Extraterrestrial life is life that is not from the planet Earth. It is alien life. It is reasonable that planets rather like ours exist, and that life might evolve there also. So far none has been found, though it is possible that life once existed on Mars.
It is not a new idea. Quite a few philosophers have speculated on the existence of other planets like ours, with the idea that what happened here could also happen there. One thing we do know is that many other star systems do have exoplanets.
- Sagan, Carl & Shklovskii I.S. 1966. Intelligent life in the Universe. Random House.
- Sagan, Carl 1973. Communication with extraterrestrial intelligence. MIT Press. ISBN 0-262-19106-7
- Crowe, Michael J. 1986. The Extraterrestrial life debate, 1750–1900. Cambridge. ISBN 0-521-26305-0
- Crowe, Michael J. 2008. The extraterrestrial life debate Antiquity to 1915: a source book. University of Notre Dame Press. ISBN 0-268-02368-9