Oumuamua

Oumuamua (actual spelling: 'Oumuamua) is a small minor-planet-like body not of the solar system, but which passed through it in 2017, the first such object identified as such. Its speed was too great for a body orbiting the Sun, i.e., more than the Sun's escape velocity, and its path was hyperbolic rather than elliptical. This, along with other characteristics led to the conclusion that it is not a solar system object. Constant changes in its brightness indicated it is asymmetric and rotating, and further, that it is elongated, apparently solid. Its longest dimension is thought to be less than a kilometer. Though it basically followed the path of a body affected Sun's gravitational field, it did show some slight discrepancy, which aroused interest: there must have been some additional source of acceleration. (Comets have shown the same characteristic, attributed to outgassing.)

Its discovery necessitated adoption of a means to designate such discovered bodies: like minor planets, they will incorporate a number (based upon discovery order if possible, thus the "1"), with the number followed by "I" for interstellar, making it 1I/'Oumuamua. It initially was given a designation indicating a comet, then one indicating an asteroid, before its interstellar trajectory was confirmed and the new convention was settled upon. As of 2024, just one more such interstellar body has been identified, 2I/Borisov in 2019.