An international team led by the University of California, Berkeley has made a final, highly intriguing selection from billions of radio signals arriving from space. The shortlist could contain traces of extraterrestrial technology - or a particularly striking silence from the cosmos.
What lies behind the 100 mysterious signals
Many people regard SETI@home as a legendary project. From 1999 onwards, millions of home computers around the world processed radio signals captured largely by the now-decommissioned Arecibo radio telescope in Puerto Rico. Each PC took on tiny chunks of data, crunched the numbers in the background, and sent the results back.
Over the years, this created an enormous stockpile of information. In total, around 12 billion narrowband radio signals were logged - so-called narrowband signals. Researchers expect this kind of tightly confined frequency bandwidth from potential technical transmitters of alien civilisations, much like Earth-based radar or satellite communications.
"After more than two decades, the scientists have whittled down these 12 billion candidates step by step to just 100 signals that cannot be explained by known sources of interference."
These 100 signals are not proof of aliens. But they are unusual enough that telescopes should deliberately point back at them for follow-up observations. Only then will it become clear whether something genuinely out of the ordinary is present - or whether it is simply exceptionally persistent terrestrial noise.
How the researchers made sense of the data deluge
The real headline is found behind the scenes: in two papers published in 2025 in The Astronomical Journal, the team set out how it managed to squeeze usable information from the flood of data. For decades, many involved did not truly know how to assess all the detections in a structured, consistent way.
In simplified terms, the newly published evaluation proceeds in three broad stages:
- Collecting: Radio telescopes such as Arecibo scan the sky for narrowband radio signals.
- Distributing: SETI@home breaks the data into small packets that millions of home computers analyse.
- Filtering: New algorithms remove known interference sources and flag the most interesting remnants.
Among other methods, the researchers developed techniques to eliminate signals that clearly originate from satellites, radar systems, or other Earth-based transmitters. Patterns indicating technical artefacts generated by the instruments themselves were also removed from the list.
"In the end, a set of signals remained that look like brief flashes of energy at a specific frequency coming from a clearly defined direction in the sky - and for which there is, so far, no convincing explanation."
The authors stress that every step is documented and the data are openly available. Other teams can reproduce the analysis, refine it, or start again from scratch. For the search for alien civilisations, this is a major move towards robust, reproducible research.
The most sensitive search for aliens ever conducted with SETI@home
With this updated analysis, SETI@home reaches a sensitivity that clearly surpasses earlier projects. Across large areas of the sky, it is now possible to state with reasonable precision the transmission power above which an extraterrestrial radio signal would have been detected.
In other words: if, at certain distances, another civilisation had been broadcasting using powerful radio technology, it should have shown up. The fact that no unambiguous hit has appeared therefore provides an important non-result as well: there do not seem to be any especially loud transmitters in those regions.
The researchers put it like this: even if no clear “hello” arrives from space, the search sets new limits. Radio astronomers can now narrow down more accurately how strong a plausible signal could be at most without having been noticed.
Why the final 100 narrowband signals are so sensitive
Despite all the technology, one obstacle remains: the final judgement still requires people to examine the most conspicuous signals in detail. Fully checking every trace in the dataset would be pure fantasy. Even with modern computing power, there is neither the time nor the staffing.
That is precisely why the list of the final 100 candidates marks a turning point. It functions as a shortlist of the most compelling cases on which future observing campaigns can focus.
| Category | Figure |
|---|---|
| Total number of signals examined | around 12 billion |
| Signals after initial filtering | in the millions |
| Final selection for follow-up observation | 100 candidates |
Whether any of them truly represent an artificial signal is completely uncertain. Even so, the effort is worthwhile: a negative outcome still provides evidence about just how quiet our cosmic neighbourhood really is.
Doubts, mistakes - and the hope of a missed greeting
The researchers involved speak candidly about possible weaknesses in their own work. In SETI@home’s early years, computing power was scarce. Many filtering choices seemed sensible at the time, but may now look too coarse. Some signals might have been discarded before anyone could recognise their true nature.
"The concern within the team: might we have simply computed away the most exciting traces?"
This is exactly where the imagination of alien enthusiasts takes off: perhaps the raw data have long contained a technical signal that older assumptions treated as “interference”. Or perhaps an algorithm was fractionally too strict and cut out precisely the most interesting frequency ranges.
The researchers emphasise that a full re-analysis would ideally require more money, more time, and more modern supercomputers. Using today’s methods, the project’s early years in particular could be examined in far finer detail.
How the search for extraterrestrial life continues
Despite SETI@home approaching its end, the hunt for extraterrestrial intelligence is entering a new phase. Future projects are increasingly relying on AI-assisted pattern recognition, more powerful telescopes, and entire networks of radio stations listening simultaneously.
SETI@home’s experience feeds directly into this next generation. Three elements are especially important:
- Better algorithms: Machine-learning techniques can detect anomalies in data patterns that the human eye would miss.
- More telescopes: Simultaneous observations from multiple sites help to rule out local interference with far greater confidence.
- Open data: If raw data remain public, independent teams can test alternative filters and models.
SETI@home also served more than science alone. For the first time, millions of users felt they could play at least a small part in the search for extraterrestrial life using their own computers. That combination of citizen participation and high technology remains a powerful model for new projects.
What “narrowband” and the famous noise actually mean
If the terminology becomes overwhelming, one simple image helps: the radio universe crackles like the chant of a packed football stadium. Most natural sources transmit broadband - spread across a wide range of frequencies.
Technical transmitters - whether a radio mast, a satellite, or (in theory) an extraterrestrial radio beacon - typically concentrate their power tightly into a narrow frequency range. Those sharp peaks are what we call narrowband signals. For SETI researchers, they stand out like a vivid, brightly coloured dot on a black-and-white picture.
However, countless narrowband disturbances are also produced by Earth-based equipment. That is why most of the work is not searching, but sorting out: what remains once everything known has been removed?
That is where the researchers now find themselves with their 100 candidates. They are not sensational in the sense of “we found aliens”. They are sensational because, after a quarter-century of data work, they cannot simply be explained away - and because each one has the potential to fundamentally change how we see life in the Universe.
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