An alien is defined as a creature from outer space, according to Reference.com. Although there is no specific confirmation of the existence of aliens, scientists do have several theories about where in outer space they are most likely to live. Different theories postulate that aliens may live in a parallel universe or that they may even be present on Earth.
Mother Nature Network says that Jupiter's moon, Callisto, is one possible place in outer space where aliens may live. This moon is more likely to have temperatures suitable for life than Jupiter's other moons because it is further away from Jupiter and experiences less tidal heating. Scientists believe that a liquid layer of water exists under the surface of Callisto. Water is essential for life, so this contributes to the likelihood that Callisto could support living beings.
Although aliens are originally from outer space, some theorize that aliens live on Earth. Sightings of aliens and alien ships have been reported on several occasions, according to About.com. Some believe that aliens live underground or in bases under the sea, and this is why they are rarely seen. Theories also propose that governments know of the existence and location of aliens, but they conceal this information from the public.