‘Oumuamua is the first confirmed interstellar object ever detected passing through our solar system. Discovered on October 19, 2017, by the University of Hawaii’s Pan-STARRS1 telescope, it had already swung past the Sun about six weeks earlier, reaching a peak speed of 196,000 miles per hour. Its Hawaiian name roughly translates to “a messenger from afar arriving first,” and it lived up to that name by baffling scientists with a set of physical properties that didn’t match anything previously observed in our cosmic neighborhood.
How It Was Found and Classified
Pan-STARRS1, a survey telescope on Maui funded by NASA’s Near-Earth Object Observations Program, spotted ‘Oumuamua as a faint, fast-moving dot. Its trajectory immediately stood out: it was traveling on a hyperbolic orbit, meaning it wasn’t bound by the Sun’s gravity and had clearly originated from outside our solar system. No object had ever been confirmed as interstellar before.
Its official designation, 1I/2017 U1, reflects that milestone. The “1I” marks it as the first interstellar object cataloged. But getting to that label was a process. Astronomers initially classified it as a comet, then reclassified it as an asteroid when telescopes found no visible gas, dust, or tail as it rounded the Sun. That classification flipped again when later tracking data revealed it was accelerating slightly as it moved away from the Sun, behavior typical of comets venting gas. So ‘Oumuamua ended up in a category of its own: something that acts partly like a comet but looks nothing like one.
An Extremely Unusual Shape
‘Oumuamua was too small and too far away for any telescope to photograph it directly. Instead, astronomers studied its brightness as it tumbled through space. The light curve, a graph of how bright the object appeared over time, showed dramatic swings of about 2.5 magnitudes. That’s roughly a tenfold change in brightness, which pointed to an object with a 10:1 length-to-width ratio. No known asteroid or comet in our solar system is anywhere near that elongated.
If ‘Oumuamua was cigar-shaped and rotating around its short axis, a reasonable model puts its dimensions at roughly 800 by 80 by 80 meters, assuming a dark surface. An alternative model suggests it could have been more like a flat disc or pancake, roughly 45 by 44 by 7.5 meters, depending on how reflective its surface was. NASA’s Spitzer Space Telescope tried to observe it in infrared but couldn’t detect it at all. That non-detection was actually useful: it capped the object’s maximum diameter at about 460 feet (140 meters) under most assumptions, and suggested ‘Oumuamua could be up to 10 times more reflective than typical comets in our solar system.
The Mystery of Its Acceleration
The single most puzzling thing about ‘Oumuamua was a tiny but unmistakable push. As it moved away from the Sun, tracking data showed it was accelerating beyond what the Sun’s gravity alone would produce. The extra acceleration was small, about 5 millionths of a meter per second squared at Earth’s distance from the Sun, but the measurement was rock solid, detected at 30 times the statistical noise threshold. For a normal comet, this kind of push comes from jets of vaporizing gas. But no telescope detected any gas, dust, or tail coming off ‘Oumuamua. That contradiction drove years of competing theories.
Leading Explanations for What It’s Made Of
Several natural-origin hypotheses have been proposed, each trying to account for the acceleration without a visible tail.
The most widely discussed explanation, published in Nature in 2023, proposes that ‘Oumuamua was a water-ice body that had been bombarded by cosmic rays during its long journey through interstellar space. Over millions of years, that radiation would have split water molecules inside the ice and trapped molecular hydrogen (the same gas that fills party balloons, in its molecular form). When ‘Oumuamua warmed up near the Sun, the trapped hydrogen slowly leaked out. Hydrogen gas is invisible to the telescopes that were watching, which explains why no one saw a comet tail. Yet the gentle release of gas could produce exactly the small push that was measured. This model doesn’t require any exotic material. It just needs ordinary water ice that spent a very long time in deep space.
A second theory suggests ‘Oumuamua was a chunk of frozen nitrogen, essentially a piece knocked off the surface of a Pluto-like world in another star system. In our own solar system, the early reshuffling of giant planets destabilized the Kuiper belt and would have blasted trillions of fragments off the surfaces of thousands of Pluto-sized bodies. About half of those fragments would have been nitrogen ice. A nitrogen ice shard from another star system that underwent a similar process would match ‘Oumuamua’s size, shape, brightness, and acceleration. Researchers behind this idea estimate ‘Oumuamua was ejected from a young star system, possibly in the Perseus arm of the Milky Way, roughly 400 to 500 million years ago. If correct, ‘Oumuamua would be the first sample of an exoplanet’s surface ever delivered to our solar system.
The Artificial Origin Hypothesis
Harvard astronomer Avi Loeb proposed a more provocative interpretation: that ‘Oumuamua could be an artifact of alien technology. Loeb pointed out that all natural explanations require invoking types of objects never observed before, whether hydrogen icebergs, nitrogen ice shards, or porous dust clouds. He argued that a thin, flat, light-sail-like object pushed by reflected sunlight would explain the acceleration without any outgassing at all. He drew a comparison to 2020 SO, a spent 1966-era NASA rocket booster that was rediscovered by Pan-STARRS in 2020 and showed a similar sunlight-driven push with no cometary tail.
Most of the astronomical community considers this hypothesis unlikely, favoring the natural explanations above. But Loeb’s argument highlighted something real: ‘Oumuamua’s combination of anomalies, no visible outgassing, extreme shape, unusual reflectivity, and non-gravitational acceleration, was genuinely unprecedented. No single conventional model explains every property without some degree of stretching.
Where ‘Oumuamua Came From and Where It’s Going
‘Oumuamua entered our solar system from the general direction of the constellation Lyra, near the bright star Vega. That doesn’t mean it came from a star system in Lyra, though. Stars move over time, and ‘Oumuamua could have been drifting for hundreds of millions of years, during which its star of origin would have shifted position considerably. Pinpointing its home system is essentially impossible with current data.
It’s now headed out of the solar system toward the constellation Pegasus. It will cross Neptune’s orbit roughly four years after its discovery and will cover one light-year of distance in about 11,000 years. It will never return. By now, it is far too faint for any existing telescope to observe, and no spacecraft was in a position to intercept it during its brief visit.
Why It Still Matters
‘Oumuamua’s visit lasted only a few weeks of useful observation time, but it reshaped how scientists think about what drifts between stars. Before 2017, interstellar objects were theoretical. Now astronomers know they pass through our solar system regularly. The second confirmed interstellar visitor, comet 2I/Borisov, was spotted in 2019 and behaved much more like a conventional comet, making ‘Oumuamua’s strangeness stand out even more.
The Vera C. Rubin Observatory, expected to begin its main survey in the mid-2020s, should detect many more interstellar objects thanks to its ability to scan the entire visible sky every few nights. When the next one appears, astronomers hope to have more warning and better tools, possibly even a spacecraft ready to chase it down and get the close-up look that ‘Oumuamua never allowed.