First published December 29, 2017.
Artist's impression of 'Oumuamua. ESO/M. Kornmesser
In December 2017, astronomers pointed one of the world's largest radio telescopes at 'Oumuamua, the first object ever seen passing through our Solar System from another star, to check whether it was sending out artificial radio signals. The answer was no. Breakthrough Listen's search with the Green Bank Telescope in West Virginia found no signs of transmissions from the object, and later searches with other telescopes came up empty as well. Most scientists today regard 'Oumuamua as a natural body, although exactly what kind of natural body is still debated.
Here is what the search did, what it could and could not rule out, and what we have learned about interstellar visitors since.
What 'Oumuamua is and why it caused a stir
The object was discovered on October 19, 2017, by Robert Weryk using the Pan-STARRS1 survey telescope on Haleakalā in Hawaii. Its name comes from Hawaiian and is usually translated as "a messenger from afar arriving first". Its official designation is 1I/2017 U1, the "I" standing for interstellar.
What made it special was its path. 'Oumuamua was moving far too fast to be bound to the Sun, following an open, hyperbolic orbit. It had come from interstellar space, swung around the Sun and was already heading back out when it was spotted, so astronomers had only a few weeks to study it.
Its brightness changed dramatically as it tumbled, by a factor of about ten, which suggested a very elongated or flattened shape. Early estimates described something like a cigar several hundred metres long and perhaps up to ten times longer than it was wide, though later work favours a flatter, pancake-like shape and gives size estimates ranging from roughly 100 to several hundred metres. No telescope ever saw it as more than a point of light, so these shapes are inferences, not photographs.
How the Breakthrough Listen search worked
Breakthrough Listen is a privately funded programme, launched in 2015 with backing from investor Yuri Milner, that searches nearby stars and galaxies for technosignatures: signs of technology such as narrow-band radio transmissions. On December 13, 2017, the team used the 100-metre Robert C. Byrd Green Bank Telescope to observe 'Oumuamua across four radio bands spanning roughly 1 to 12 gigahertz.
The observation of 'Oumuamua itself lasted about two hours, within a six-hour session, and produced about 90 terabytes of raw data. The team looked for very narrow signals, the kind that natural sources rarely produce, whose frequency drifted in a way that matched the object's motion relative to Earth.
The Green Bank Telescope. NRAO/AUI
The first results, announced within days, covered only part of the data and found nothing. Andrew Siemion, director of the Berkeley SETI Research Center, said in the announcement that it was "great to see data pouring in from observations of this novel and interesting source." The full analysis, published in 2018, confirmed the non-detection.
What a non-detection does and does not prove
A radio search can only rule out signals it was able to detect. According to the team, the Green Bank observations could have picked up a transmitter on the object with a power of well under a watt in some bands, weaker than a typical mobile phone. Nothing of the sort was there.
That does not prove 'Oumuamua was natural in every respect. A silent spacecraft, one transmitting at other frequencies, or one pointing its signals elsewhere would not have shown up. But science works by testing claims that can be tested, and the specific idea that the object was broadcasting at radio frequencies was checked and not supported. Other teams reached the same result: the Allen Telescope Array in California and the Murchison Widefield Array in Australia also found no signals.
The search also illustrates a crucial safeguard in SETI. Earth is full of radio noise from satellites, phones and radar. Any candidate signal has to appear when the telescope points at the target and vanish when it points away, otherwise it is almost certainly human-made interference.
The alien probe idea and the natural explanations
The question of an artificial origin was raised openly by Harvard astronomer Avi Loeb, who served as science theory director for the Breakthrough Initiatives. He described 'Oumuamua to IFLScience as "a very peculiar object indeed, which made me wonder – could it be an artificially produced interstellar probe?"
The debate sharpened in 2018 when a study in Nature reported that 'Oumuamua was speeding up slightly as it left the Sun, more than gravity alone could explain. Loeb and a colleague suggested it might be a thin light sail pushed by sunlight. Most astronomers were not persuaded and proposed natural explanations instead, including:
- Outgassing from an icy body, as comets do, even though no visible dust tail was seen.
- A fragment of nitrogen ice chipped from the surface of a Pluto-like world in another planetary system.
- Release of hydrogen trapped in water ice that had been altered by cosmic rays during its long journey, an idea published in Nature in 2023.
None of these has been confirmed, and 'Oumuamua is now far too distant and faint to observe. The honest summary is that it was an unusual natural object whose exact composition we may never know.
The interstellar visitors that followed
'Oumuamua turned out to be the first of a series. In 2019 the amateur astronomer Gennadiy Borisov discovered 2I/Borisov, an interstellar object that looked very much like an ordinary comet, with a clear tail. On July 1, 2025, the ATLAS survey found 3I/ATLAS, a third interstellar comet.
Breakthrough Listen gave 3I/ATLAS the same treatment as 'Oumuamua, observing it with several radio telescopes including Green Bank, which looked at it on December 18, 2025, shortly before its closest approach to Earth. The method was unchanged: hunt for narrow-band signals that follow the object's motion, then rule out interference from Earth. Again, no technosignatures were found.
New survey telescopes, especially the Vera C. Rubin Observatory in Chile, are expected to spot many more of these visitors, giving astronomers the chance to study them earlier and for longer than they could with 'Oumuamua.
Sources: Breakthrough Initiatives: initial results from observations of 'Oumuamua; Enriquez et al.: Breakthrough Listen observations of 1I/'Oumuamua with the GBT; Wikipedia: 'Oumuamua; Berkeley SETI Research Center: Breakthrough Listen and 3I/ATLAS; arXiv: Breakthrough Listen observations of 3I/ATLAS with the Green Bank Telescope; ScienceDaily, April 2018: Murchison Widefield Array search of 'Oumuamua; IFLScience. Images: artist's impression by ESO/M. Kornmesser; Green Bank Telescope photo by NRAO/AUI.
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