Astronomers around the world are watching a remarkable visitor from beyond our Solar System — the interstellar object 3I/ATLAS (also designated C/2025 N1 (ATLAS)). First spotted on July 1, 2025 by the survey telescope of the Asteroid Terrestrial‑impact Last Alert System (ATLAS) in Chile, this object is only the third confirmed interstellar resident we’ve ever found.
What makes 3I/ATLAS especially intriguing is its odd chemistry and behaviour. Observations using the James Webb Space Telescope (JWST) and the Very Large Telescope (VLT) show that its coma (the surrounding cloud of gas and dust) is dominated by carbon dioxide (CO₂) rather than water (H₂O) — a ratio never before seen in a comet from our own Solar System. Scientists measured a CO₂/H₂O mixing ratio of approximately 8 ± 1 — far higher than typical.
Even more startling: Spectroscopic data reveal the presence of nickel (Ni) emission lines but almost no detectable iron (Fe) lines in the outgassing plume of 3I/ATLAS. Since nickel and iron are usually found together in natural comets (formed in the same supernova processes), this decoupling is highly unusual and has led some researchers to propose exotic chemical pathways — for example, the release of nickel from nickel-carbonyl compounds.
Why this matters: If 3I/ATLAS formed in a star-system very different from our own, its materials may reflect conditions we have never sampled. Its trajectory is hyperbolic (meaning it is not bound to our Sun) and its high speed suggests it has been travelling through interstellar space for perhaps billions of years.
Because it was discovered well before its closest approach to the Sun (its perihelion), astronomers have been able to track it carefully as it entered our inner Solar System. That makes it a rare opportunity to study an object formed elsewhere.
The unusual nickel/iron ratio and CO₂-rich coma do not yet amount to proof of artificial origin, but they do raise fascinating questions. Natural processes can be surprising, especially when dealing with materials from a completely different stellar environment. One hypothesis is that low-temperature organometallic compounds (such as nickel carbonyl) might exist in the dust of this interstellar object and release nickel gas more efficiently when heated or exposed to radiation.
What should we watch for next?
More detailed spectroscopy as 3I/ATLAS moves closer to the Sun (and then away). This may reveal water, methane, ammonia, or other common cometary gases.
Imaging of the nucleus to determine its size and shape more precisely; current estimates of its diameter vary widely (from a few hundred metres to several kilometres) because the coma makes it hard to observe the core.
Monitoring for any changes in its outgassing behavior or tail‐formation — such features can tell us about its structure and history.
In short: 3I/ATLAS isn’t just another comet. It’s a messenger from another star system, bringing with it clues about how things form and evolve far away. Whether it ends up being a textbook natural interstellar comet or something more surprising, one thing is clear: our understanding of the neighbourhood beyond our Solar System just got a lot richer.