Astronomers manning an asteroid warning system caught a glimpse of a large, bright object zipping through the solar system late on July 1, 2025. The object’s potentially interstellar origins excited scientists across the globe, and the next morning, the European Space Agency confirmed that this object, first named A11pl3Z and then designated 3I/ATLAS, is the third ever found from outside our solar system.

Current measurements estimate that 3I/ATLAS is about 12 miles (20 kilometers) wide, and while its path won’t take it close to Earth, it could hold clues about the nature of a previous interstellar object and about planet formation in solar systems beyond ours.

On July 2 at 3 p.m. EDT, Mary Magnuson, an associate science editor at The Conversation U.S., spoke to Darryl Z. Seligman, an astrophysicist at Michigan State University who has been studying 3I/ATLAS since its discovery.

What makes 3I/ATLAS different from its predecessors?

We have discovered two interstellar objects so far, ’Oumuamua and Comet 2I/Borisov. ’Oumuamua had no dust tail and a significant nongravitational acceleration, which led to a wide variety of hypotheses regarding its origin.

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