Q&A WITH AN INTERSTELLAR OBJECT INVESTIGATOR

A seven-billion-year-old object has just entered our Solar System. We ask what comet 3I/ATLAS is doing here - and what it reveals about our Galaxy's past

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What is 3I/ATLAS and what makes it so interesting?

3I/ATLAS is an interstellar object that was discovered passing through the Solar System by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey. During the survey, we built a simulation: the Õtautahi-Oxford interstellar object population model. Among other things, this can tell us about the age of transiting bodies like 3I/ATLAS. I noticed that 3I/ATLAS was bobbing up and down on its orbit around the Milky Way, which was a lightbulb moment for me, because I knew that older stars also followed a similar pattern in their orbits. When we ran all the statistics and looked at the correlations, we found that 3I/ATLAS was probably over seven billion years old, about twice the age of our Solar System.

Why have we only discovered three of these interstellar interlopers?