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( Extraterrestrial life)
Extraterrestrial life is life originating outside of the Earth. It is the subject of astrobiology, and its existence remains hypothetical. There is no credible evidence of extraterrestrial life that has been widely accepted by the scientific community. There are several hypotheses regarding the origin of extraterrestrial life if it exists. One proposes that it may have emerged, independently, in different places in the universe. An alternative hypothesis is panspermia, which holds that life emerging in one location then spreads between habitable planets. These two hypotheses are not mutually exclusive. The study and theorization of extraterrestrial life is known as astrobiology, exobiology or xenobiology. Speculative forms of extraterrestrial life range from sapient or sentient beings to life at the scale of bacteria. Suggested locations that might have once developed or continue to host life include the planets Venus[1] and Mars, moons of Jupiter and Saturn (e.g. Europa,[2] Enceladus and Titan). Gliese 581 c and d, recently discovered to be near Earth-mass extrasolar planets apparently located in their star's habitable zone, and having the potential to have liquid water.[3] Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary, or morphological viewpoint. All life on Earth requires carbon, hydrogen, oxygen, nitrogen, and phosphorus; it also requires water as the solvent in which biochemical reactions take place. Sufficient quantities of carbon and the other major life-forming elements along with water may enable the formation of living organisms on other planets with a chemical make up and average temperature similar to Earth. Because Earth and other planets are made up of "star dust" ie, relatively abundant chemical elements formed from stars which have ended their life as supernova, it is very probable that other planets may have been formed by elements of a similar composition as Earth. The combination of carbon and water in the chemical form of carbohydrates (e.g., sugar), can be a source of chemical energy on which life depends, and also provide structural elements for life (such as ribose, in the molecules DNA and RNA and cellulose in plants). Plants derive energy through the conversion of light energy into chemical energy via photosynthesis. Life requires carbon in both reduced (methane derivatives) and partially-oxidized (carbon oxides) states. It also requires nitrogen as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins, and phosphorus oxidized to phosphates in genetic material and in energy transfer. Adequate water as a solvent supplies adequate oxygen as constituents of biochemical substances.
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