3I/ATLAS is only the third confirmed interstellar object ever detected passing through our solar system. It may be older than the Sun itself, and Webb observations show it is carrying an unusually carbon dioxide-rich coma as it swings past once before leaving for interstellar space forever.

On 1 July 2025, the ATLAS survey in Chile picked up a faint object moving on a path that did not belong to the solar system. Within days it was confirmed as 3I/ATLAS, only the third known interstellar object identified passing through our system, after 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. It is a comet, it appears to be very old, and it has already made its single pass through the inner solar system and is heading back out to interstellar space, never to return.

The most distinctive thing about it, beyond where it came from, is what the James Webb Space Telescope found in the gas streaming off it.

What it is and where it is going

3I/ATLAS arrived on a strongly hyperbolic orbit, with an eccentricity of 6.14, the highest of the three known interstellar objects, and an excess speed of about 58 kilometres per second. That trajectory is the signature of something not bound to the Sun. It reached its closest approach to the Sun, perihelion, around 30 October 2025, passing at about 1.4 astronomical units, just inside the orbit of Mars. Its closest approach to Earth came on 19 December 2025, at roughly 270 million kilometres, far enough to pose no threat of any kind.

It is also only the second interstellar object to show an obvious coma, the halo of gas and dust that defines a comet. ‘Oumuamua showed none, which is part of why it remained so puzzling. 3I/ATLAS, by contrast, behaves unmistakably like a comet, which made it a far richer target while the window lasted.

What Webb saw

The notable result came from Webb’s near-infrared spectrograph, which first observed the comet on 6 August 2025, while it was still 3.32 astronomical units from the Sun. A team led by Martin Cordiner reported the first Webb results in a paper on the arXiv preprint server: the coma was dominated by carbon dioxide, with roughly eight times as much carbon dioxide as water, among the highest ratios ever measured in a comet and well above the usual trend for comets at that distance.

Alongside the carbon dioxide, Webb detected water, carbon monoxide, carbonyl sulfide, water ice and dust. The simplest reading, which the authors offer cautiously, is that the nucleus is intrinsically rich in carbon dioxide, perhaps because its ices were exposed to more radiation than those of comets formed in our own system. A second look after perihelion, in late December, added methane, methanol and ethane, and allowed measurements of the comet’s carbon and hydrogen isotopes.

How old, and how they can tell

Those isotopes feed the claim that 3I/ATLAS is ancient. Its speed and galactic motion give a kinematic age estimate of somewhere between 3 and 11 billion years, already older than the previous two interstellar objects. The chemistry pushes in the same direction. As Sky & Telescope reported, the comet’s low level of heavy carbon suggests it formed before moderate-mass stars had enriched the galaxy with that element, and its water carries more deuterium than any comet previously studied.

Put together, several independent lines point to a formation time well before the Sun came into being about 4.6 billion years ago, with the isotopic analysis indicating something like 10 to 12 billion years. These are inferences from motion and composition rather than a measured date, and the range is wide. But the direction is consistent: this is most likely an object older than our own star, possibly by a large margin.

On the speculation

It is worth addressing directly, because it travelled widely. The arrival of a fast, unusual interstellar object prompted speculation that 3I/ATLAS might be artificial. The evidence does not support that. A coma rich in carbon dioxide and water, jets of dust, organic molecules and outgassing that responds to sunlight are the properties of a comet, and every measurement made of 3I/ATLAS has fit that description. In June 2026 the SETI Institute reported that more than seven hours of radio scans with the Allen Telescope Array had turned up no technosignatures, with every candidate signal tracing back to transmitters on Earth or in orbit. The interesting thing about the object is not that it might be a probe. It is that it is a natural body from another planetary system, and an old one.

What to watch

The comet is now outbound and fading, and because its orbit is not closed, the observations gathered during this single pass are all anyone will ever get of it. The remaining work is analysis: refining the composition, the isotope ratios and the age estimates from the data already collected by Webb, Hubble, ground-based telescopes and the spacecraft at Mars that caught it near perihelion.

The longer arc is that three confirmed interstellar objects in eight years is the start of a sample, not the whole of one. The Vera C. Rubin Observatory is expected to find such visitors far more often, which would let astronomers compare them rather than studying each as a singular event. For now, 3I/ATLAS is the best-characterised piece of another star system we have seen, and it is already leaving.