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 and Mars, moons of Jupiter and Saturn (e.g. Europa, 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.
Communication with Extraterrestrial Intelligence
From the nineteenth century and onwards there were many books and articles about the possible inhabitants of other planets. Many people believed that intelligent beings might live on the Moon, Mars, and Venus; but since travel to other planets was not yet possible, some people suggested ways to signal the extraterrestrials even before radio was discovered. But when the Martian canals proved illusory, it seemed that humans were alone in the solar system.
CETI (Communication with Extraterrestrial Intelligence) is a branch of SETI research that focuses on composing and deciphering messages that could theoretically be understood by another technological civilization. The best-known CETI experiment was the 1974 Arecibo message composed by Frank Drake and Carl Sagan.
CETI research has focused on four broad areas: mathematical languages, pictorial systems such as the Arecibo message, algorithmic communication systems (ACETI) and computational approaches to detecting and deciphering 'natural' language communication.
See also: Extraterrestrial Life
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