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.
Some people believe that certain unidentified flying objects come from extraterrestrial intelligence.
A conceptual approach often used in discussing the possibility of extraterrestrial intelligence is the Drake Equation, presented in 1961 by Frank Drake of the National Radio Astronomy Observatory. It can only be considered on the lines of a thought experiment. It gives a context for estimating
N, the number of civilization with intelligent life that communicates.
N = N* fp ne fl fi fc fL
- N* represents the number of stars in the Milky Way Galaxy, estimated at 100 billion
- fp is the fraction of stars that have planets around them, with estimates in the 20-50%range
- ne is the number of planets per star that are capable of sustaining life; a common estimate is 1 to 5 per system with planets
- fl is the fraction of planets in ne where life evolves; highly controversial estimates range from 100% (where life can evolve it will) down to close to 0%.
- fi is the fraction of fl where intelligent life evolves, again with guesses between
- fc is the fraction of fi that have the means and desire to communicate, which Drake suggested as 10-20%
- fL is fraction of the planet's life during which the communicating civilizations live. "This is the toughest of the questions. If we take Earth as an example, the expected lifetime of our Sun and the Earth is roughly 10 billion years. So far we've been communicating with radio waves for less than 100 years. How long will our civilization survive? Will we destroy ourselves in a few years like some predict or will we overcome our problems and survive for millennia? If we were destroyed tomorrow the answer to this question would be 1/100,000,000th. If we survive for 10,000 years the answer will be 1/1,000,000th."