What Kind of Planet Can Support Life?

You may have read stories about extraterrestrials or aliens, and alien fiction is often seen in films. At present, however, life is not known to exist on any planet other than Earth. Scientists continue to research whether life or aliens inhabit other planets and celestial bodies outside our solar system. Even if life exists on planets millions of light-years away, it cannot be observed directly; scientists estimate its possibility by studying atmospheric conditions and other planetary features.
Scientists have determined the conditions under which life is possible on a planet. A planet must not be so close to its star that it experiences extreme heat, nor so far that it suffers extreme cold. The planet must have water. Since living organisms are carbon-based, the planet must also contain carbon, along with solid ground for organisms to inhabit—some planets are merely giant balls of gas and clouds. Scientists have identified thousands of candidate planets meeting such conditions, known as extrasolar planets (exoplanets). However, a planet inhabited by alien life has not yet been found.