Our solar system has a new mysterious visitor—what is it?

On July 1, scientists operating a planetary defense facility saw something glimmering in the shadow of Jupiter, about 420 million miles from Earth. It certainly wasn’t a near-Earth asteroid, nor was it a regular comet. Within a matter of hours, it became clear that this voyager wasn’t even from our solar system: Its orbit was too steep, its speed was too great. Astronomers quickly concluded that this was an interstellar object—a sojourner from another star.

These entities are, for the moment, vanishingly rare. This interstellar object is just the third ever confirmed. Serendipitously, scientists caught as it was making its way into the solar system, which means they will get to study it for several months and uncover its many secrets.

“Astronomers around the world will be pointing their telescopes, large and small, at this object… providing us with clues about its home planetary system,” says Sarah Greenstreet, an astronomer at the University of Washington.

Here's everything we know so far about our solar system’s latest interstellar visitor—and how scientists plan on resolving its mysteries in the coming weeks and months.

This image shows the observation of comet 3I/ATLAS when it was discovered on July 1, 2025. The NASA-funded ATLAS survey telescope in Chile first reported that the comet originated from interstellar space. Photograph by ATLAS/University of Hawaii/NASAThis animation shows the observations of comet 3I/ATLAS when it was discovered on July 1, 2025. The NASA-funded ATLAS survey telescope in Chile first reported that the comet originated from interstellar space. Animation by ATLAS/University of Hawaii/NASA

And then there were three

Like asteroids and comets, interstellar objects are thought to be remnants from a time when planets were first forming or the ruins of wrecked worlds and moons. But instead of originating around our sun, they hail from another planetary system entirely. As such, finding them is of paramount importance to astronomers.

“The more we can learn about other planetary systems from these interstellarvisitors, the better we can understand how similar or different our own solar system is to the vast number of other planetary systems that stretch across the Milky Way,” says Greenstreet.

(Did a 2014 meteor come from interstellar space?)

Only two have been discovered so far. The first, ‘Oumuamua, was spotted in 2017, and it was considerably strange: a pancake or cigar-shaped object that behaved like a comet but didn’t display any clear signs of cometary activity. Because it was detected as it was already leaving the solar system, astronomers didn’t get much time to study it. Theories about its origin run the gamut from plausible (a piece of a dead planet or a particularly odd comet) to the considerably less plausible (an alien spaceship).

Then in 2019, astronomers spied a second object called 2I/Borisov, which looked and acted a lot more like a comet. Astronomers managed to scope it out as it was making its way into the solar system, and they got to examine it more closely.

A sample size of two makes it difficult to know what interstellar objects are like in general. Astronomers hope that number will increase with the new Vera C. Rubin Observatory’s comprehensive 10-year survey of the night sky launching later this year. But for now, this third interstellar object is a nice surprise.

The new visitor was detected by the Asteroid Terrestrial-impact Last Alert System (ATLAS) facility, a network of four autonomous NASA-funded telescopes designed to spot potentially hazardous near-Earth asteroids. ATLAS’s station in Chile spotted the interloper during a routine survey.

Initially, “we did not suspect anything unusual about its orbit,” says Larry Denneau, one of the principal investigators of ATLAS at the University of Hawai‘i. But that quickly changed when astronomers at other telescopes glimpsed the object, and scientists plotted out its orbit with more precision.

(Asteroids with “unstable orbits” hide around Venus—do they threaten Earth?)

Asteroids and comets can have all sorts of orbits, but they all still circumnavigate the Sun. This object, though, was moving too rapidly and in a very elongated manner, which indicated it “is not bound to the Sun's gravity and will never pass through our solar system again,” says Greenstreet.

With everyone in agreement, the object was confirmed and officially given a name: 3I/ATLAS. So, what’s it like?

The knowns and unknowns of 3I/ATLAS

A new paper uploaded to the pre-print server arXiv summarizes what scientists know about the interstellar object so far. It clearly has a coma—a hazy bubble of gas created when ice turns into vapor as it’s warmed by sunlight. “Compared to the first two interstellar objects we've discovered, 3I/ATLAS is cometary like 2I/Borisov,” says Greenstreet, meaning, for now, ‘Oumuamua remains the weirdo of the group.

There are also hints that the object is somewhat red—a tell-tale sign that it’s made of primeval matter that coalesced during the early days of its planetary system. So it’s probably an extremely old geologic time capsule.

“At the moment, it's difficult to say how big it is,” says Greenstreet. Based on how much sunlight it’s reflecting, it could be anywhere from 6 to 15 miles long. That’s far bigger than ‘Oumuamua (about one-quarter mile long) and 2I/Borisov (two-thirds of a mile long). But its coma and current distance obscure the true size of its likely smaller solid core.

Not too much else is known about 3I/ATLAS just yet, but its trajectory through the solar system works in astronomers’ favor. “It'll be moving inwards for a few more months before it starts heading back out again,” says Aster Taylor, a graduate student of astrophysics at the University of Michigan. “it’s moving fast… [but] we’ll have time to observe it.” Observatories all around the world, and even those in space, will spend time tracking it.

As it dives toward the sun, 3I/ATLAS will also come within 18 million miles of Mars in early October. Several spacecraft are currently orbiting the Red Planet, and they may be able to use their cameras to study 3I/ATLAS’s chemistry and other features as it rushes by.

Astronomers expect to see plenty more cometary activity. Comets are often made of various types of ice, including frozen carbon monoxide, carbon dioxide, and water. In the next few weeks, 3I/ATLAS will get close enough to the sun for its water ice to start vaporizing—expanding its coma and lengthening its luminescent tail.

“Be on the lookout for images in the coming months that show off its beautiful cometary features as it makes its relatively shortjourney through our corner of the galaxy,” says Greenstreet.