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. 2I/Borisov was very clearly a comet, though it has a somewhat unique composition compared to comets in our solar system.
All of our preparation for the next interstellar object was preparing for something that looked like a ’Oumuamua, or something that looked like Borisov. And this thing doesn’t look like either of them, which is crazy and exciting.
This object is shockingly bright, and it’s very far away from the Earth. It is significantly bigger than both of the interstellar objects we’ve seen – it is orders of magnitude larger than ’Oumuamua.
For some context, ’Oumuamua was discovered when it was very close to the Earth, but this new object is so large and bright that our telescopes can see it, even though it is still much farther away. This means observatories and telescopes will be able to observe it for much longer than we could for the two previous objects.
It’s huge and it’s much farther away, but it is also much faster.
When I went to bed last night, I saw an alert about this object, but nobody knew what was going on yet. I have a few collaborators who figure out the orbits of things in the solar system, and I expected to wake up to them saying something like “yeah, this isn’t actually interstellar.” Because a lot of times you think you may have found something interesting, but as more data comes in, it becomes less interesting.
Then, when I woke up at 1 a.m., my colleagues who are experts on orbits were saying things like “no, this is definitely interstellar. This is for real.”
How can astronomers tell if something is an interstellar object?
The eccentricity of the object’s orbit is how you know that it’s interstellar. The eccentricity refers to how noncircular an orbit is. So an eccentricity of zero is a pure circle, and…
