General
Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking,[6] as well as well-regarded thinkers such as Winston Churchill,[7][8] it would be improbable for life not to exist somewhere other than Earth.[9][10] This argument is embodied in the Copernican principle, which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle, which states that there is nothing special about life on Earth.[11] The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old.[12][13] Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia.[14][15] In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth.[16] According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.[16]
Since the 1950s, scientists have proposed that "habitable zones" around stars are the most likely places to find life. Numerous discoveries in such zones since 2007 have generated numerical estimates of Earth-like planets —in terms of composition—of many billions.[17] As of 2013, only a few planets have been discovered in these zones.[18] Nonetheless, on 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way,[19][20] 11 billion of which may be orbiting Sun-like stars.[21] The nearest such planet may be 12 light-years away, according to the scientists.[19][20] Astrobiologists have also considered a "follow the energy" view of potential habitats.
