'Oumuamua (formally 1I/2017 U1) is the first object confirmed to have entered our solar system from interstellar space. Discovered on 19 October 2017 by Robert Weryk with the Pan-STARRS1 telescope on Haleakalā, Hawaii, it was a small, reddish, strongly elongated or flattened body, roughly 100 to 400 metres long, that swept past the Sun on a hyperbolic orbit, showed no coma or tail, and yet accelerated slightly in a way gravity alone could not explain.
That last detail turned a scientific first into one of the most argued-over objects in modern astronomy. Most researchers now favour a natural explanation, while Harvard astrophysicist Avi Loeb has argued since 2018 that an artificial origin should stay on the table. The debate has shaped how astronomers study interstellar visitors and how the public reads every new arrival, including 2I/Borisov in 2019 and 3I/ATLAS in 2025.
What 'Oumuamua is and how it was found
'Oumuamua is a small body from another star system that passed through the inner solar system in 2017 and is now heading back out into interstellar space. Robert Weryk spotted it in Pan-STARRS1 survey images on 19 October 2017, by which time it had already made its closest approaches to the Sun and to Earth and was moving away.
Its orbit was the giveaway. Comets and asteroids native to the solar system follow closed, elliptical paths around the Sun, with an orbital eccentricity below 1. 'Oumuamua's eccentricity was about 1.2, an open hyperbola: it was moving too fast to be bound by the Sun's gravity and could only have come from outside. It had reached perihelion on 9 September 2017 at 0.25 astronomical units (AU) from the Sun, inside the orbit of Mercury, and passed closest to Earth on 14 October at about 0.16 AU, roughly 24 million kilometres away.
Its label changed several times: first a comet, then an asteroid when no coma appeared, and finally a category of its own. The International Astronomical Union introduced the prefix I, for interstellar, making the object 1I/2017 U1. The name 'Oumuamua is Hawaiian and is usually rendered as a messenger from afar arriving first, or simply a scout.
Why 'Oumuamua looked so strange
'Oumuamua looked strange because almost every measurement astronomers could make fell outside the range of familiar small bodies. No telescope could resolve it; it appeared only as a point of light, so its properties had to be read from how that light changed over time.
- Extreme brightness swings. Its brightness varied by about a factor of ten as it rotated, far more than a typical asteroid. That implies a highly elongated cigar shape or a flattened pancake, with estimates of roughly 100 to 400 metres long and 35 to 100 metres wide. Later modelling found that a flattened disk fits the data at least as well as the famous cigar.
- Tumbling. Rather than spinning neatly about one axis, it tumbled, a sign it may have been disturbed, possibly by a collision, at some point in its history.
- Reddish colour. Its surface was reddish, similar to icy bodies in the outer solar system whose surfaces have been darkened by long exposure to radiation.
- No coma, no tail. Deep images showed no dust cloud and no tail, which is why it was first treated as an asteroid rather than a comet.
None of these features on its own required anything exotic, but together they described an object that fit no single category neatly. Even the size figures are uncertain, because its reflectivity was never measured directly.
The acceleration problem
The acceleration problem is the central puzzle: 'Oumuamua moved away from the Sun slightly faster than gravity alone predicted, yet it showed no sign of the gas or dust that normally causes that effect. The finding was published by Marco Micheli and colleagues in Nature in 2018, based on precise tracking with ground-based telescopes and the Hubble Space Telescope.
Comets routinely show this kind of non-gravitational push. Sunlight warms their ices, the ices turn to gas, and the escaping gas acts like a weak rocket. Micheli's team concluded that comet-like outgassing was the most plausible cause. The difficulty was that nobody could detect the gas. Water, carbon monoxide and carbon dioxide are the usual suspects, and observations, including a search with the Spitzer Space Telescope, found no sign of enough of them. Other researchers noted that jets strong enough to produce the push should also have changed the object's spin noticeably.
That gap, a comet-like push with no visible comet activity behind it, is the space every competing explanation has tried to fill.
Natural explanations: hydrogen, nitrogen, dust and dark comets
Natural explanations try to identify an invisible substance or an unusual structure that could produce the push without leaving a detectable trace. Four ideas have received the most attention.
Hydrogen ice
In 2020 Darryl Seligman and Gregory Laughlin proposed that 'Oumuamua was made largely of frozen molecular hydrogen, which would escape as gas without producing an easily detectable signature. Critics, including Thiem Hoang and Avi Loeb, argued that a hydrogen-ice body would evaporate in interstellar space long before it could reach us, and the idea remains disputed.
A nitrogen-ice fragment
In 2021 Alan Jackson and Steven Desch suggested that 'Oumuamua was a chip of nitrogen ice knocked off the surface of a Pluto-like world in another planetary system, much like the frozen nitrogen that covers parts of Pluto today. Amir Siraj and Loeb challenged the proposal, arguing that the galaxy could not produce enough such fragments to make finding one this early plausible. The site's coverage of that exchange is in [Harvard scientists have debunked the notion that 'Oumuamua was a nitrogen iceberg](/harvard-scientists-have-debunked-the-notion-that-oumuamua-was-a-nitrogen-iceberg).
A fractal dust aggregate
Other researchers proposed a highly porous, fluffy aggregate of dust and ice, so light for its size that the pressure of sunlight alone could nudge it outward.
A dark comet releasing hydrogen
The explanation most researchers now regard as the leading natural candidate came from Jennifer Bergner and Darryl Seligman in Nature in 2023. They proposed that 'Oumuamua began as an ordinary icy body rich in water ice. During its long journey between the stars, cosmic rays split water molecules in its outer layers and produced molecular hydrogen that stayed trapped in the ice. When the Sun warmed the object, that hydrogen escaped. The amount needed to explain the acceleration is small enough to have gone undetected, and the idea fits a newly recognised class of so-called dark comets in our own solar system, which show non-gravitational acceleration without visible tails.
Earlier, in 2019, the International Space Science Institute's 'Oumuamua team, in a review led by Michele Bannister in Nature Astronomy, reached the conclusion that still reflects the mainstream view: the observations are consistent with a natural object, and nothing in the data compels an artificial explanation.
The alien-probe hypothesis and Avi Loeb
The alien-probe hypothesis holds that 'Oumuamua may have been a piece of technology, specifically a thin lightsail pushed by sunlight. Shmuel Bialy and Avi Loeb, then chair of Harvard's astronomy department, set out the idea in a 2018 paper in The Astrophysical Journal Letters. They calculated that radiation pressure from the Sun could account for the extra acceleration if the object were extremely thin, well under a millimetre, and very light for its surface area, and they raised the possibility that such a sail could be artificial.
Loeb took the argument to a general audience in his 2021 book Extraterrestrial: The First Sign of Intelligent Life Beyond Earth. His case rests on the combination of anomalies: the unusual shape, the missing outgassing and a push of the kind a sail would naturally produce. His broader point is about method. He argues that science should keep an artificial origin as a testable hypothesis rather than rule it out because it is unfamiliar, and that astronomers risk missing evidence if they decline to look.
Most researchers disagree with his conclusion rather than with his right to ask. They point out that each anomaly has at least one natural explanation, and that no signal was ever detected: the Breakthrough Listen project observed 'Oumuamua with the Green Bank Telescope in West Virginia in December 2017 and found no artificial radio emission. The artificial hypothesis is best described as a minority position that has not been excluded with certainty, rather than an established finding. The question of whether the object could have been a probe is explored further in [Was it an interstellar probe, or did 'Oumuamua contain signs of extraterrestrial technology?](/was-it-an-interstellar-probe-or-did-oumuamua-contain-signs-of-extraterrestrial-technology)
In 2021 Loeb founded the Galileo Project, a research effort to build telescopes and instruments that search systematically for physical evidence of extraterrestrial technology, both in Earth's skies and among interstellar objects.
What came after: 2I/Borisov and 3I/ATLAS
Two more interstellar objects have been confirmed since 'Oumuamua, and both behaved like comets. The second, 2I/Borisov, was discovered in August 2019 by the amateur astronomer Gennady Borisov in Crimea. Unlike 'Oumuamua, it had an obvious coma and tail, making it the first unmistakably cometary visitor from another star system and a reminder that interstellar objects can be ordinary in form while foreign in origin.
The third, 3I/ATLAS, was discovered on 1 July 2025 and named for the ATLAS survey that found it. It is an active comet and reached perihelion in October 2025. Loeb again raised the question of whether an artificial origin should be considered, and the debate played out in public a second time. Observations showed cometary activity, which most astronomers took as confirmation that 3I/ATLAS, like Borisov, is a natural comet.
Each new discovery changes the statistics. With one object, 'Oumuamua was a puzzle with no comparison group. With three, researchers can begin to judge what is typical and what is not, and the Vera C. Rubin Observatory in Chile is expected to find many more.
Billy Carson's reading
Billy Carson treats 'Oumuamua as a case study in keeping scientific questions open. His interest is in the hypothesis rather than a verdict: he has interviewed Avi Loeb, and on the 4biddenknowledge Podcast he has followed the story from 'Oumuamua to its successor, in episodes such as A New Interstellar Visitor: 3I/ATLAS vs 'Oumuamua and a later conversation covering Loeb's Harvard update on the 3I/ATLAS mystery.
In Carson's framing, the significance of Loeb's work is that a senior Harvard astrophysicist was willing to put the artificial hypothesis into the peer-reviewed literature and argue that it should be tested. That fits a larger theme in Carson's work, that the possibility of other intelligences visiting or observing Earth deserves serious attention, a thread that also runs through his coverage of [ancient astronaut theory](/ancient-astronaut-theory-explained-origins-evidence-and-criticism) and the [Black Knight satellite](/black-knight-satellite-explained). He presents these conversations as open questions for his audience to weigh, and the mainstream view that 'Oumuamua and 3I/ATLAS are natural objects is part of that conversation rather than something to be waved away.
Watch, read and go deeper
Carson's full run of conversations, including his interstellar-object episodes, is on [the 4biddenknowledge Podcast on 4BK TV](/watch/billy-carson-4biddenknowledge-podcast). For audio only, start with the 3I/ATLAS vs 'Oumuamua episode, which sets the two visitors side by side. For the wider cosmology behind these discussions, the [Holographic Universe topic hub](/topics/holographic-universe) collects the site's coverage of physics, reality and the cosmos.
Frequently asked questions
Was 'Oumuamua an alien spacecraft?
There is no evidence that it was. Avi Loeb and Shmuel Bialy proposed in 2018 that it could have been an artificial lightsail, but most astronomers favour natural explanations, most prominently the 2023 proposal that it was a water-ice body releasing hydrogen produced by cosmic rays. Radio searches in 2017 found no artificial signals.
What does the name 'Oumuamua mean?
The name is Hawaiian, chosen because the object was discovered from Haleakalā on Maui. It is usually translated as a messenger from afar arriving first, or a scout. Its formal designation is 1I/2017 U1, where 1I marks it as the first interstellar object ever catalogued.
Where is 'Oumuamua now?
It is leaving the solar system on its hyperbolic path and will never return. It became too faint to track in early 2018, only a few months after its discovery, and has since travelled beyond the orbits of the planets.
How many interstellar objects have been found?
Three had been confirmed and given interstellar designations by the end of 2025: 1I/'Oumuamua in 2017, 2I/Borisov in 2019 and 3I/ATLAS in 2025. The two later objects were both active comets. New survey telescopes such as the Vera C. Rubin Observatory are expected to raise the count.
Why did 'Oumuamua speed up?
Its small extra acceleration away from the Sun, measured by Micheli and colleagues in 2018, is the core of the mystery because no escaping gas was detected. The leading natural explanation is that trace amounts of hydrogen, created inside its water ice by cosmic rays, were released as the Sun warmed it. Loeb's alternative is that sunlight pushed on a thin, sail-like structure.
Sources and further reading
- Karen Meech et al., A brief visit from a red and extremely elongated interstellar asteroid, Nature, 2017.
- Marco Micheli et al., Non-gravitational acceleration in the trajectory of 1I/2017 U1 ('Oumuamua), Nature, 2018.
- Shmuel Bialy and Abraham Loeb, Could Solar Radiation Pressure Explain 'Oumuamua's Peculiar Acceleration?, The Astrophysical Journal Letters, 2018.
- The 'Oumuamua ISSI Team (Michele Bannister et al.), The natural history of 'Oumuamua, Nature Astronomy, 2019.
- Darryl Seligman and Gregory Laughlin, Evidence that 1I/2017 U1 ('Oumuamua) was composed of molecular hydrogen ice, The Astrophysical Journal Letters, 2020.
- Alan Jackson and Steven Desch, 1I/'Oumuamua as an N2 ice fragment of an exo-Pluto surface, Journal of Geophysical Research: Planets, 2021.
- Jennifer Bergner and Darryl Seligman, Acceleration of 1I/'Oumuamua from radiolytically produced H2 in H2O ice, Nature, 2023.
- Avi Loeb, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, Houghton Mifflin Harcourt, 2021.
