Interstellar objects are rare: only three have ever been confirmed passing through our solar system, and 3I/ATLAS is the third. It was tracked by the James Webb Space Telescope in August and December 2025 and again in April 2026, giving astronomers three snapshots as it approached the Sun, passed through the asteroid belt at about 2.5 astronomical units, and receded. Each observation added a layer to a picture that turned out stranger than expected.

Webb is not taking pretty pictures here. Its NIRSpec instrument splits the comet's infrared light into a spectrum, like a chemical fingerprint, revealing which gases are escaping from its nucleus and what its icy grains are made of. For an object born around another star, that fingerprint is the only direct sample we will ever get — the comet will leave the solar system and never return.

The headline result concerns methane. By measuring the ratio of deuterium — heavy hydrogen — to ordinary hydrogen in the comet's methane, researchers could infer the conditions where it formed. Deuterium ratios act like a thermometer and a birth certificate combined: they record the temperature and chemistry of the gas cloud that made the object, billions of years ago.

3I/ATLAS showed by far the heaviest methane of any body in our solar system: its deuterium-to-hydrogen ratio was 13 times higher than that of comet 67P/Churyumov-Gerasimenko, the only other local comet with a secure measurement. The implication, published in Nature Astronomy, is that 3I/ATLAS may have formed in a stellar nursery older than our Sun — possibly more than 7 billion years old — in a colder, more deuterium-rich environment than the cloud that made our solar system.

A separate study led by the Southwest Research Institute examined the comet's water-ice grains and found a puzzle. Water ice normally leaves identifiable absorption features near 1.5, 2.0 and 3 microns in infrared light. 3I/ATLAS showed a strong feature near 3 microns, but the 1.5 and 2.0 micron features were barely visible or missing entirely — a pattern that does not match clean water ice.

Modelling showed that tiny grains, about one micron or smaller, made of water ice mixed with a large amount of dark rocky dust, explain the pattern. As lead author Silvia Protopapa put it, the grains are about the same size as those in our own comets; what sets them apart is how much dust is mixed into the ice. In other words, 3I/ATLAS is dust-rich in a way that makes its ice look different even though its building blocks are familiar.

The three Webb observations also caught change over time. Spectra from April 2026 hinted at larger, more ice-rich grains than the earlier observations showed clearly. Researchers are debating two explanations: solar radiation may be physically altering the surface grains as the comet journeys through the inner solar system, or the comet may be shedding its outer layers to expose fresher material underneath.

Both explanations remain on the table, and that uncertainty is itself informative. Comets are not static snowballs; they evolve as they heat up, with gas emissions of carbon dioxide and carbon monoxide joining the water vapour as different ices sublimate at different distances from the Sun. Watching one interstellar comet evolve over eight months is like watching a geological process in fast-forward, and it tells us how much a comet's appearance can change between its first and last observations.

When compared with trans-Neptunian objects in our own Kuiper Belt, 3I/ATLAS feels less like an alien anomaly and more like a distant relative: similar grain sizes, similar ices, different proportions and a much older birthplace. That similarity supports a powerful idea — that planet formation works broadly the same way around other stars, starting from icy dust grains that stick together, grow into comets and asteroids, and eventually build planets.

Every interstellar visitor is therefore a free sample return mission from another star system, delivered without a spacecraft. Webb's contribution is to read that sample in chemical detail: the heavy methane points to an ancient birthplace, the dusty ice points to a different mix of rock and ice, and the changing spectra point to a dynamic, evolving body. The next interstellar visitor will be studied faster and better because 3I/ATLAS taught astronomers exactly which fingerprints to look for.