The discovery of extremophilic life lends major support to the first part of the Rare Earth Hypothesis. The almost ubiquitous presence of extremophiles on
Earth in regions previously thought too hot, cold, acidic, basic, or saline shows that (at least in microbial form) life can exist in a much wider range of habitats than previously thought. This is the strongest evidence that life might be widespread in the Universe (and thus perhaps widespread in the solar system). But there is a second major implication of the discovery of ex-tremophiles: They show that life can exist well above and below the temperature range (32-212°F) that allows for the existence of liquid water at a pressure of 1 atmosphere, the conditions found in what has been called the habitable zone. The extremophiles have rendered the original concept of the habitable zone obsolete. In our solar system, surface water exists only on Earth (and perhaps on Europa), so if we assume that we will find life only on planets with water, then we would have to conclude that only these two bodies should harbor life of any sort. The discovery of the extremophiles requires us to revise that thinking. Let us keep this in mind as we examine, in Chapter 2, the concept of habitable zones.
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