Abstract:

One of the big puzzles of mankind is the question: Are we alone in the universe? Presently there is no contact to extraterrestrial organisms and no trace of them has ever been found. Despite of this it is possible to pose convincing arguments for the existence of intelligent societies in the vast expanse of our universe. An estimate of the number of planets is given on which life is possible. This estimate is based on recent insights into the theory of planet formation from which can be concluded that there must be a very large number of so-called habitable planets in the universe. The conditions for the formation and evolution of life and especially of intelligent life are discussed. From these arguments it appears highly probable that life and intelligent life does occur outside the solar system. Why then have we not been visited by extraterrestrial beings and why have the ongoing search programs for  extraterrestrial intelligence (SETI) failed?