The comet passing through our solar system is probably older than the Sun

3I/ATLAS formed at a temperature below minus 240C. That single reading is how astronomers worked out roughly where it came from, and it is the closest thing we have to a geology sample from another star system.

How they read it

3I/ATLAS is only the third confirmed interstellar object found in our neighbourhood. A paper in Monthly Notices of the Royal Astronomical Society, led by Dr Lea Ferellec using the William Herschel Telescope's WEAVE instrument, puts the formation temperature below minus 240C.

The evidence is chemical. The team detected ions of dinitrogen, carbon monoxide, carbon dioxide, water and hydrocarbons. The high dinitrogen level is the key reading, because that molecule only survives in regions cold enough to keep it locked in ice. A temperature that low places the comet's birth in the distant icy outskirts of its home system, the equivalent of our own Kuiper Belt or Oort Cloud.

Why the age claim follows

Those chemical fingerprints point to an old star system, which makes 3I/ATLAS plausibly older than the Sun. The reasoning runs through composition rather than direct dating: the mix of ices records the conditions where it formed, and those conditions are characteristic of an older, less metal-rich environment.

These are the first measurements of this kind ever made for a comet formed outside our solar system. Everything we previously knew about the raw material of planet formation came from our own neighbourhood, which is a sample size of one.

What it adds

We have never been able to sample another star system's building blocks before. Missions such as the ones probing exoplanet atmospheres like LHS 1140b work at extraordinary distance and infer composition indirectly. This one arrived on its own and let a ground telescope read its chemistry directly.

For anyone who wants to start following this sort of thing from a back garden, the entry point is usually a decent pair of astronomy binoculars rather than a telescope. Much of what makes the sky interesting, including the wide structures visible in images like the Lion Nebula and the decagon at Saturn's south pole, rewards a wide field first.

What to watch

More interstellar objects. Three confirmed detections in a decade suggests the rate is higher than anyone assumed, and each one carries a chemical record of somewhere we will never visit.

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