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
 as well as well-regarded thinkers such as
 it would be improbable for life not to exist somewhere other than Earth.
 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.
chemistry of life may have begun shortly after the
13.8 billion years ago, during a habitable epoch when the
universe was only 10–17 million years old.
 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
 In any case,
may have formed in the
protoplanetary disk of
dust grains surrounding the
Sun before the formation of Earth.
 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.
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.
 As of 2013, only a few planets have been discovered in these zones.
 Nonetheless, on 4 November 2013, astronomers reported, based on
Kepler space mission data, that there could be as many as 40 billion
planets orbiting in the
habitable zones of
Sun-like stars and
red dwarfs in the
 11 billion of which may be orbiting Sun-like stars.
 The nearest such planet may be 12 light-years away, according to the scientists.
 Astrobiologists have also considered a "follow the energy" view of potential habitats.
A study published in 2017 suggests that due to how complexity evolved in species on Earth, the level of predictability for alien evolution elsewhere would make them look similar to life on our planet. One of the study authors, Sam Levin, notes "Like humans, we predict that they are made-up of a hierarchy of entities, which all cooperate to produce an alien. At each level of the organism there will be mechanisms in place to eliminate conflict, maintain cooperation, and keep the organism functioning. We can even offer some examples of what these mechanisms will be."
 There is also research in assessing the capacity of life for developing intelligence. It has been suggested that this capacity arises with the number of potential
niches a planet contains, and that the complexity of life itself is reflected in the information density of planetary environments, which in turn can be computed from its niches.