Life can be distinguished from nonliving things by its ability to reproduce and evolve. All terrestrial life is based on carbon atoms and water, but it may be possible that there are extraterrestrial life-forms based on substances other than carbon and water.
Life on Earth began within a billion years or less after the Earth formed. We have no conclusive evidence that life originated elsewhere in the solar system though it seems possible that life may have developed on Mars and Europa.
Spectroscopic observations have recently led to the discovery of Jupiter-like planets orbiting many nearby solar-type stars. We can now be confident that many single stars like the Sun have planetary systems.
Stars like the Sun have habitable zones large enough that they are likely to encompass habitable planets in many planetary systems. Our current ideas about the origin of life on Earth suggest that life may have begun on many planets in the galaxy. We are now too uncertain about the durability of life and the inevitability of evolution of intelligence to make reliable estimates of how common life and intelligent life are in the universe.
Radio communication using powerful transmitters and large radio telescopes seems to be the best way for civilizations in different planetary systems to communicate with each other and with us. In order to detect signals from extraterrestrials, however, we need to know what frequency to use and in what direction or directions to search. There have been many attempts, without success, to detect radio signals from other civilizations. The distances between us and the nearest civilizations are likely to make it difficult to carry out a conversation.
