3 min read
There is a lump of ice and rock older than the Sun currently barrelling through our solar system, and it did not come from around here. Its name is 3I/ATLAS, it is only the third interstellar visitor humanity has ever caught in the act, and the more astronomers study it, the older and stranger it turns out to be.
What “interstellar” actually means
Almost everything in the night sky, every planet, comet and asteroid, was born alongside our Sun from the same cloud of gas and belongs to our solar system. An interstellar object is different: it formed around another star entirely, got flung out into the galaxy, and is merely passing through ours on a one-way trip. We have only ever confirmed three: 1I/’Oumuamua in 2017, 2I/Borisov in 2019, and now 3I/ATLAS, first spotted in July 2025 by a survey telescope in Chile.
Older than the Sun
Here is the part that stops you dead. By measuring the chemical fingerprints in the gas streaming off the comet, astronomers using some of the world’s best instruments have concluded that 3I/ATLAS is probably older than our own Sun, which itself is about 4.6 billion years old. The clue is in its carbon. The ratio of carbon-12 to carbon-13 is unusually high, and because carbon-13 slowly builds up across the galaxy over cosmic time, a shortage of it points to an object that formed very long ago, in the outskirts of an ancient star system, before that heavier carbon had a chance to accumulate. In other words, this thing predates the ground you are standing on by a comfortable margin.
The other clues
The James Webb Space Telescope has found ratios of carbon and heavy hydrogen in the comet unlike anything in our own back garden of comets. Hubble has imaged a teardrop-shaped cocoon of dust peeling off its solid, icy nucleus. And in a lovely twist, astronomers digging back through images from the new Vera Rubin Observatory found 3I/ATLAS lurking in the data before it was officially discovered, a cosmic gatecrasher caught in old photographs after the fact.
And no, it is probably not aliens
Every time an interstellar object turns up, someone suggests it might be an alien spacecraft, a tradition that dates back to the weird, cigar-shaped ‘Oumuamua. It is a fun idea, it makes for great headlines, and the evidence here points firmly and boringly to “extremely old natural comet”. The truth is arguably better anyway. We do not need it to be a spaceship for it to be astonishing. It is a physical object from another star, forged before the Sun existed, close enough for our telescopes to read its chemistry. That is not a let-down. That is one of the more remarkable things a person can be told on a Tuesday.
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Did you know: the naming is refreshingly literal. The “3I” in 3I/ATLAS simply means the third confirmed interstellar object, with “I” for interstellar, and “ATLAS” credits the survey that found it. Its two predecessors, ‘Oumuamua and Borisov, are still the only other interstellar visitors ever logged, which makes catching a third, and an ancient one at that, a properly rare event in the history of astronomy.