'Oumuamua

'Oumuamua is the first object confirmed to have entered our solar system from interstellar space. Detected in October 2017, it showed a set of unusual properties that most astronomers attribute to natural causes and that Harvard astronomer Avi Loeb argued could indicate an artificial origin. The debate it started reshaped how science talks about searching for extraterrestrial technology.

On October 19, 2017, astronomer Robert Weryk spotted a fast-moving object in data from the Pan-STARRS1 telescope in Hawaii. Its trajectory showed it was not bound to the Sun. It had come from interstellar space and would return there. The International Astronomical Union designated it 1I/2017 U1 and it was named 'Oumuamua, Hawaiian for "a messenger from afar arriving first." By the time it was detected it had already passed its closest point to the Sun and was leaving the solar system at over 87 kilometers per second.

Weryk and the Pan-STARRS team made the discovery. Observatories worldwide scrambled to study the object during the few weeks it remained observable. Avi Loeb, then chair of Harvard's astronomy department, co-authored a 2018 paper with Shmuel Bialy proposing that its anomalous acceleration could be explained if it were a thin artificial structure, such as a light sail. Mainstream teams led by researchers including Karen Meech and the International Space Science Institute's 'Oumuamua working group published natural-origin analyses.

The anomalies are not disputed, only their interpretation. 'Oumuamua's brightness varied by a factor of about ten, implying an extremely elongated or flattened shape unlike any known asteroid or comet. It accelerated slightly as it left the Sun's vicinity, the kind of push comets get from outgassing, yet no telescope detected a coma, tail, or gas emissions. It was small, estimated at 100 to 400 meters, and tumbling. Proposed natural explanations include a hydrogen or nitrogen ice body, a fluffy dust aggregate, and a fragment of a tidally disrupted planetesimal.

There is no government file on 'Oumuamua. It is a fully public astronomical event. The 2019 ISSI working group paper in Nature Astronomy concluded there is "no compelling evidence to favor an alien explanation," while acknowledging the object remains unusual. NASA describes it as a natural interstellar object of undetermined composition.

Most astronomers say anomalous does not mean artificial, and that a sample size of one interstellar object makes "unusual" statistically meaningless. Critics of Loeb's argument, including Meech and astrophysicist Steve Desch, say the natural models fit the data without invoking technology, and some accused Loeb of bypassing peer consensus for public advocacy. Loeb answered that dismissing the hypothesis without testing it is the unscientific position, and founded the Galileo Project in 2021 to search for interstellar artifacts systematically. The second and third confirmed interstellar objects, comet 2I/Borisov in 2019 and 3I/ATLAS in 2025, behaved like ordinary comets, which both sides cite: skeptics as evidence interstellar visitors are natural, Loeb as evidence 'Oumuamua was different.