Aliens - 1

Alien life, such as bacteria, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.[1][2] This argument is embodied in the Copernican principle, which states that the Earth does not occupy a unique position in the Universe, and the mediocrity principle, which holds that there is nothing special about life on Earth.[3] 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.[4] In any case, complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth based on computer model studies.[5] According to these studies, this same process may also occur around other stars that acquire planets.[5] (Also see Extraterrestrial organic molecules.)
Suggested locations at which life might have developed include the planets Venus[6] and Mars, Jupiter's moon Europa,[7] and Saturn's moons Titan and Enceladus.[8] 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".[9][10] 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.[11] In December 2011, scientists working with NASA’s Kepler space telescope announced the discovery of Kepler-22b, an exoplanet that appears to be orbiting a sun-like star within the habitable zone.[12]
No widely accepted evidence of extraterrestrial life has been found; however, various controversial claims...