A Signal from Proxima

In April 2019, a radio telescope in Australia listening towards Proxima Centauri, the nearest star to the Sun, recorded a narrow signal that slipped through every routine interference check. Could it be alien technology?³

The line in the data

The observation came from Murriyang, the Parkes radio telescope in New South Wales, whose 64-metre dish stands on a tower north of the town, on Wiradjuri Country. Completed in 1961, it had been gathering radio waves for nearly sixty years. Its sensitivity is what makes it useful for hunting distant signals. It also makes it very good at picking up technology much closer to home.⁴

The target was what made the signal tantalising. Proxima Centauri is a faint red dwarf just over four light-years away. In 2016, astronomers announced evidence for a planet, Proxima b, orbiting it roughly every eleven days, at a distance that would allow liquid water under the right conditions. Nobody had found an ocean, an atmosphere or life. But if you were going to look for neighbours, Proxima was the obvious place to start.⁵

The 2019 observations were actually part of a campaign to study stellar flares. The same recordings could also be searched for the kind of narrow signal a transmitter would produce, so a dataset gathered for one purpose became evidence for another. Nobody at the dish heard a voice from the stars. The clue surfaced later, when the recorded data were processed.¹⁰

Shane Smith, an undergraduate researcher with the Breakthrough Listen project, spotted the feature in 2020. His mentor, Danny Price, shared a plot with the group, and Sofia Sheikh volunteered to lead the investigation. In her own account, Sheikh described a trace her colleagues could not immediately explain. There was no decoded message and no identified sender.⁶

The initial search had thrown up more than four million detections, signals above the search threshold, nearly all of them interference. According to Smith and his colleagues, 5,160 survived a directional test; visual inspection and further filtering left just one. It was nicknamed BLC1, short for Breakthrough Listen Candidate 1.¹

Even “candidate” was more than the team wanted to claim, so they preferred “signal of interest”, meaning something worth examining with the hard work still to come. A genuine transmission from another civilisation would have to survive explanations involving the telescope, terrestrial electronics and human spacecraft, and the label marked the start of those tests.¹

The papers that eventually followed attributed BLC1 to human interference. The trace itself was real enough, because something had entered the receiving system and left a measurable pattern, so the puzzle was how an earthly signal had passed checks designed to tell it apart from something in the sky.⁷

A signal that knew where to appear

The signal sat near 982 megahertz, but what caught the eye was its width of less than four hertz. Its frequency also drifted slowly, appearing as a slanting line on a plot of frequency against time. Signals that narrow are typically engineered, which left the question of who had engineered it.²

A radio receiver can find artificial signals without finding extraterrestrial intelligence, as Breakthrough Listen’s principal investigator, Andrew Siemion, stressed in an interview in February 2021, before the final papers appeared. The task was to separate signals made by our own technology from any that might come from another civilisation. By then he already considered a human source overwhelmingly likely, though its exact origin had not been found.¹²

The telescope’s movements supplied a sterner test. A signal genuinely coming from the target should fade sharply when the dish points elsewhere, so observers alternate between the star and reference positions, known as on-source and off-source observations. Local interference usually shows up in both and can be thrown out. BLC1 seemed to appear only when the telescope pointed at Proxima.⁹

That was what set it apart, but it also left room for a misleading coincidence, because the direction was not the only thing changing between observations. Time was passing too, and a source of interference that changed strength, or switched on and off during the sequence, could mimic the pattern of a fixed point in the sky. The telescope would then seem to locate something it had not located at all.²

The detailed analysis recorded detections over about five hours, including five half-hour observations of the target. It was not a continuous five-hour broadcast, and a narrow line carries no return address. The pattern told the investigators where to look in the data. Linking it to a planet, a transmitter or an intention would take much more.²

Reading the trace: a signal is measured radio energy; a source is what produced it; a message contains information that can be interpreted. BLC1 supplied a signal. Neither an alien source nor a message had been established.

A clean line on a plot invites the imagination to supply a beacon, and a beacon might well be narrow, but so might an accidental emission from ordinary electronics. To be worth anything, a test had to tell the two apart, not merely confirm that the trace looked like what the search had been built to notice.

What else could make it?

Human radio interference was the obvious suspect; the difficulty was finding a version of it that could behave like BLC1. Sheikh and her colleagues checked possible transmitters, local conditions and the working of the equipment. Her account describes going through instrument temperatures, telescope logs and nearby activity, and checking whether the dish had been pointing at a building. Nothing matched.⁶

That made the signal unusual. It did not put it at Proxima. A list of known transmitters is a starting point, not a complete inventory of everything electronics can emit. Devices leak unintended signals as well as the ones they were built to send, and finding nothing licensed at a given frequency rules out neither leakage, nor signals mixing, nor a fault.

Could the drift have come from something moving? Relative motion shifts the frequency received, so aircraft, satellites and human spacecraft all had to be examined, and the verification framework tests drift together with duration and plausible motion. A changing frequency says nothing about interstellar distance.⁹

The electronic alternative

There was another way to produce a drift, because an electronic oscillator’s frequency can wander as its operating conditions change without anything travelling anywhere. Ruling out a particular moving transmitter therefore did not rule out electronics, and the investigators had to think about how the signal was generated, not just where it came from.

The rival explanation was the one that made headlines, an engineered source beyond Earth. The telescope’s direction made Proxima the obvious candidate, but pointing at a star is not the same as identifying a planet, and the pattern would have to hold up in further observations and against comparison with other signals. An alien explanation could be tested like any other, but it did not win by default just because the familiar earthly one was proving hard to pin down.

In the contemporary ICRAR account, Price summed up why the event had been worth investigating:

Passing those checks justified more work. It did not mean the filters could catch every unusual form of interference. A filter is built around the characteristics researchers already know how to recognise, and an unfamiliar fault can slip through, especially in a search of millions of signals. The decisive clue would come from looking beyond the one trace that had survived.

The signals that had been thrown away

Sheikh widened the search. Instead of asking only whether BLC1 looked unusual, the team hunted for other signals that behaved the same way, on other dates and in other parts of the receiver’s frequency range. A signal that looked unique in one carefully filtered plot might turn out to belong to a family once the filter was relaxed.⁶

It did. The search turned up dozens of lookalikes, and unlike BLC1, they also appeared when the telescope was pointing away from Proxima. That marked them as interference, and their resemblance to BLC1 tied the interesting trace to something already known to be local. The case no longer rested on a general hunch that strange detections usually have human causes.²

The spacing of the frequencies supplied another link. The researchers found relationships consistent with the clock frequencies that keep time inside electronic devices. When signals interact in a circuit, the output can contain new frequencies, a process called intermodulation. It offered a way for apparently separate traces to belong to one interference problem, and explained why looking only near the original frequency had not been enough.²

Think of two marks on separate sheets of paper that turn out to share the same spacing, slope and small irregularities. Neither means much alone; together, they point to a common origin. Here the comparison was numerical, made on recorded radio measurements, and the explanation had to connect the traces through their behaviour, not just their roughly diagonal look.

The overlooked material mattered precisely because the first search had done its job. It had discarded signals that appeared away from the target. Those rejects were poor alien candidates but excellent comparisons, and keeping only the survivor would have thrown away the evidence needed to understand it. In the end, the routine interference explained the exceptional detection.

The explanation has a gap. The team did not find a particular appliance and recreate BLC1 by switching it on, and the published announcement left the specific source unidentified. That gap is far narrower than the original mystery. Evidence linking a signal to human electronics does not become neutral between Earth and another star simply because the offending device has not been found.⁷

Investigators can identify a mechanism, or a class of source, without a street address or a serial number. Finding the device would complete the account and might show exactly how the emissions interacted. Not finding it does not make the family of related signals, or their behaviour away from Proxima, disappear.

Could it still have been a beacon?

A deliberate transmission remained a hypothesis worth testing. It could explain both the narrow width and a drifting frequency. A distant transmitter might also switch on and off, so the idea survived the fact that the signal was artificial and did not immediately repeat. It ran into much harder trouble once BLC1 was linked to lookalikes typical of earthly interference.

An alien explanation now had to account for the whole pattern: the relationships between the frequencies and the similar signals that appeared away from the target. It could not treat BLC1 as though those comparisons had never been made. Another civilisation might well use clocks or multiple frequencies, but proposing that it could does not show that it produced these measurements.

The star’s closeness prompted a different kind of argument. In January 2021, Amir Siraj and Abraham Loeb published a statistical case against a neighbouring inhabited system being the source. Their calculation used the Copernican principle, the idea that Earth holds no special position in the universe. It was an argument about prior odds under a model, not an identification of the transmitter, and it was not the work that traced BLC1’s source.¹¹

The two kinds of evidence work differently. A population model can make an explanation look improbable before anyone examines the signal. Measurements can show that a particular signal has the hallmarks of local interference, which speaks to this event directly. The two pointed the same way, but they are not interchangeable, and a model’s assumptions matter even when its conclusion sounds decisive.

Detecting an emission is not the same as reading a message. BLC1 carried no greeting, no decoded mathematical sequence and no sign of anyone trying to reach Earth. None of that is needed to detect distant technology in the first place, but calling BLC1 an alien message adds content the observation never contained.

The extraordinary explanation made the investigation worth doing. It could not be shielded from unfavourable tests. If every resemblance to local interference were read as an alien technique for resembling local interference, the theory would stop distinguishing between anything. A useful alternative needs some evidence that could favour it over the ordinary mechanism, and some observation that could count against it.

The telescope went back

The researchers returned to Proxima with the same instrument. The follow-up observations ran in November 2020, January 2021 and an anniversary campaign in April and May 2021. A separate paper reported 39 hours of reobservation without BLC1 reappearing. Some sequences changed the balance between target and comparison pointings; the anniversary campaign also repeated the original observing pattern.¹⁰

That was far more informative than a quick second look. Changing the procedure could expose a pattern produced by the procedure itself; repeating the original conditions showed whether the signal would return under the circumstances in which it was first found. Either way, BLC1 never came back.¹⁰

The failure to repeat did not prove a human transmitter, since a distant source could be intermittent or no longer operating. What mattered was how the follow-up fitted with everything else, including the related interference, the electronic relationships and the absence of any convincing astronomical origin. Together they made the case, though no single result carried it alone.

Looking in another light

Could a different kind of light offer an independent check? A separate study in 2021 searched 107 archived optical spectra of Proxima, recorded between 2004 and 2019. They included 29 observations made between March and July 2019, around the time Parkes recorded BLC1. The author reported no evidence of technological signals in them. The search looked for laser emission, not for the radio signal; it was a different investigation with its own limits.¹⁵

An optical blank cannot settle every radio hypothesis. A civilisation with a radio transmitter would not necessarily aim a laser at Earth, and archived spectra capture particular moments, not every instant. The value of such a search is that it puts a different question to different evidence. It found nothing to support a transmitter, and it should be described for what it was, not folded into a sweeping claim that the system has been fully examined.

The wider verification procedure also calls for observations with other telescopes when a signal survives the early checks. Instruments at different sites, built to different designs, can separate a phenomenon in the sky from one local to a receiving station. BLC1 never emerged from that process as a confirmed interstellar source. It became a case study in how a promising detection can fail the larger investigation.⁹

The public archive and published methods made the result more useful than a bare announcement that the signal was not alien. Other researchers could inspect the work, question its choices and design different searches. Access to the records is not an independent reanalysis, but it supplies the material for one. For a claim of this weight, a label on a plot is no substitute for the observations behind it.³

Why the alien idea keeps returning

The alien interpretation did not vanish once the papers appeared. In October 2024, renewed claims suggested that an announcement was imminent, but when IFLScience asked Siemion about them, he said that no redetections or other developments had changed the conclusions of the 2021 papers. There was no new beacon, only new publicity.¹³

Why does the case keep coming back? Its most exciting facts are still there to be repeated: a nearby star, a narrow signal and a directional pattern that looked convincing at first. Retold without the lookalikes, the story returns to its earliest stage, and the later explanation starts to look like one opinion set against the original excitement, not a conclusion drawn from further work on the data.

The unidentified device gives that kind of claim an opening. It is reasonable to ask what produced the interference. It does not follow that failing to name the device revives every explanation once considered. A serious challenge to the published finding would have to deal with its strongest evidence, and a promised announcement, a fresh description of an old plot or a claim of private knowledge does not do that.

Later searches belong to separate stories. In 2023, another Breakthrough Listen project applied machine learning to data from the Green Bank Telescope and identified new signals of interest. Its public account stated that follow-up observations did not detect them again and that they had not met the criteria for genuine candidates. A new method and new targets are not a return of BLC1.¹⁴

What remains at Proxima

As of 6 November 2026, the published explanation of BLC1 remains human-generated radio interference, although the investigation never identified the individual transmitter. The 2024 response reported no development that overturned the 2021 finding, and no independently verified alien transmission has been reported.⁷¹³

None of this shows that Proxima’s planets are lifeless. A search covers particular times, frequencies, sensitivities and directions, and cannot rule out every possible technology or form of life, but what it showed is specific. The line in the Parkes data was made on Earth, by electronics that for a few hours in 2019 did a convincing impression of a voice from the nearest star, and nobody has yet found the device.²⁷

Linked case files

Case file 001

Bella in the Wych Elm

Case file 008

The Typewriter Was Listening

Case file 009

A Ghost Made to Order

Case file 011

The Stone That Wouldn’t Sink