Alien Imagination

Suggested locations on which life might have developed, or which might continue to host life today, include the planets Venus and Mars; moons of Jupiter and Saturn such as Europa, Enceladus and Titan; and extrasolar planets such as Gliese 581 c 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.
Beliefs that some unidentified flying objects are of extraterrestrial origin (extraterrestrial hypothesis), along with claims of alien abduction, are considered spurious by most scientists. Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes. Some sightings have remained unexplained, in some cases having been reported by trained professionals.
Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint.