Extraterrestrial intelligence
Extraterrestrial intelligence is a general term for the existence of intelligent life that originated on other than the Earth. The term is occasionally used to refer to paranormal intelligence that exists on Earth and elsewhere, but the major scientific emphasis is on life forms completely separate from the Earth. Search for Extraterrestrial Intelligence programs are systematic observations to detect such life.
The scientific argument for searching for life, according to the SETI Institute, is that
Over the last half-century, scientists have developed a theory of cosmic evolution that predicts that life is a natural phenomenon likely to develop on planets with suitable environmental conditions. Scientific evidence shows that life arose on Earth relatively quickly, suggesting that life will occur on similar planets orbiting sun-like stars. With the recent discoveries of extrasolar planetary systems, and the suggestive evidence that life may once have existed on Mars, this scenario appears even more likely.
Additionally, one should keep in mind that we are only one planet around a very ordinary star. There are roughly 400 billion other stars in our Galaxy, and nearly 100 billion other galaxies. It would be extraordinary if we were the only thinking beings in all these enormous realms.[1], III. Background and Rationale