[BrifsGefel]

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.)





http://en.wikipedia.org/wiki/Extraterrestrial_life
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An hypothesis that could well be shown to be valid before this century is out, and one that echos my own thoughts on the subject, with a leaning towards life arising independently in many places and spread further via "Panspermia".

Again the sheer vastness and great distances between stellar objects and there planets, makes contact difficult.
Again time and distance are the barriers, but also a sufficiently advanced civilisation could overcome these to a certain extent......Maybe a class 2 or 3 civilisation.
We of course have yet to get to class 1, but barring catastrophic calamity I think we will make it before the millenium is half way over.