'Oumuamua

Summary

In 2017 an object passed through the Solar System on a trajectory that showed it had come from outside it. It was the first such visitor ever detected. It was extraordinarily elongated, it tumbled, and as it left it accelerated: it picked up speed that gravity alone cannot account for.

Comets do that, because they outgas, and the escaping gas pushes. But 'Oumuamua had no visible coma, no tail, and no detectable outgassing of any kind. It accelerated like a comet while showing none of the evidence of being one. Since 2023 there has been a mechanism that reconciles the two, and it is contested.

What is documented

The trajectory, which is the one thing beyond dispute. 1I/'Oumuamua was moving too fast, on too hyperbolic a path, to be bound to the Sun. It came from interstellar space, passed through, and left. It is the first object of interstellar origin ever observed, and it is the origin of the "1I" designation, an entirely new class.

The shape. Its brightness varied enormously as it tumbled, by a factor of about ten. That implies an extreme axis ratio: something long and thin, or flat and wide. Nothing in the Solar System has that shape.

The acceleration. As it departed, 'Oumuamua's motion deviated from a purely gravitational trajectory. The deviation is small, and it is unambiguous.

The absence, which is the anomaly. Every standard tracer of cometary activity was looked for and none was found. No coma. No dust tail. No emission from the molecules comets usually shed. The infrared limits were tight.

So the object accelerated as though it were venting something, and nothing could be seen venting.

The window closed. 'Oumuamua was identified on the way out. It was observable for a matter of weeks, by a handful of instruments, and then it was gone. It will not come back. Every question about it must be answered from a small and fixed body of data, and that constraint is permanent.

Leading explanations

Trapped hydrogen, released by warming (leading, contested). In March 2023, Jennifer Bergner and Darryl Seligman published a mechanism in Nature that requires no exotic material and no exotic origin.

A body of ordinary water ice, drifting through interstellar space for a very long time, is bombarded by cosmic rays. Laboratory work from the 1970s, 1980s and 1990s had already established what happens: the radiation splits water molecules, producing molecular hydrogen, and that hydrogen becomes trapped inside the ice. Cosmic rays penetrate tens of metres, and a substantial fraction of the water can be converted this way.

When such a body approaches a star, the ice warms and reorganises, and the trapped hydrogen escapes. That escaping hydrogen provides the push.

The mechanism explains the anomaly precisely, because molecular hydrogen is close to invisible. It neither emits nor reflects appreciably at the wavelengths anyone was watching. An object outgassing hydrogen would accelerate exactly as 'Oumuamua accelerated, and would show exactly the empty coma that 'Oumuamua showed.

The objection. The calculation has been challenged. A subsequent analysis argued that Bergner and Seligman had neglected the cooling caused by the evaporating hydrogen itself, and that including it lowers the surface temperature substantially, which in turn reduces the thrust the mechanism can deliver. The dispute is live. The registry records the hypothesis as the leading one and does not record it as settled.

Hydrogen and nitrogen icebergs (superseded). Before 2023, the proposals that could produce enough thrust required the object to be made of solid molecular hydrogen, or solid nitrogen chipped off an exo-Pluto. Both would work. Both require formation conditions that have never been observed and that are difficult to arrange. The strength of the radiolytic hydrogen model is that it needs only water ice and time, both of which are abundant.

Artificial origin. Avi Loeb of Harvard has argued that the acceleration is consistent with radiation pressure on a thin, artificial structure, and has pressed the case in public and in print. It is a hypothesis the data does not exclude, and it is not the leading one. The registry notes that a natural mechanism now exists which accounts for the observations, and that the existence of such a mechanism is the ordinary reason to prefer it.

Dark comets, which may be the real discovery. Working from the same problem, Bergner, Seligman and colleagues went looking for other small bodies that accelerate without a visible coma. They found them. Several have now been identified, and one of them, 1998 KY26, spent decades classified as an asteroid before being recognised as a comet in December 2022. It is a target of the Hayabusa2 extended mission.

If that holds, 'Oumuamua was not a freak. It was the first member of a class we had been failing to notice, and the anomaly was in our categories rather than in the object.

Current status

Partially explained. There is a strong, published, natural mechanism that accounts for the two facts that made 'Oumuamua strange, and it is under active challenge in the literature. The object itself is gone and cannot be re-observed, so the case will be settled, if it is settled, by studying the other members of the class it turned out to belong to.

The registry keeps it because of what it teaches about anomalies in general. 'Oumuamua looked unique. It was probably just first.

Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.