in the coma of interstellar comet 3I/ATLAS,” The Astrophysical Journal Letters, 2026. DOI: 10.3847/2041-8213/ae433b

Tags: astronomy, ALMA, 3I/ATLAS, cometary chemistry, interstellar comet, methanol

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