Before one can successfully search for life on other planets, one must understand the characteristics of a system that allow it to be habitable for life. In general, the habitability of a planet or moon depends on its size and the distance from the star that it orbits. Planets must be large enough to maintain an atmosphere and liquid solvents. Planets must also have a suitable temperature under which life can survive. Therefore, the distance at which a planet orbits its sun and the spectral class of the central star are significant in determining the habitability of a planet. It turns out that there is a particular region about any given star in which the temperatures neither rise so high that water cannot exist as a liquid on the surface nor drop so low that the planet freezes permanently. This region is called the habitable zone and can be visualized as a spherical shell around a star. For more detailed information about habitable zones around stars, check out the following websites:
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