In October 2017, astronomers using the Pan-STARRS survey telescope in Hawaii detected a small object moving through the inner solar system on a hyperbolic path, too fast for the Sun to hold. It had been found on 19 October, after its closest approach to the Sun in September, so it was already on its way out when it was spotted. It was the first confirmed interstellar object detected in our solar system. They named it ʻOumuamua, a Hawaiian term NASA renders as a messenger from afar arriving first.
Its path should have been simple to read. Then, as it receded from the Sun, it sped up very slightly, by an amount gravity alone could not account for. What made that strange was the absence of the obvious cause. A comet accelerates this way when it vents gas and dust, but ʻOumuamua showed no coma and no tail. That gap, between a measured push and a missing exhaust, is where the whole argument lives.
What was actually observed
The non-gravitational acceleration was reported by Marco Micheli and colleagues in a 2018 paper in Nature. It was small, and it faded with distance from the Sun in roughly the way sunlight or outgassing would predict, which is part of why it drew attention rather than being dismissed as measurement error.
The other oddities were the shape and the silence. ʻOumuamua tumbled as it went, and its brightness varied so much as it rotated that its light curve implied an extreme shape, though whether that meant a cigar-like body or a flattened, pancake-like one depends on the model. And despite searches with large telescopes, no one detected gas or dust coming off it. Those searches set upper limits on how much material it could be shedding, and the limits were low.
The argument for technology
This is where Avi Loeb, an astronomer at Harvard, made the case he is now known for. With his colleague Shmuel Bialy, he showed in a 2018 paper in the Astrophysical Journal Letters that solar radiation pressure could in principle supply the extra acceleration, but only if the object had a very low mass-to-area ratio, equivalent to a sheet less than a millimetre thick. Since nature is not known to make objects like that, they went on to discuss a possible light-sail origin, and Loeb later argued, in follow-up work and his book Extraterrestrial, that ʻOumuamua might be the product of an extraterrestrial technology.
The shape of that argument is an inference from absence. The push was real and the usual explanation was missing, so an unusual explanation was put on the table. That is not the same as evidence of manufacture, and Loeb has generally framed it as a hypothesis worth taking seriously rather than a settled conclusion. Most astronomers have not accepted it.
The natural explanations, including the leading one
The reason most astronomers reject the technology reading is not that they find it offensive.
It is that natural explanations kept arriving, and none of them required anything new about the universe.
Several were floated early: a body made largely of hydrogen ice, whose outgassing would be nearly invisible; a fragment of nitrogen ice chipped off a Pluto-like world; an extremely porous, fluffy structure that sunlight could push. The most prominent recent natural explanation came in 2023, when Jennifer Bergner and Darryl Seligman published a model in Nature in which ʻOumuamua began as a water-rich icy body bombarded by cosmic rays during its long passage through interstellar space. That processing could split water and trap molecular hydrogen inside the ice. Warmed by the Sun, the ice could then release hydrogen, giving the object a small push without carrying off enough dust to form a visible coma.
Bergner and Seligman presented this as a natural and economical account. Seligman and colleagues have since identified, in work published in PNAS, several other small bodies in the solar system that show non-gravitational acceleration without any visible coma, now called dark comets. That does not prove ʻOumuamua was a dark comet, but it does show that small bodies can behave in this awkward middle ground between asteroid and comet.
Why it has not fully settled
The honest complication is that the leading natural explanation is itself disputed. Loeb, with Thiem Hoang, responded that the 2023 model miscalculated the object’s surface temperature, which would undercut the hydrogen mechanism. Separately, the chemist Niels Ligterink argued, in a formal response to the paper, that it is unlikely enough hydrogen could be produced to do the job unless the object had an unusual composition or was far older than assumed.
None of this means ʻOumuamua was artificial. It means the most popular natural account is strong but not airtight, which is a different and more ordinary kind of uncertainty.
The deeper problem is that the object is gone.
ʻOumuamua left the solar system within months of discovery and is now too far and too faint to study. There is no prospect of new data on it. Whatever it was, the case has to be argued from the limited observations gathered during a short window in late 2017, and limited evidence is exactly the soil in which a claim like Loeb’s stays alive.
What to watch
The way this gets resolved is not by re-litigating ʻOumuamua but by finding more of its kind. The second confirmed interstellar object, 2I/Borisov, turned up in 2019 and behaved like a perfectly ordinary comet, with an obvious tail, which told us interstellar visitors are not all strange. A third, 3I/ATLAS, was found on 1 July 2025 and showed clear cometary activity, with Hubble imaging dust coming off its icy nucleus.
More are coming. The Vera C. Rubin Observatory in Chile, built for a decade-long survey of the sky, should sharply increase the rate at which these objects are found, on Seligman’s estimate something like one to three ʻOumuamua-like bodies a year, and more with visible comae. If a future object with a faint coma and a small non-gravitational push can be caught early and tracked closely, the questions left hanging by ʻOumuamua become answerable. The first interstellar visitor arrived before we were ready to study it. The next ambiguous one may not.
