With the help of more than 2 million citizen scientists and the legendary Arecibo Observatory, one of the largest searches for alien information in history is nearing completion.
Launched in 1999, the SETI@Home project enlisted the help of millions of volunteers around the world to identify anomalous radio signals in data from the Arecibo Observatory, a giant radio telescope in Puerto Rico that collapsed in 2020 due to a cable failure. Although the project ended prematurely due to the disappearance of the telescope, citizen scientists still identified over 12 billion interesting signals from 21 years of data.
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As of now, there is no conclusive evidence that aliens were transmitted from these radio sources. However, the research team is keen that its vast dataset could help make future searches for extraterrestrial life even more effective.
“Even if we didn’t find ET, all we can say is that we established a new level of sensitivity. If there was 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 we should have done differently and things we should have done differently in future air survey projects.”
ET joins 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 is that if humans have reached this point technologically, hypothetical alien life forms may also be able to achieve it.
The Arecibo telescope has been a star player in the SETI field. In 1974, a team of scientists including Carl Sagan and Frank Drake transmitted radio transmissions from Arecibo to a nearby star cluster in hopes of reaching an intelligence community audience. The famous “Arecibo Message,” sent 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. (Unfortunately, ET has not yet called home about this.)
One of the big challenges for SETI is that space is full of radio waves. Everything from cold hydrogen molecules to exploding stars emits some form of radio energy. Amid this cosmic noise, it is nearly impossible to meaningfully detect radio signals from intelligent aliens.
To narrow down their search, SETI@Home’s co-founders turned to crowdsourcing. The researchers asked volunteers to download a free software program onto their home computers, and each computer’s processing power was used to analyze Arecibo’s latest scan of the night sky.
Starting in the mid-1990s, the team planned the project with 50,000 volunteers in mind. But within a year of the project’s launch, more than 2 million users in 100 countries were running SETI@Home on their computers.
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“It far exceeded our initial expectations,” Anderson said. “We want to let that community and the world know that we actually did science.”
Expand your search
In two papers published in The Astronomical Journal in 2025, Anderson and his colleagues describe the massive dataset their contributors collected and how their team analyzed it to obtain top candidate signals.
The project focused on radio signals coming from the Milky Way around the 21-centimeter radio wavelength, the wavelength used to map hydrogen gas in the galaxy. Astronomers routinely observe the universe at this frequency. The researchers explained that a hypothetical alien civilization would know about it and use that frequency to increase its chances of being discovered.
Using a supercomputer provided by Germany’s Max Planck Institute for Gravitational Physics, the researchers eliminated billions of false signals and sources of radio interference on Earth, reducing the number of candidates to one million. The team then manually analyzed the 1,000 most promising radio sources and narrowed it down to the top 100 candidates.
So far, there’s nothing unusual about the results.
“We are arguably the most sensitive and narrow-band searcher of a large part of the sky, so we have the best chance of finding something,” astronomer and SETI@Home project director Eric Korpela said in a statement. “Yes, I’m a little disappointed that I couldn’t see anything.”
But what’s possible with today’s calculations is far greater than what was possible in 1999, when the project began, Korpera added. Similar surveys are being conducted at FAST and other radio telescopes around the world. The search for alien intelligence will continue, and data analysis will become faster and more reliable.
“It’s still possible that the data contains ET, but we missed it by a small margin,” Korpela concluded.
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