What Makes a World Habitable? [pdf]

I think this is based a lot around Earth paradigms. Microbial life is probably very common through the universe, more complex varieties less so. There does appear to be some evidence of life on/in Mars and in the upper atmosphere of Venus, although this is disputed.

Even on Earth we can find life in harsh environments, a mile down in rock, a mile up in the air, and in the deepest parts of the ocean. Some life tolerates extreme heat and cold, acid/alkaline conditions and high salt levels.