This SETI program is chasing down its final 100 signals. Could one of them be from aliens?

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Astronomers are using China’s powerful FAST radio telescope to track 100 intriguing signals detected by the SETI@home project, managed by SETI (Search for Extraterrestrial Intelligence). scientists.

SETI@home, which ran from 1999 to 2020, saw millions of users around the world donate their CPU time to downloadable software that leveraged data collected by the Arecibo radio telescope in Puerto Rico. In the end, 12 billion candidate narrow-band signals were identified. These signals appeared as “momentary flashes of energy at a particular frequency coming from a particular point in the sky,” David Anderson, a computer scientist at the University of California, Berkeley and co-founder of the SETI@home project, said in a statement. statement.

FAST, the Five Hundred Meter Aperture Spherical Telescope, has been patiently tracking this century of candidate extraterrestrial signals since July 2025. Although observations and analyzes are still ongoing, bitter experience has taught the SETI@home team to expect that they will all turn out to be local radio frequency interference (RFI) rather than true extraterrestrial beacons.

But whatever their origin, they represent the culmination of one of the largest citizen science projects ever undertaken. It took years to figure out how to properly examine this vast amount of data.

“Until about 2016, we didn’t really know what we were going to do with these detections that we had accumulated,” Anderson said. “We didn’t understand how to do the whole second part of the analysis.”

“It’s impossible to do a complete investigation of every possible signal you detect, because it always requires a person and eyeballs,” added Berkeley astronomer Eric Korpela, co-founder of SETI@home with Anderson and Dan Werthimer, an astronomer and electrical engineer also at Berkeley.

Eventually, at the supercomputer facilities at the Max Planck Institute for Gravitational Physics in Germany, algorithms designed to spot RFI sorted the wheat from the chaff, reducing those 12 billion to 1 million, then to 1,000. These 1,000 signals then had to be inspected manually, by eye, before being narrowed down to 100 that merited a second look.

Arecibo was the world’s largest single dish radio telescope, with an aperture of 305 meters, until the arrival of FAST in 2016. Because Arecibo collapsed and was destroyed in December 2020, FAST is now the only radio telescope capable of picking up these candidate signals.

“If we don’t find ET, what we can say is that we have established a new level of sensitivity. If there was a signal above a certain power, we would have found it,” Anderson said.

The scale of the project far exceeded the dreams of Anderson or anyone on his team when SETI@home started in 1999. They thought they could attract 50,000 users if they were lucky. By the end of the first week, they had 200,000 users, and within a year, they had 2 million.

“I would say it performed well beyond our initial expectations,” Anderson said.

The SETI@home data came from Arecibo’s regular astronomical observations and covered billions and billions of planets. stars in the Milky Way.

“We are arguably doing the most sensitive narrow-band search over large parts of the sky, so we had the best chance of finding something,” Korpela said. “So yeah, it’s a little disappointing that we didn’t see anything.”

As the mammoth project nears completion, assuming no actual extraterrestrial signals appear among the final 100 candidates, Korpela views the project not only with pride, but as a learning experience for future SETI investigations.

“We need to do a better job of measuring what we exclude,” he said. “Are we throwing the baby out with the bathwater? I don’t think we know that for most SETI research and it’s really a lesson for SETI research everywhere. In a world where I had money, I would reanalyze it in the right way, which means I would correct the mistakes we made. And we made mistakes. Those were conscious choices because of the speed of computers in 1999.”

Indeed, Korpela wonders if one day a new project could be launched in the same vein as SETI@home to look at all the data again, but with modern crowdsourced computing power and machine learning looking for anything that was missed the first time.

“It’s still possible that ET is present in this data and we just missed it.”

The overall results of SETI@home presented in two articles in 2025 in The Astronomical Journal: an article on data analysis and resultsand another on data acquisition and processing.