Plate Nº 57 · recorded October 31, 2026
Space & AstronomyReported finding
Interstellar Comet 3I/ATLAS Carries Methane Up to 10 Billion Years Old
New James Webb Space Telescope observations show comet 3I/ATLAS holds thirteen times more heavy methane than comet 67P, hinting at ice formed before our Sun existed.
By Priya Raman4 min read726 words
In brief
- 3I/ATLAS contains 13 times more heavy methane (by D/H ratio) than comet 67P/Churyumov-Gerasimenko
- Observations were made with the James Webb Space Telescope in late December 2025, at 2.5 times the Earth-Sun distance
- Its ices may have formed more than 7 billion years before the Sun ignited
- The findings were published in the journal Nature Astronomy
- Prestellar cores where such ice can form reach about -263 degrees Celsius
The interstellar comet 3I/ATLAS holds thirteen times more heavy methane than any securely measured comet in our Solar System — and its ice may have formed more than 7 billion years before the Sun ignited. A team led by scientists from NASA's Solar System Exploration Division reported the finding in the journal Nature Astronomy, based on observations with the James Webb Space Telescope in late December 2025.
At that moment, the comet was about 2.5 times as far from the Sun as Earth is, racing through the asteroid belt between Mars and Jupiter. Solar radiation had begun to sublimate its frozen volatile chemicals — turning ice directly into gas — causing its atmosphere, called a coma, to swell substantially. Viewing the molecular emission from that escaping gas, the astronomers could read the comet's chemistry like a shipping manifest from another star system.
What did the researchers measure?
The team focused on the deuterium-to-hydrogen ratio, or D/H ratio — the proportion of deuterium, a heavier version of hydrogen, to ordinary hydrogen within a molecule. In this case, they measured the ratio inside methane, which reveals how much heavy methane the comet has retained since its frozen dust grains first formed.
The result: 3I/ATLAS holds significantly more deuterium-rich methane than any body in the Solar System. The previous record holder among local comets, 67P/Churyumov-Gerasimenko, showed a "secure" signature only because the Rosetta spacecraft took in-situ measurements — direct readings on location. 3I/ATLAS surpasses it thirteenfold, detected from across the Solar System.
"The elevated D/H ratio for methane in 3I/ATLAS is consistent with the hypothesis that its ices formed in a cold, low-metallicity interstellar or protostellar environment under a high cosmic ray irradiation rate," the researchers explain.
Where might such ancient ice form?
One candidate birthplace is the pancake-shaped planet-forming disk of dust and gas surrounding a very young or forming star. These environments are difficult to study directly because they are thickly opaque, hiding the cool chemical reactions within; astronomers often must rely on astrochemical models instead.
Another possibility is a prestellar core — a dense region of super-cold gas, shielded from starlight, that represents the earliest stage of star formation. These cores act as cryo-preservers. At temperatures around -263 degrees Celsius (-440 degrees Fahrenheit), gases freeze onto dust grains, and the process favors deuterium because it is heavier than ordinary hydrogen. That selective freezing produces the distinctive D/H ratios seen in molecules such as methane.
This is precisely what makes 3I/ATLAS so valuable to science. It is a pristine sample ejected from the hidden depths of an ancient cosmic nursery, possibly around one of the Milky Way's oldest stars, delivered to our doorstep by the caprices of gravity.
How old could this material be?
The comet's low carbon-13 content points to extraordinary age. Carbon-13 is a heavier variant of carbon, and its scarcity suggests the object originated in an ancient Universe that had not yet been enriched by generations of stars fusing heavier elements and scattering them through space in supernova explosions.
The exosolar system that built 3I/ATLAS may have assembled its ices more than 7 billion years before our Sun ignited into life — possibly even before 3I/ATLAS's own parent star erupted. The comet may have been drifting through interstellar space for those entire 7 billion years before the Solar System existed.
Put plainly: this visitor from who knows where may have formed before many of the atoms comprising our Sun and planets were forged in the hearts of massive stars — stars that later died and left behind the remnants that seeded our existence.
What are the caveats?
The findings describe a hypothesis, not a confirmed biography. The chemistry is consistent with formation in a cold, radiation-bathed environment, but the researchers cannot yet pinpoint the exact birthplace. The D/H comparison also rests on a small sample: 67P/Churyumov-Gerasimenko remains the only other comet with a secure methane measurement, and that required a spacecraft mission.
Still, as time capsules from the cosmos at large go, 3I/ATLAS is the closest thing to a courier hand-delivering an ancient parcel. The comet will warm itself briefly at our solar hearth before speeding back into darkness — leaving astronomers with data that outlast the visit.
via science.nasa.gov (Original)
More from Priya Raman
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Senior reporter covering people, launches and day-to-day practice in general science news at SciBeat.
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