Extraterrestrial intelligence (often abbreviated
ETI) refers to hypothetical
intelligent extraterrestrial life. The question of whether other inhabited worlds might exist has been debated since ancient times. The modern form of the concept emerged when the
Copernican Revolution demonstrated that the Earth was a planet revolving around the Sun, and other planets were conversely, other worlds. The question of whether other inhabited planets or moons exist was a natural consequence of this new understanding. It has become one of the most speculative questions in science and is a central theme of science fiction and popular culture.