Weirdest interstellar comets to visit our solar system and the alien theories they fuelled

Our solar system is filled with strange objects, but those visiting from interstellar space have exhibited behaviors that have caused even the most brilliant minds to scratch their heads. As of today, there are three known interstellar objects to visit our solar system, namely ‘Oumuamua, comet 2I/Borisov, and comet 3I/ATLAS. Apart from sparking curiosity, they’ve also triggered controversy in the scientific community due to bold statements from experts like Harvard physicist Avi Loeb, who stated that ‘Oumuamua and 3I/ATLAS might actually be alien spacecraft. While Loeb’s claims were not proven, the latter did have characteristics that were quite uncommon in comets of our solar system.

Image showing Comet 2I/Borisov (left), 'Oumuamua (middle), and Comet 3I/ATLAS (right).

The mystery object ‘Oumuamua was discovered on October 19, 2017, by the University of Hawaii’s Pan-STARRS1 telescope as the first confirmed object from another star to visit our solar system. It was a rocky, cigar-shaped object which was perhaps 10 times as long as it was wide—something that NASA said is unlike objects seen in our solar system. Interestingly, scientists don’t really know what it looks like, as it was photographed as just a speck of light, and its shape was determined by studying its unique rotation. Another interesting finding during the observation campaign was that ‘Oumuamua experienced an unexpected non-gravitational boost that altered its trajectory as it zoomed past the Sun at 196,000 miles per hour.

What’s weird is that Oumuamua has been categorized as a comet, but it lacked a visible tail or a coma because it had no ice or water on its surface. Scientists had also confirmed that it was reddish in color due to the effects of irradiation from cosmic rays.

This comet is the second confirmed interstellar object in our solar system. It is also the first confirmed comet and the least controversial one. On August 30, 2019, this comet was discovered barreling into our solar system at about 110,000 miles per hour, and it had a lot of dust around a bright nucleus. It was estimated to be about 3,200 feet (975 meters) across, and observations by NASA’s Hubble telescope revealed that its nucleus was a loose cluster of ices and dust particles. In 2020, scientists noticed that Borisov’s appearance had changed.

Pictures from Hubble indicated that the comet’s nucleus had fragmented as it moved through the solar system. As more data poured in on the comet, astronomers were puzzled by Borisov’s activity and how different it was from ‘Oumuamua. And since it formed in a different star system, they think it could be older than the Sun—which formed over 4.6 billion years ago.

This comet gained notoriety for its unusual characteristics that were not seen in our solar system comets. Discovered by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope on July 1, 2025, this comet initially lacked a coma and had an unusual abundance of metals like nickel, which fueled theories of its extraterrestrial origin. NASA, however, explained that such variations from known characteristics are expected because 3I/ATLAS may have formed in a different environment than ours. It was also found that the relative water, carbon dioxide, and carbon monoxide production rates on 3I/ATLAS were different from typical comets.

NASA had launched an extensive observation campaign to study 3I/ATLAS and photographed it using a large number of spacecraft and telescopes.

In a 2021 paper, Loeb had stated that ‘Oumuamua had “half a dozen anomalies relative to comets or asteroids in the Solar system” and insisted that “an artificial origin for 'Oumuamua must be considered.” He mentioned facts like the lack of a cometary tail as an anomaly, along with its initial velocity, which he wrote “was unusual for a natural object that originated from a nearby star.” Loeb also wrote that ‘Oumuamua exhibited an excess push away from the Sun which would have required it to lose approximately 10 percent of its mass like other natural comets, but any such cometary activity was ruled out by NASA’s Spitzer telescope. Sharing more explanations on the said anomalies, Loeb said that ‘Oumuamua might be artificial in origin, possibly a light sail built by aliens.

Then came 3I/ATLAS, which drew more attention toward Loeb and his alien theories. He published multiple papers and articles explaining the anomalies that separated 3I/ATLAS from other comets and how its unusual brightness, lack of coma, sunward tail, and nickel/iron ratio call for investigation while considering its possible extraterrestrial origin. He had even stated in an article that if 3I/ATLAS did not fragment during its closest pass to the Sun (perihelion), it would be a major indication that 3I/ATLAS may not be a natural comet. The comet reached perihelion in late October 2025, and while there was no confirmation of its fragmentation, its extraterrestrial origin hasn’t been proven either.