"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.
[3][4] 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.
"
Taken right from wikipedia.