A Visitor From a Colder Star: What 3I/ATLAS's Strange Water Reveals

For only the third time in history, astronomers have watched a confirmed visitor from another star system pass through our cosmic neighbourhood — and this one left behind a chemical fingerprint that has no match anywhere in our Solar System. The object, 3I/ATLAS, was first spotted on 1 July 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey telescope in Río Hurtado, Chile. Its steep, hyperbolic path immediately gave it away: it was not bound to the Sun, and tracing the orbit backwards showed it clearly came from beyond the Solar System.

What is an interstellar object?

Almost everything we see in the night sky orbits our own Sun. An interstellar object is different: it formed around a different star, was flung out into the galaxy, and is merely passing through. Only two had ever been confirmed before — 1I/'Oumuamua in 2017 and 2I/Borisov in 2019 — which is why the European Space Agency calls these bodies "absolutely foreign" carriers of information about how planets form around other stars. 3I/ATLAS is the third, hence the "3I" prefix.

Because it was caught early, NASA and ESA had months to aim a fleet of instruments at it. Pre-discovery images pushed sightings back to 14 June 2025, and Hubble, the James Webb Space Telescope, the Parker Solar Probe and even Mars-orbiting spacecraft tracked it. Hubble pinned its nucleus somewhere between about 440 metres and 5.6 kilometres across. It reached perihelion — closest approach to the Sun, at roughly 1.4 astronomical units, inside the orbit of Mars — around 29-30 October 2025, moving at about 250,000 km/h, and made its closest, harmless pass by Earth in December at some 270 million kilometres.

The clue hidden in heavy water

As the comet heated near the Sun it vented gas and dust, including water, carbon dioxide, carbon monoxide and carbonyl sulphide. Six days after perihelion, a team using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile measured something remarkable in that water: its deuterium-to-hydrogen (D/H) ratio.

Deuterium is a heavy form of hydrogen — an ordinary hydrogen atom with an extra neutron. "Heavy water" (HDO) builds up preferentially in extremely cold, dark conditions. The relative amount of it in a comet's water therefore records the temperature of the place and time the ice first froze. The study, led by PhD student Luis E. Salazar Manzano with principal investigator Teresa Paneque-Carreño of the University of Michigan, was published in Nature Astronomy on 24 April 2026.

The highest ratio ever seen in a comet

The result was extreme. 3I/ATLAS's water carries deuterium at a level over 40 times higher than Earth's oceans and at least 30 times the average for Solar System comets — the highest D/H ratio ever recorded in any comet, as reported by Phys.org and the Smithsonian magazine. The chemistry needed to enrich heavy water this much, the team says, requires environments colder than about 30 Kelvin (roughly minus 243°C). In other words, 3I/ATLAS's ice froze in a star system far colder and more isolated than the cloud that gave birth to our Sun.

One honest caveat matters here. Ordinary water vapour itself fell below ALMA's detection threshold, so the team did not measure H₂O directly. They detected the heavy-water and methanol signals and inferred the D/H ratio through modelling — a robust approach, but an indirect one.

How old, and from where?

This is, in a real sense, a chemical sample from another planetary system delivered to our doorstep. Where did it come from? Its very high galactic velocity suggests it belongs to the Milky Way's "thick disk" of ancient stars. Separate dynamical work — distinct from the ALMA study — has estimated it could be up to about 11 billion years old, which would make it more than twice the age of the Sun and possibly the oldest comet ever observed, as CNN reported. That age figure is an estimate from its motion, not a direct measurement, and researchers still have not pinpointed its home star.

What about the 'alien spacecraft'?

Early on, Harvard astrophysicist Avi Loeb suggested 3I/ATLAS might be artificial, citing features such as its size, trajectory and unusual chemistry. The wider scientific community pushed back hard: the overwhelming consensus, emphasised by NASA scientist Tom Statler, is that it is a comet. As the object departed, Loeb himself acknowledged it is "most likely" of natural origin. The genuinely interesting anomalies — its odd water chemistry, high nickel and rapid brightening — are exactly what you would expect from a new kind of comet, born around a star very unlike our own.