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Published online by Cambridge University Press: 10 August 2009
This chapter gives an overview about planetary habitability, which is based on the assumption that a habitable planet is one that supports liquid water on at least part of its surface. The factors that have kept Earth habitable throughout most of its life are reviewed, as well as those that made present-day Mars and Venus uninhabitable. These serve also as indicators for the expected width of the habitable zone around solar-like stars. The last two sections cover the causes for low abiotic O2 abundances expected in Earth's early atmosphere, and the (biological) origin of the current high concentrations of O2 and O3. Implications for the detectability of biological activity on extrasolar planets are discussed.
Introduction
In this chapter, I have tried to present an overview of the topic of planetary habitability. This topic can be broken down into three related questions: (1) what are the factors that have kept the Earth habitable throughout most of its lifetime? (2) what has caused our neighbouring planets, Mars and Venus, to be uninhabitable? and (3) what are the chances that habitable planets exist around other main sequence stars, and how might we tell if they are inhabited? I will briefly address each question, recognizing that it will be impossible to do justice to any of them in the space of one short chapter. References to the relevant literature are provided, and this should allow the interested reader to pursue these topics further.
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