Oumuamua: a brief history.

 ʻOumuamua is the first known interstellar object detected passing through the Solar System. Formally designated 1I/2017 U1, it was discovered by Robert Weryk using the Pan-STARRS telescope at Haleakalā Observatory, Hawaii, on 19 October 2017, approximately 40 days after it passed its closest point to the Sun on 9 September. When it was first observed, it was about 33 million km (21 million mi; 0.22 AU) from Earth (about 85 times as far away as the Moon), and already heading away from the Sun.

ʻOumuamua is a small object estimated to be between 100 and 1,000 metres (300 and 3,000 ft) long, with its width and thickness both estimated to range between 35 and 167 metres (115 and 548 ft). It has a red color, similar to objects in the outer Solar System. Despite its close approach to the Sun, ʻOumuamua showed no signs of having a coma, but it did exhibit non‑gravitational acceleration. Nonetheless, the object could be a remnant of a disintegrated rogue comet (or exocomet), according to astronomer Zdenek Sekanina. The object has a rotation rate similar to the average spin rate seen in Solar System asteroids, but many valid models permit it to be more elongated than all but a few other natural bodies. While an unconsolidated object (rubble pile) would require it to be of a density similar to rocky asteroids, a small amount of internal strength similar to icy comets would allow a relatively low density. ʻOumuamua's light curve, assuming little systematic error, presents its motion as "tumbling", rather than "spinning", and moving sufficiently fast relative to the Sun that it is likely of an extrasolar origin. Extrapolated and without further deceleration, ʻOumuamua's path cannot be captured into a solar orbit, so it would eventually leave the Solar System and continue into interstellar space. ʻOumuamua's planetary system of origin and the age of its excursion are unknown.

In July 2019, astronomers concluded that ʻOumuamua is most likely a natural object. A small number of astronomers suggested that ʻOumuamua could be a product of alien technology, but evidence in support of this hypothesis is weak. In March 2021, scientists presented a theory based on nitrogen ice that ʻOumuamua may be a piece of an exoplanet similar to Pluto, from beyond our solar system.