Largest crowd-sourced hunt for alien intelligence reveals 12 billion 'signals of interest' in collapsed Arecibo Observatory data

One of the largest searches for alien intelligence in history is nearing completion, thanks to the help of more than 2 million citizen scientists and the legendary Arecibo Observatory.

Launched in 1999, the SETI@Home project enlisted millions of volunteers around the world to help identify unusual radio signals in data from the Arecibo Observatory — a massive radio telescope in Puerto Rico that collapsed in 2020 due to a cable failure. Though the project ended prematurely with the telescope's demise, citizen scientists nonetheless identified more than 12 billion signals of interest in 21 years of data.

Now, the researchers behind the project have narrowed that daunting list down to the top 100 candidate signals, which they are studying in detail with China's Five-hundred-meter Aperture Spherical Telescope (FAST) — now the world's largest single-dish radio telescope, following the death of Arecibo.

So far, there is no smoking-gun evidence of alien transmissions from any of these radio sources. However, the team is enthusiastic that their vast dataset will help make future hunts for extraterrestrials even more effective.

"If we don't find ET, what we can say is that we established a new sensitivity level. If there were a signal above a certain power, we would have found it," computer scientist and project co-founder David Anderson said in a statement. "We have a long list of things that we would have done differently and that future sky survey projects should do differently."

ET enters the group chat

The search for extraterrestrial intelligence (SETI) is a branch of science that aims to detect and communicate with advanced alien civilizations using radio signals — the idea being that, if humans have made it this far technologically, hypothetical alien lifeforms might have too.

The Arecibo telescope was a star player in the SETI field; in 1974, a team of scientists including Carl Sagan and Frank Drake sent a radio transmission from Arecibo to a nearby star cluster in hopes of reaching an intelligence audience. The famous "Arecibo Message," transmitted in binary code, included a human stick figure, a double-helix DNA structure, a model of a carbon atom and a diagram of a telescope. (Sadly, E.T. has yet to phone home about it.)

One big challenge for SETI is that space is overflowing with radio waves; everything from cold hydrogen molecules to exploding stars emits some form of radio energy. Finding a meaningful detection of radio signals from intelligent aliens among all this cosmic noise borders on the impossible.

To help narrow the search, the co-founders of SETI@Home turned to crowd sourcing. The team asked volunteers to download a free software program to their home computers, borrowing each computer's processing power to analyze Arecibo's latest scans of the night sky.

Starting in the mid-1990s, the team planned their project with 50,000 volunteers in mind. But within a year of the project starting, more than 2 million users in 100 countries were running SETI@Home on their computers.

"It went way, way, way beyond our initial expectations," Anderson said. "I would like to let that community and the world know that we actually did some science."

Expanding the search

In two papers published in 2025 in The Astronomical Journal, Anderson and his colleagues describe the vast dataset their contributors collected, and how the team analyzed it for the top candidate signals.

The project focused on radio signals coming from the Milky Way near the radio wavelength of 21 centimeters, which is the wavelength used to map hydrogen gas in the galaxy. Astronomers routinely observe the universe at this frequency; a hypothetical alien civilization would know that, and make use of that frequency to boost their chances of being detected, the researchers explained.

Using a supercomputer provided by the Max Planck Institute for Gravitational Physics in Germany, the team eliminated billions of false signals and Earth-based sources of radio interference, dropping the candidate pool down to a million. The team then analyzed the most promising 1,000 radio sources manually, whittling them down to the top 100 contenders.

So far, nothing unusual has jumped out of the results.

"We are, without doubt, the most sensitive narrow-band search of large portions of the sky, so we had the best chance of finding something," astronomer and SETI@Home project director Eric Korpela said in the statement. "So yeah, there's a little disappointment that we didn't see anything."

However, what is computationally possible today far outpaces what was possible in 1999, when the project began, Korpela added. Similar surveys are being conducted by FAST and other radio telescopes around the world; the hunt for alien intelligence will continue, and the data analysis will only get faster and more reliable going forward.

"There's still the potential that ET is in that data and we missed it just by a hair," Korpela concluded.