ABSTRACT
Theories regarding extraterrestrial intelligent life-forms must be supported by evidence of potentially habitable sites, such as stable planets in a moderate temperature zone. This chapter surveys contemporary claims concerning potentially habitable planets, theories of planetary formation and methods of observing ex-solar planets. The investigation then focuses on the development of life, stressing an important distinction between conditions which may support the origins of life and conditions which life can tolerate, reorganize or adapt. Given the fact that there are probably many more sites where life can survive than those where life can be produced from scratch, the thesis that life on Earth is the result of seeding is defended as a more plausible account of the development of life on Earth. Arguments concerning the contingency of life and the tenacity of life are assessed and the emergence of intelligent life and exotic life-forms is also considered.
