Enthusiasts Used Their Home Computers to Search for ET as Scientists Narrow the Hunt to 100 Intriguing Signals
For more than two decades, one of the most ambitious citizen-science experiments ever attempted quietly ran in the background of millions of home computers around the world. From 1999 to 2020, volunteers downloaded a simple program, let it run during idle time, and unknowingly helped process one of the largest scientific datasets in astronomy. That project, known as SETI@home, was designed to search for signs of extraterrestrial intelligence by analyzing radio signals from space.
Now, years after the program stopped distributing tasks to volunteers, scientists have completed a massive final analysis. Out of 12 billion signal detections, researchers have narrowed the list down to about 100 signals that are interesting enough to deserve another look using one of the world’s most powerful radio telescopes.
What SETI@home Was Really Doing
SETI@home stands for Search for Extraterrestrial Intelligence at Home, and it was launched by researchers at UC Berkeley as an experiment in both astronomy and distributed computing. At the time, supercomputers were expensive and scarce. The idea was simple but radical: instead of relying on a single powerful machine, why not split the work into tiny pieces and send them to millions of personal computers across the internet?
Participants downloaded SETI@home software that analyzed radio data collected by the Arecibo Observatory in Puerto Rico. The program looked for narrow-band radio signals, which scientists believe would be the most obvious type of artificial signal produced by a technologically advanced civilization.
Within days of its launch in 1999, the project had hundreds of thousands of users. Within a year, that number grew to millions of volunteers from over 100 countries, making SETI@home one of the earliest and most successful crowd-sourced science projects on the internet.
The Source of the Signals: Arecibo Observatory
The radio data analyzed by SETI@home came from the 300-meter Arecibo radio telescope, once the largest single-dish radio telescope in the world. Rather than dedicating telescope time exclusively to SETI, the project used a technique called commensal observing. This meant SETI@home passively recorded data while Arecibo was being used for other astronomical studies.
Over the life of the project, this approach turned out to be remarkably effective. Roughly one-third of the sky, specifically the portion visible from Puerto Rico, was observed at least a dozen times, and some regions were observed hundreds or even thousands of times. This repeated coverage made it possible to compare signals across long periods and identify patterns that might stand out from random noise.
From Billions of Blips to a Manageable List
Each volunteer computer analyzed small chunks of data, searching for momentary spikes of energy at specific frequencies and sky locations. These detections were not signals from aliens by default. Most were expected to be natural astrophysical sources, random noise, or radio frequency interference caused by human technology.
By the end of the distributed computing phase, SETI@home had logged an astonishing 12 billion detections. For years, the team did not yet have a clear plan for how to thoroughly analyze and filter this enormous dataset.
That changed after 2016, when researchers began a focused effort to clean and organize the detections. Using a powerful supercomputing cluster provided by the Max Planck Institute for Gravitational Physics in Germany, they eliminated obvious interference and grouped detections that appeared to come from the same sky position and frequency.
This process reduced the dataset from billions of detections to about one million candidate signals, and eventually to roughly 100 top candidates that stood out statistically from the rest.
Testing the Search Itself
One of the most important insights from SETI@home had less to do with aliens and more to do with how searches are conducted. To test whether their filtering methods were accidentally discarding real signals, researchers inserted around 3,000 artificial test signals, known as birdies, into the data pipeline.
The team did not know which signals were fake during analysis. By seeing which birdies survived the filtering process, they were able to measure the true sensitivity of the search and identify weaknesses in their algorithms. This approach revealed that many SETI searches may not fully understand what kinds of signals they are unintentionally excluding.
Why 100 Signals Still Matter
The final list of around 100 signals does not mean scientists believe these are messages from extraterrestrials. In fact, the researchers involved say they do not expect any of them to turn out to be alien in origin. Still, these signals are statistically interesting enough that they deserve further observation.
Since July 2025, astronomers have been using China’s Five-hundred-meter Aperture Spherical Telescope (FAST) to re-observe these targets. FAST has about eight times the collecting area of Arecibo, making it far more sensitive. Each candidate region of the sky is observed for roughly 15 minutes, looking to see whether the signal appears again.
A genuine extraterrestrial signal would likely repeat or show consistent behavior over time. So far, the FAST data are still being analyzed.
How SETI Searches for Technosignatures
Most modern SETI efforts focus on technosignatures, observable signs of advanced technology rather than biological life. Radio signals are a prime target because they travel long distances through space and are relatively easy to detect.
Scientists often assume that an advanced civilization would transmit a strong, narrow-band beacon, possibly near the 21-centimeter hydrogen line, a frequency commonly used by astronomers. This frequency would be a logical choice because any civilization studying the galaxy would likely recognize its significance.
If such a beacon were ever detected, it would trigger an immediate global response, with radio and optical telescopes worldwide focusing on the same region of the sky.
Lessons for Future SETI Projects
Even without finding extraterrestrial intelligence, SETI@home is widely regarded as a success. It demonstrated that citizen science can produce real, publishable results and that distributed computing can rival traditional supercomputers for certain tasks.
The project also exposed limitations. Filtering billions of signals without losing something important is extremely difficult, and many early design decisions were shaped by the slower computers and internet connections of the late 1990s.
Today, with faster hardware and broadband internet, researchers believe a modern version of SETI@home could analyze far larger datasets more efficiently. Platforms like BOINC, which still supports projects such as Einstein@home and LHC@home, make this technically feasible.
The biggest challenge is not technology but funding, as large-scale SETI projects require sustained staffing and long-term support.
The Broader Impact of SETI@home
SETI@home helped popularize public participation in science and inspired countless people to engage with astronomy. For many volunteers, simply knowing their computer was helping search for life beyond Earth was reason enough to participate.
Scientifically, the project established new sensitivity limits for radio SETI searches. Researchers can now say with confidence that if powerful, narrow-band transmitters exist in large portions of the Milky Way, they are either rare, intermittent, or very well hidden.
There is also the possibility that something was missed by a small margin. Some scientists believe a reanalysis of the data, using modern techniques, could still uncover surprises.
Research Papers
David P. Anderson et al., SETI@home: Data Analysis and Findings, The Astronomical Journal (2025)
https://doi.org/10.3847/1538-3881/ade5ab
E. J. Korpela et al., SETI@home: Data Acquisition and Front-end Processing, The Astronomical Journal (2025)
https://doi.org/10.3847/1538-3881/ade5a7
