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The Habitable Zone
Habitable Zone Illustration From the center of the Earth to the far-flung galaxies, we find evidence that life arose from cosmic processes. The iron in our blood and the calcium in our bones was made inside stars. All silver and gold was forged by stars that exploded long ago. Terrestrial life is embedded in a cosmic web, and it seems reasonable to speculate that life is cosmically commonplace.

If life is in fact cosmically commonplace, where might we find it? Our search begins within the solar system, as we try to locate three ingredients upon which life depends: water, energy, and organic molecules (or carbon). Recent discoveries inform us that these requisites may exist well beyond the planets closely orbiting the sun. This area — where conditions might potentially support life — is called The Habitable Zone.

Previous Definition
The habitable zone first encompassed the orbits of Venus to Mars, planets close enough to the sun for solar energy to drive the chemistry of life — but not so close as to boil off water or break down the organic molecules on which life depends.
Expanding Definition
But the habitable zone may be larger than originally conceived. The strong gravitational pull caused by large planets may produce enough energy to sufficiently heat the cores of orbiting moons. Life has proven itself tough here on Earth. Perhaps it could thrive in more extreme environments.
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