Alien life, such as bacteria, has been theorized to exist in the Solar System and quite possibly throughout the Universe. This theory relies on the vast size and consistent physical laws of the observable Universe. According to this argument, supported by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.This argument is embodied in the Copernican principle, which states that the Earth does not occupy a favored position in the Universe, and the mediocrity principle, which holds that there is nothing special about life on Earth. Life may have emerged independently at many places throughout the Universe. Alternatively life may form less frequently, then spread between habitable planets through panspermia or exogenesis.

Suggested locations at which life might have developed, or which might continue to host life today, include the planets Venus and Mars, Jupiter's moon Europa,  and Saturn's moons Titan and Enceladus. In May 2011, NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".Life may appear on extrasolar planets, such as Gliese 581 c, g and d, recently discovered to be near Earth mass and apparently located in their star's habitable zone, with the potential to have liquid water.

No samples of extraterrestrial life have been found. However, various controversial claims have been made for evidence of extraterrestrial life. Beliefs that some unidentified flying objects are of extraterrestrial origin (see Extraterrestrial hypothesis), along with claims of alien abduction, are dismissed by most scientists. Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.