维基百科上的,很科学: Extraterrestrial life is defined as life which does not originate from planet Earth. The existence of such life is theoretical and all assertions about it remain disputed. Hypotheses regarding the origin(s) of extraterrestrial life, if it exists, are as follows: one proposes that it may have emerged, independently, from different places in the universe. An alternative hypothesis is panspermia or exogenesis, which holds that life emerges from 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. Speculated forms of extraterrestrial life range from life with the simplicity of bacteria to sapient or sentient beings. Suggested locations which might have once developed, or presently continue to host life similar to our own, include the planets Venus [1] and Mars, moons of Jupiter and Saturn (e.g. Europa, [2] Enceladus and Titan) and Gliese 581 c and d, recently discovered to be near Earth-mass extrasolar planets apparently located in their star's habitable zone, and with the potential to have liquid water. [3] To date, no credible evidence of extraterrestrial life has been discovered which has been generally accepted by the mainstream scientific community. All other proposals, including beliefs that some UFOs are of extraterrestrial origin (see extraterrestrial hypothesis) and claims of alien abduction, are considered hypothetical by most scientists. UFO sightings are sightings of unidentified flying objects that may or may not be connected with extraterrestrial intelligent life. Most of these sightings can be dismissed as sightings of Earth based aircraft or known astronomical objects, or perpetration of hoaxes. Some sightings have remained unexplained, in some cases having been reported by trained professionals.
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