Of the approximately 1,500 exoplanets found so far, only a few are in the so-called habitable zone around their parent star. It's also called the "Goldilocks zone," a coveted sweet-spot where the amount of radiation coming from a star is "just right" for liquid water to exist on the surface of a rocky planet.
For example, Earth is within the present habitable zone around our sun, which is a comparatively narrow swath 18 million miles across.
Read More: http://news.discovery.com/space/planetary-habitable-zones-defined-by-alien-biochemistry-111118.html
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