'Oumuamua
‘Oumuamua is the first known interstellar object, which passed through the Solar System in 2017. Because of its unusual shape and its motion deviating slightly from gravity, astrophysicist Avi Loeb also raised the possibility of an artificial origin, but most scientists consider it a natural object.
‘Oumuamua (official designation: 1I/2017 U1) is the first object observed in the Solar System that is considered proven to have come from outside the Solar System. It was discovered on October 19, 2017, with the Pan-STARRS1 telescope in Hawaii. The name is of Hawaiian origin and means roughly a first messenger arriving from afar.
What was observed
The object passed through the Solar System at high speed on a hyperbolic trajectory. When it was discovered, it had already passed its perihelion and was moving away, so it could only be observed for a few weeks. Its brightness changed by roughly a factor of ten over a few hours, from which researchers inferred a highly elongated, cigar-shaped or flat, pancake-like body tumbling as it rotated. Its motion deviated slightly from a purely gravitational trajectory, but no tail or outgassing characteristic of comets was detected.
Interpretations
Most of the scientific community considers it to be of natural origin. Several explanations have been proposed for the trajectory deviation: according to some, it may have been a fragment of nitrogen ice broken off the surface of a Pluto-like body. According to a 2023 study, hydrogen formed in the ice by cosmic radiation and then released in the Sun's warmth may have caused the acceleration. Many now consider the latter the most convincing explanation, but none of them is universally accepted.
According to Avi Loeb, an astrophysics professor at Harvard University, an artificial origin, for example a thin light sail, cannot be ruled out either. In 2021, Loeb also launched the Galileo Project, which searches for unidentified aerial phenomena using scientific instruments. Most researchers do not share his view. According to NASA's official finding, there is no evidence that the object is artificial.
Since then, further interstellar objects have been discovered: comet 2I/Borisov in 2019, and 3I/ATLAS in 2025. In the case of the latter, Loeb again raised the possibility of an artificial origin.
The topic has also received considerable attention among UFO/UAP enthusiasts because ‘Oumuamua is one of the few phenomena about which recognized scientists have publicly debated the possibility of a non-natural origin.