Last summer, an object from beyond our solar system drifted close enough to Earth for scientists to listen for signals from an alien civilization. For seven hours, researchers at the SETI Institute aimed the Allen Telescope Array at Hat Creek Radio Observatory in Northern California toward 3I/ATLAS, the third interstellar visitor ever detected, scanning frequencies from 1 to 9 gigahertz. They found no technosignatures—no hints of extraterrestrial radio broadcasts, no signs of alien technology. What they did find, however, reveals something equally important about how we'll recognize an artificial object if one ever truly arrives.
Discovered in July 2025, 3I/ATLAS joins a rare club. Only two other objects from beyond our star system have entered our solar system: 1I/'Oumuamua in 2017 and 2I/Borisov in 2019. Each represents a precious window into how planetary systems form and evolve across the galaxy. Yet they also present something more tantalizing—the possibility, however remote, of stumbling upon evidence of life elsewhere in the universe.
This is why Dr. Sofia Sheikh and her team acted so quickly. Less than a day after 3I/ATLAS was announced, the ATA began its observations. The speed matters. If an artificial object ever reaches our solar system, we need to know what it might look like—or sound like—against the background of natural space debris. As Sheikh explains in the paper published in The Astronomical Journal, "Eventually, our own Voyager spacecraft will be extraterrestrial artifacts in other stellar systems. 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."
The team's search was exhaustive. Across their seven-hour observation, nearly 74 million narrowband signals appeared in the data—the kind of focused radio emissions that don't occur naturally in space. After removing human interference and filtering for signals that matched 3I/ATLAS's movement through our sky, only about 200 remained worthy of closer inspection. Every single one traced back to Earth: either human technology on the ground or our own satellites orbiting overhead.
The results set new boundaries on what might be hiding near 3I/ATLAS. Any radio transmitter on or near the object would have to operate at less than 10 to 110 watts—roughly equivalent to a household appliance—to remain undetected by the ATA's sensitive equipment. That constraint alone rules out many possible scenarios, strengthening the case that 3I/ATLAS is simply what it appears to be: a comet-like natural body drifting through space.
Yet the team's work carries a forward-looking message. Valeria Garcia Lopez, co-author of the study, notes that "the results from 3I/ATLAS show how realistic it is to detect a signal with the technology we have today. That is why it is important to keep searching for technosignatures, even from objects we might not expect to have signals." As more interstellar objects are discovered—and telescopes grow sharper—each new arrival becomes another opportunity to listen. The Allen Telescope Array has proven itself ready to answer that call.