Transcript: If planetary scientists are asked to speculate on the most possible sites for life within the solar system, they will generally give five places: Mars, Venus, Europa, Titan, and Io. These five places are significant. Mars is a traditional place where we might imagine life could have existed, and Venus is our sister planet in many ways. However the other three sites are moons in the outer solar system, beyond the habitable zone. Their selection as possible sites for life indicates two things, first, that life can get its energy from a variety of mechanisms. Direct sunlight from a star is not the only possibility. Geothermal energy or tidal heating are also possible. Second, we are informed by the existence of extremophiles on Earth that can subsist under a wide range of physical conditions that the range of conditions in the solar system that could harbor biochemical life is also quite large, so we should be generous in including sites for life where conditions might be quite extreme.