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Comets are generally thought to originate in our Solar System, made up of the leftover gas and rocks thrown out as the planets formed. The recent arrival of two interstellar objects—a rock named ‘Oumuamua and a flashy comet called Borisov—have challenged that assumption.

Tom Hands, an astrophysicist at the University of Zurich’s Institute for Computational Science and his co-author Walter Dehnen at the University of Leicester used mathematical models to estimate just how many long-period comets—those that take 200 years or longer to circle the Sun—could be interstellar visitors. Their research was published last month in the Monthly Notices of the Royal Astronomical Society. Science talked to Hands to find out more about these mysterious icy visitors. This interview has been edited for clarity and length.

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