How Probable Is It That Life Exists Somewhere Else in the Universe?

What is the chance of success in the search for extraterrestrial intelligence? The answer to this question depends on a series of probabilities. My methodology consists in asking a series of questions which narrow down the probability of success.

Even most skeptics of the SETI project will answer the above question affirmatively. Molecules that are necessary for the origin of life, such as amino acids and nucleic acids, have been identified in cosmic dust, together with other macromolecules, so that it would seem quite conceivable that life could originate elsewhere in the universe. Some of the modern scenarios of the origin of life start out with even simpler molecules, which makes an independent origin of life even more probable. Such an independent origin of life, however, would presumably result in living entities that are drastically different from life on Earth.

Where Can One Expect To Find Such Life?

Obviously only on planets. Even though we have up to now secure knowledge only of the nine planets of our solar system, there is no reason to doubt that in all the galaxies there must be millions if not billions of planets. The exact figure, e.g. for our own Galaxy, can only be guessed.