Sorry, true believers: It appears the interstellar comet 3I/ATLAS is indeed just a comet.
Radio astronomers recently hunted for "technosignatures" coming from 3I/ATLAS, which is just the third confirmed interstellar object ever seen in our solar system. As expected, that search came up empty — but it still gathered valuable data, the scientists said.
"The results from 3I/ATLAS show how realistic it is to detect a signal with the technology we have today," study co-author Valeria Garcia Lopez, of Furman University in South Carolina, said in a statement Wednesday (June 3). "That is why it is important to keep searching for technosignatures, even from objects we might not expect to have signals."
3I/ATLAS was first seen on July 1, 2025 by the Deep Random Survey remote telescope in Chile, which is part of the ATLAS (Asteroid Terrestrial-impact Last Alert System) project.
That explains "ATLAS" in the object's name. "3I" indicates that it's the third known interstellar interloper, after 1I/'Oumuamua and 2I/Borisov, which were discovered zooming through our neck of the cosmic woods in 2017 and 2019, respectively.
Telescope observations strongly suggest all three of these visitors are natural objects — namely, comets that were born in other solar systems and subsequently booted into interstellar space, likely via gravitational interactions.
However, it's still worth studying such bodies in detail, according to lead author Sofia Sheikh, who's based at the SETI (Search for Extraterrestrial Intelligence) Institute in California.
"Eventually, our own Voyager spacecraft will be extraterrestrial artifacts in other stellar systems," Sheikh said in the same statement. "Given that, it is important that we understand the natural distribution of interstellar objects so that we will be able to identify any anomalies that could one day be signs of an artificial interstellar object."
There's also the possibility, however remote, that one or more of these interstellar objects could actually be the product of alien intelligence. Indeed, that hypothesis has been advanced to explain 'Oumuamua's perceived weirdness.
So Sheikh and her team studied 3I/ATLAS using the SETI Institute's Allen Telescope Array, a network of radio dishes at the Hat Creek Radio Observatory in Northern California, not long after the comet's discovery. They collected more than seven hours of observations, looking for "narrowband" radio signals — emissions that aren't produced by any known natural phenomenon.
The researchers identified nearly 74 million such signals, which they then culled to about 200 after eliminating human interference and filtering out artifacts caused by 3I/ATLAS' movement. They were then able to explain away those final 200 as well, as signals generated here on Earth or by orbiting satellites.
The new study, which was published online Wednesday in The Astronomical Journal, "sets new constraints reinforcing that 3I/ATLAS is a natural object," SETI Institute officials wrote in the same statement.
"The observations place upper limits on the power of any radio transmitter on or near 3I/ATLAS, ruling out signals stronger than about 10-110 watts, approximately the power of a household appliance, over the detected frequencies," they added.
