Named 3I/ATLAS, this comet is the third-ever interstellar visitor to our solar system to be observed by astronomers. By the end of this month, it’ll pass behind Mars and be blocked from our view, reemerging in December. Somewhere between 5.6 kilometers and 320 meters across, 3I/ATLAS is traveling at 209,000kph. That’s fast enough to go from Earth to the moon in about an hour and a half.
For most astronomers, the chance to observe 3I/ATLAS is an opportunity to better refine our understanding of the objects that pass through our solar system. But for one controversial Harvard physicist, it represents something more: intelligent extraterrestrial life.
This diagram shows the comet’s predicted path, passing by Mars on its way through our solar system. Photo: NASA
An alien emissary?
Avi Loeb, a somewhat notorious Harvard science professor, was quick to jump on the story of 3I/ATLAS. Only days after it was first sighted, he was questioning whether it was a comet or “something else.” Two weeks after finding out about it, he released a preprint (meaning it has not been formally published or peer reviewed) paper. Raising the possibility that the comet was alien in origin, he outlined several key points.
These points included the fact that the orbital plane of 3I/ATLAS was very close to Earth’s, within five degrees, which he felt was an astounding coincidence. Second, the object seemed to be extremely large, over 20 kilometers in diameter, which is vanishingly rare for a comet. There was also the fact that it was going to pass near Venus, Jupiter, and Mars, which he again cited as an unlikely coincidence. Finally, he claimed that there wasn’t good evidence for the cometary gas trail we expect from a comet.
Relying on the “Dark Forest” solution to the famous Fermi Paradox, Loeb has gone on to suggest that the intelligence behind 3I/ATLAS may be malicious and planning on targeting Earth. Its path worked out so that it would pass behind the sun, hiding it from Earth, and at the exact point where it could use the Sun’s gravity to swing back around toward Earth, presumably to destroy us all.
For months, he’s been publishing blog posts on Medium almost daily, outlining his theories as more information comes out about 3I/ATLAS. But other astronomers, and now NASA, have come out with information debunking his theories.
Taken on July 1st 2025 by Chile’s ATLAS telescope, this was our first glimpse of the outsider. Photo: ATLAS/University of Hawaii/NASA
An interesting exercise
Even Loeb himself is unwilling to commit to his theory. In the recent paper, Loeb et al claim to “not necessarily ascribe,” to the theory, but feel it is “an interesting exercise in its own right, and is fun to pursue.”
He also pursued this exercise in 2017, when he claimed that ‘Oumuamua, the first confirmed interstellar object to enter our solar system, was possibly of artificial origin. He tasked radio astronomers with watching for alien signals coming from the object. They did, and there weren’t.
Three years before that, Loeb thought a meteor entering the atmosphere could be the wreckage of an alien spaceship, citing seismic data he gathered from the crash site. Last year, planetary seismologist Benjamin Fernando led a team that found the cause of the odd readings. It was a truck driving by the sensor.
In a follow-up interview with The New York Times, Fernando said the two takeaways were “One, if you want to do seismic analysis, it’s ideal if you check with a seismologist first. The other is, it’s not aliens.”
Point one goes a long way in explaining why Loeb’s “interesting exercises” have received so much skepticism (and outright derision) from his fellow astronomers. Loeb isn’t actually an expert in any of the science he’s doing. Astronomy is a big field, and a man who is (like Loeb) a scholar of galaxy dynamics may not be qualified to speak on, say, comets.
Once respected within his field, Loeb’s papers now mostly remain in preprint, with colleagues like Professor of Astrophysics Steve Desch saying that Loeb is “conflating the good science we do with this ridiculous sensationalism and sucking all the oxygen out of the room.”
Behold! An alien spacecraft. Photo: Wikimedia Commons
If it looks like a comet and quacks like a comet
The more we find out about the comet, the more absurd and hasty Loeb’s claims become. For one thing, it isn’t nearly as large as Loeb claimed. Observations from the Hubble telescope now show that 31/ATLAS is somewhere around, or smaller than, 2.8 kilometers. A far cry from Loeb’s 46 kilometers. In fact, Loeb himself cited the paper which showed it’s 2.8 kilometers, and simply… chose to ignore that.
As on the previous occasions, other astronomers were unimpressed with the science behind Loeb’s bold theory. Steve Desch called the paper “sloppy work beneath the level of an undergraduate” and graded it accordingly.
Sorry it took me so long to grade your August 20 assignment, Avi Loeb. Are you sure you’re ready for a class on comets? You might need to learn about things like bow shocks and chemistry.
— Steve Desch (@deschscoveries.bsky.social) 9 September 2025 at 20:20
The comet does have interesting properties. It comes from outside our solar system and may have a different chemical composition than we’re used to seeing. But as Tom Statler, NASA’s lead scientist for solar system small bodies, told The Guardian, “It looks like a comet…It does comet things. It very, very strongly resembles, in just about every way, the comets that we know. It’s a comet.”
To Loeb’s argument that 3I/ATLAS brightens strangely, Statler agreed — because it’s normal for comets to do that. Even ones from within our solar system can react unpredictably as they grow closer to the radiation of the sun. Comets are mostly ice, and the heat of the sun melts chunks of ice, causing changes in size, brightness, and composition.
It isn’t merely boring alien haters bashing on the visionary who dares to dream the impossible dream. Many of the scientists criticizing Loeb’s work are from institutes like SETI, which is dedicated to the search for extraterrestrial life. They feel that Loeb’s attention-grabbing, unsupported claims discredit the whole field, making it harder to do actual science.
You can follow the comet’s path live on NASA’s website.
