A team led by scientists from NASA's Solar System Exploration Division turned the James Webb Space Telescope on comet 3I/ATLAS in December 2025, when it was traveling through the asteroid belt roughly 2.5 times farther from the sun than Earth.

Webb's infrared vision let them analyze the glowing coma of gas released as the comet's frozen ices warmed. The focus was the deuterium-to-hydrogen ratio measured in methane — a chemical fingerprint that reveals where and how an object formed.

The findings, published in Nature Astronomy, surprised the researchers: the comet contains far more heavy methane than any known object in our solar system.

The comet's deuterium-to-hydrogen ratio in methane is 13 times higher than that of comet 67P/Churyumov-Gerasimenko, the only other comet near the sun with a reliable measurement of this kind — a reading that required the European Space Agency's Rosetta spacecraft to visit 67P in person.

The researchers say the elevated D/H ratio is consistent with ices forming in a cold, low-metallicity interstellar or protostellar environment under high cosmic ray irradiation.

The chemistry points to an object far older than our solar system. According to the study, 3I/ATLAS may have formed more than 7 billion years before the sun even existed, making it a frozen relic from the early days of the Milky Way.

A second clue reinforces the age: the comet contains low amounts of carbon-13, a heavier form of carbon.

That detail suggests it formed in the early universe, before generations of stars had lived, died in supernova explosions and filled space with heavier elements — meaning it may predate many of the atoms that make up the sun, Earth and the other planets.

3I/ATLAS is only the third interstellar object ever confirmed passing through our solar system, first spotted on July 1, 2025.

Researchers describe it as a pristine sample ejected from the hidden depths of an ancient cosmic nursery, possibly around one of the Milky Way's oldest stars, and delivered to our doorstep by gravity.

The comet passed its closest approach to the sun around October 30, at about 1.4 au — roughly 130 million miles, just inside the orbit of Mars — and poses no threat to Earth. Its brief visit offers a rare chance to study material from regions too thick and dark to observe directly.