Scientists Study 100 Possible Alien Signals as Arecibo’s Historic SETI Search Concludes

Scientists are closely examining 100 possible alien radio signals, marking the final chapter of one of the world's largest searches for extraterrestrial intelligence. The signals come from data gathered by the now-collapsed Arecibo Observatory and analysed through SETI@Home, a global citizen science project launched in 1999. The huge amounts of cosmic data were processed by over two million volunteers in 21 years. Even as Arecibo fell in 2020, the project it enabled lives on to inform searches for intelligent life elsewhere in the universe.

From Billions to 100: Astronomers Zero In on Prime Alien Signal Candidates

According to a 2025 report in The Astronomical Journal, scientists narrowed over 12 billion signal detections to just 100 strong candidates after removing natural sources and Earth-based interference. Source: The Astronomical Journal. These shortlisted signals are now being rechecked using China's FAST telescope, currently the world's most powerful single-dish radio observatory.

SETI@Home listened for radio signals close to the 21-centimetre wavelength, which is frequently chosen to study hydrogen within the Milky Way. Researchers explained that an advanced alien civilisation may prefer this band, as they would know it is widely watched from Earth and would be easier to detect across interstellar distances.

Millions of Home Computers Power SETI@Home, Redefining the Search for Alien Signals

To manage the massive load of data, volunteers invited a small program to run on home computers and share with the project their unused processing power. What sounded like a small project soon expanded to users in more than 100 countries, far exceeding original expectations.

So far, no signal shows clear signs of an intelligent origin. Still, scientists said the project set new sensitivity limits and proved what kinds of strong alien signals do not exist nearby, helping guide smarter and more focused searches ahead.