What Is Habitable

For present purposes, we shall enlarge on our definition of a habitable planet (a planet on which large numbers of people could live without needing excessive protection from the natural environment) to mean that the human population must be able to live there without dependence on materials brought from other planets. In other words, a planet that is habitable can supply all of the physical requirements of human beings and provide an environment in which people can live comfortably and enjoyably. At present, the Earth is the only such planet that we know. Yet it is reasonable to suppose that there may be many other planets in the universe that have the proper combination of surface environmental conditions and on which human beings could live as pleasantly and as freely as they do on the Earth. It is one objective of this book to specify in a quantitative manner, insofar as possible, precisely what astronomical conditions must be met for a planet to be called "habitable." First, let us itemize the specific requirements necessary for planets to be habitable for man.

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