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.
Author
- Darryl Z. Seligman
Assistant Professor, Michigan State University
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 as the eccentricity increases, it becomes what's known as an ellipse - a stretched out circle.
And then once you get past an eccentricity of one, you go from an ellipse to a hyperbolic orbit , and that is unbound. So while an elliptical orbit is stretched out, it still orbits and comes back around. An object with a hyperbolic orbit comes through and it leaves, but it never comes back. That type of orbit tells you that it didn't come from this solar system.
When researchers are collecting data, they're getting points of light on the sky, and they don't know how far away they are. It's not like they see them and can just tell, "oh, that's eccentric." What they're seeing is how far away the object is compared with other stars in the background, what its position is and how fast it's moving. And then from that data, they try to fit the orbit.
This object is moving fast for how far away it is , and that's what's telling us that it could be hyperbolic. If something is moving fast enough, it'll escape from the solar system. So a hyperbolic, unbound object inherently has to be moving faster.
This is a real-time process. My collaborators have preexisting software, which will, every night, get new observations of all the small bodies and objects in the solar system. It will figure out and update what the orbits are in real time. We're getting data points , and with more data we can refine which orbit fits the points best.
What can scientists learn from an interstellar object?
Objects like this are pristine, primordial remnants from the planet formation process in other planetary systems. The small bodies in our solar system have taught us quite a lot about how the planets in the solar system formed and evolved. This could be a new window into understanding planet formation throughout the galaxy.
As we're looking through the incoming data, we're trying to figure out whether it's a comet. In the next couple of weeks, there will likely be way more information available to say if it has a cometary tail like Borisov, or if it has an acceleration that's not due to a gravitational pull , like 'Oumuamua.
If it is a comet, researchers really want to figure out whether it's icy. If it contains ices, that tells you a ton about it. The chemistry of these small bodies is the most important aspect when it comes to understanding planet formation , because the chemical composition tells you about the conditions the object's solar system was in when the object formed.
For example, if the object has a lot of ices in it, you would know that wherever it came from, it didn't spend much time near a star, because those ices would have melted. If it has a lot of ice in it, that could tell you that it formed really far away from a star and then got ejected by something massive, such as a planet the size of Jupiter or Neptune.
Fundamentally, this object could tell astronomers more about a population of objects that we don't fully understand, or about the conditions in another solar system.
We've had a couple of hours to get some preliminary observations. I suspect that practically every telescope is going to be looking at this object for the next couple of nights, so we'll get much more information about it very soon.
Darryl Z. Seligman is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-2303553. This research award is partially funded by a generous gift of Charles Simonyi to the NSF Division of Astronomical Sciences. The award is made in recognition of significant contributions to Rubin Observatory's Legacy Survey of Space and Time.
