Planetary habitability is the measure of a planet’s or a natural satellite’s potential to have habitable environments hospitable to life or its ability to generate life endogenously.
A planet in the habitable zone has no guarantee of actually being habitable, and habitable environments do not need to contain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a hypothetical process known as panspermia.
As the existence of life beyond Earth is unknown, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favorable to life’s flourishing—in particular those factors that have sustained complex, multicellular organisms and not just simpler, unicellular creatures.
Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology.
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