|MadSci Network: Astronomy|
As far as we know, life would most likely be found on a planet in a stable orbit around a star with a long and steady lifespan. Presumably this would be a star like the Sun, with a lifespan of about 10 billion years. Each star is believed to have a habitable zone, i.e., a distance at which a planet would be neither too hot nor too cold for liquid water to exist for long periods of time. In the case of the Sun, the habitable zone probably extends from somewhat outside of Venus' orbit to about Mars' orbit. (It certainly has to include the Earth!) Although a number of stars with planets around them are now known, in many cases the planets appear to be too close to their star to be in that star's habitable zone. (However, with the possibility that Europa has an ocean under an icy crust, some astronomers have been suggesting that the size of the Sun's habitable zeon much be much larger than originally thought.) No Earth-like planets inside a star's habitable zone have yet been found, but this is mainly because human telescopes and equipment are not yet powerful enough to find them.
The whole question of what is needed for life to get started, and to allow evolution and growth, is part of the science of astrobiology and is being actively studied by NASA and other Institutes around the world.
Highly technological life on the other hand might more probably be found off the surface of planets in outer space, since it is easier to expand using undiluted solar energy and raw materials mined from small planets and comets. (Also, it is hypothesized that a civilization's growth may ultimately be limited by how much energy, like sunlight, it can obtain.) These civilizations would be found not on the surfaces of large planets, but in asteroid sized semi-artifical habitats detectable by their waste heat radiated out into deep space!
Civilizations like ours might be quite fragile. In our case, there is some concern that we are in growing danger from natural and human disasters.
Expanding into space would also protect the human species from extinction by an asteroid or comet impact. A wise cook does NOT keep all his eggs in one basket!
Carl Sagan & Josef Schlovskii "Intelligent Life in the Universe" (Holden Day: London) 1966 (many of Carl Sagan's later books also address these issues)
Gerard K. O'Neill "The High Frontier" (Anchor Books, Doubleday: New York) 1974
Michael Martin-Smith "Salto nello Spazio" to be published in Rome Dec 1999 by Tre Editori, via Principe Umberto 35, 00185 Roma, Italy
Try the links in the MadSci Library for more information on Astronomy.