Earth is unique in this solar system – it is the only planet that seems to support life. Its hospitable ecosphere stands in stark contrast to the empty, lifeless landscapes of the Moon, Mars, Venus, Mercury and other worlds probed by our spacecraft. Recent arguments suggest that while planets may be common in the universe, habitable worlds may not be. Internally, a candidate planet's proper composition, tectonic dynamics and very narrow but extremely long-period thermal stability may be rare. External biosphere-destroying natural processes, from interstellar dust clouds to sterilizing radiation sources, may periodically rid vast regions of a galaxy of planet-bound life forms.

Such a severe limitation on life-supporting worlds significantly impacts discussions of the Search for Extraterrestrial Intelligence at both ends of the question, the search for causes and the search for consequences. On the former issue, it seriously reduces the stage on which the formation and evolution of life may take its chances against the odds: few candidate planets means even fewer ultimate successes. At the other end of the question, it suggests strategies for searching for the few successful technologies that do evolve, by identifying potential technological activities they would choose to engage in, activities that may have very long range of detectability. The significance to human searches for ETI may be profound.

The issues of ‘probability of intelligence arising’ are dealt with in other chapters. My purpose is to address the question of final consequences.