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- HD 89389 is a sunlike star about 100 light-years from Earth. It is slightly brighter and more massive than our sun.
- A recent optical SETI search found a pair of very fast, identical pulses coming from the star. Optical SETI looks for light signals such as laser instead of radio signals. What caused them?
- An object passing in front of the star is the most likely explanation. But what it was is unknown, and the data suggests it was closer to us than the star itself.
Unusual starlight pulses
We typically think of SETI (Search for Extraterrestrial Intelligence) as looking for radio signals from distant alien civilizations. But in more recent years, scientists have been broadening that search to include other possible technosignatures (signs of alien technology). Optical pulses might be one of these technosignatures, from powerful lasers or directed-energy technology. This search is called optical SETI or LaserSETI. So far, astronomers haven’t found anything definitive. But now a new survey of 1,300 sunlike stars has revealed something interesting: two very fast, identical pulses from a sunlike star called HD 89389, 100 light-years away, seen on May 14, 2023. Former NASA scientist Richard Stanton led the new survey.
These pulses are also similar to two pulses from a different star, HD 217014 (51 Pegasi). Astronomers found these other pulses on September 30, 2019. And astronomers detected a third set of similar pulses on January 18, 2025. But those are not included in the current paper.
Stanton, now retired, has been using a 76.2-cm (30-inch) telescope in Big Bear, California, for the past several years. The telescope uses a multi-channel photometer that Stanton designed himself. He has been specifically searching for optical signals that could possibly originate from alien intelligence. He hadn’t found anything too unusual … until now.
The star in question is HD 89389, located in the constellation Ursa Major. It is similar to our sun, an F-type main sequence star, but slightly brighter and more massive. The telescope detected two fast pulses of light coming from the star. The pulses were identical to each other, which was unusual, and spaced only 4.4 seconds apart.
Stanton published the peer-reviewed findings in the journal Acta Astronautica on April 30, 2025 (to be included in Volume 233, August 2025).
Pulsing stars: a possible candidate for SETI research? www.universetoday.com/articles/not…
— Matthew S Williams (@houseofwilliams.bsky.social) 2025-05-12T21:51:01.197Z
Starlight pulses unlike any seen before
Stars themselves can be known to pulse, and there are also pulsars, of course, so what made these observations unusual? HD 89389 is a sunlike star and doesn’t behave anything like a pulsar. But it was how the two pulses were identical to each other – and spaced only 4.4 seconds apart – that stood out. Single pulses that astronomers have seen before didn’t resemble them either. As the paper describes them:
Years spent searching more than 1,300 sunlike stars for optical SETI signals have finally yielded unexpected results. A ‘signal’ of two fast identical pulses, separated by 4.4s, was discovered in the light of HD89389. No single pulses, even remotely resembling these, have been found in these searches. Close examination of this signal reveals that several unique features of the first pulse are repeated almost exactly in the second.
What makes the pulses unique?
The first oddity is how similar the two pulses are to each other. In the main set from HD 89389, the primary focus of the paper, they are separated by 4.4 seconds. In the third event, in January 2025, they were separated by only 1.2 seconds. Astronomers haven’t found any single pulses that resemble these. The “fine-structured light” in the first pulse repeats almost exactly the same in the second pulse.
Second, the star changed in overall brightness – brighter, fainter, brighter again and then back to normal – in only 0.2 seconds.
Third, a camera imaged the star and a background sensor monitored it at the time the pulses were recorded. Nothing was detected moving in the field of view, such as aircraft, satellites, meteors or birds.
Narrowing down the possibilities
Stanton said that several possible Earth-bound possibilities don’t appear to explain the pulses. The paper states:
Comparison of this signal with those of airplanes, satellites, meteors, lightning, atmospheric scintillation and system noise, emphasizes their uniqueness. During the re-examination of historical data, another pair of similar pulses was found in an observation of HD217014 made four years earlier. Not fully explained at the time, this signal had been dismissed simply as ‘birds.’ After all pulses were examined in detail, and shown that they could not have been made by birds, several theories are proposed that might explain their origin. A theory based on edge diffraction is discussed in some detail. If correct, this theory should enable future observations to measure the distance to the occulting object, and using arrays of telescopes, determine its size, shape and velocity.
Gravitational waves – ripples in spacetime – are a possible explanation. Or a shockwave within Earth’s atmosphere. Or edge diffraction – where light or other waves bends around the edges of an object, creating an interference pattern – caused by a straight-edged object (whatever that would be). When collimated light – consisting of parallel beams of light – is diffracted by a straight edge, it produces ripples in the shadow of that edge. And, of course, there is the possibility of extraterrestrial intelligence (straight-edged object again?), which is what optical SETI is searching for. But a lot more study is needed before that kind of conclusion could be reached, or even other conclusions.
An object in our own solar system?
Right now, the consensus is that an object in space occulting – passing in front of (eclipsing) – the star is the most likely explanation. But what kind of object?
If there is indeed an object passing in front of the star, then how far away is it? Since the pulses and changing brightness of the star happened so quickly, it suggests the object is relatively nearby, closer to us than the star. The paper says:
The star gets brighter-fainter-brighter and then returns to its ambient level, all in about 0.2 seconds. This variation is much too strong to be caused by random noise or atmospheric turbulence. How do you make a star, over a million kilometers across, partially disappear in a tenth of a second? The source of this variation can’t be as far away as the star itself.
In fact, Stanton said the object would likely be within our own solar system. So could it be starlight diffraction from an asteroid? Maybe. The characteristics of the pulses are still hard to explain, though. And it would need to be a double asteroid to explain the two pulses in quick succession. Stanton said:
None of these explanations are really satisfying at this point. We don’t know what kind of object could produce these pulses or how far away it is. We don’t know if the two-pulse signal is produced by something passing between us and the star or if it is generated by something that modulates the star’s light without moving across the field. Until we learn more, we can’t even say whether or not extraterrestrials are involved!
What’s next?
If astronomers can detect the pulses again, or find more that are similar, that would help to better understand what might be causing them. Stanton said:
Look for events using arrays of synchronized optical telescopes. If the object is moving between the star and us, this approach should tell us how fast it is moving normal to the line of sight, and potentially its size and distance.
[Also,] it would be very interesting if the star’s light is modulated without an object moving across the field. Observing events with telescopes separated by a few hundred kilometers might show that any separation in the time each pulse arrives is due only to differences in the light time from the star to each telescope. Then, unless the variation could somehow be attributed to the star itself, we would have even more to explain!
There is also a good, short video overview of the discovery on John Michael Godier’s YouTube channel.
Bottom line: A SETI search of 1,300 stars for optical alien signals has found something odd: two fast, identical starlight pulses from a sunlike star 100 light-years away.
Source: Unexplained starlight pulses found in optical SETI searches
Read more: New SETI search goes beyond our Milky Way
Read more: SETI looks to Milky Way’s heart for alien signals