Interstellar Comet 3I/ATLAS Carries 13 Times More Heavy Methane Than Any Solar System Comet

The interstellar comet 3I/ATLAS appears to be made of unusually ancient material, according to a new study in the journal Nature Astronomy. Researchers led by scientists from NASA’s Solar System Exploration Division found that its methane holds far more heavy hydrogen than anything measured in our Solar System, ScienceAlert reported.

The result suggests the comet may have formed in a very cold, radiation-soaked region of an older star system, possibly more than 7 billion years before the Sun ignited, the outlet said.

The key numbers behind the finding

• 13 times: how much more heavy methane the comet holds than comet 67P/Churyumov-Gerasimenko, the only other comet in our Solar System with a secure reading of this kind. That measurement required instruments on the Rosetta spacecraft at the comet itself.
• 2.5 times: how far from the Sun, compared with Earth, the comet was when the James Webb Space Telescope observed it in late December 2025. At that distance it was passing through the asteroid belt between Mars and Jupiter.
• More than 7 billion years: how long before our Sun ignited the comet’s home system may have assembled it. ScienceAlert said that might even predate the eruption of the comet’s own star.
• Around -263 degrees Celsius (-440 degrees Fahrenheit): the temperature at which gases freeze onto dust grains and favor heavier hydrogen, producing the distinctive ratios seen in molecules such as methane.

How the team read the comet’s chemistry

As solar radiation warms a comet, its icy chemicals turn to gas and puff up a hazy atmosphere called a coma. The team used the infrared capabilities of the James Webb Space Telescope to study the chemical mix in that coma.

They focused on the deuterium-to-hydrogen ratio. Deuterium is a heavier form of hydrogen, and its share in a molecule shows how much of it an object kept from the time its frozen dust grains formed. Here the researchers measured the ratio in methane. They found it was significantly higher than in any body in the Solar System.

The authors wrote that the elevated ratio fits the idea that the comet’s ices formed in a cold, low-metallicity interstellar or protostellar environment with a high rate of cosmic ray irradiation.

Why the comet is thought to be so old

A second clue points the same way. According to ScienceAlert, the comet has a low carbon-13 content, which suggests it comes from an early Universe that had not yet been enriched by generations of stars. Those stars fused heavier elements and spread them through space when they exploded as supernovae.

The isotope ratios also hint that the comet was exposed to a lot of radiation. They fit with a birth in a prestellar core, a dense region of super-cold gas in the earliest stages of star formation, protected from starlight and cold enough to preserve ices.

One possible birthplace is the pancake-shaped disk of dust and gas around a very young or forming star. These regions are thickly opaque, which makes the chemistry inside them hard to observe, so researchers often rely on astrochemical models instead.

Why it matters: a sample from a hidden nursery

The significance, ScienceAlert noted, is that 3I/ATLAS gives astronomers a rare physical sample from the hidden depths of an ancient stellar nursery, possibly around one of the Milky Way’s oldest stars. It reached our neighborhood by chance, carried by gravity.

The work also shows how vast cosmic timelines are. The comet could have traveled for 7 billion years before our Solar System existed, and it may have formed before many of the atoms that make up the Sun and planets were forged in the hearts of massive stars.

The conclusions come from one study and rest on interpreting isotope ratios, so they describe a hypothesis about the comet’s origin that fits the data rather than a settled answer. The comet’s home star and the place it came from remain unknown.

Around Prague, based on reporting by ScienceAlert