An Earth analog (also referred to as an Earth analogue, Earth twin, or Earth-like planet, though this latter term may refer to any terrestrial planet) is a planet or moon with environmental conditions similar to those found on Earth. The possibility is of particular interest to astrobiologists and astronomers under reasoning that the more similar a planet is to Earth, the more likely it is to be capable of sustaining complex extraterrestrial life. As such, it has long been speculated and the subject expressed in science, philosophy, science fiction and popular culture. Advocates of space colonization have long sought an Earth analog as a "second home", while advocates for space and survival would regard such a planet as a potential "new home" for humankind.
Before the scientific search for and study of extrasolar planets, the possibility was argued through philosophy and science fiction. The mediocrity principle suggests that planets like Earth should be common in the universe, while the Rare Earth hypothesis suggests that they are extremely rare. The thousands of exoplanetary star systems discovered so far are profoundly different from our solar system, supporting the Rare Earth hypothesis.
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