Pre-modern[ edit ] Cosmic pluralismthe assumption that there are many inhabited worlds beyond the human sphere predates modernity and the development of the heliocentric model and is common in mythologies worldwide. The 2nd century writer of satires, Lucian, in his True History claims to have visited the moon when his ship was sent up by a fountain, which was peopled and at war with the people of the Sun over colonisation of the Morning Star.
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 as well as well-regarded thinkers such as Winston Churchill  it would be improbable for life not to exist somewhere other than Earth.
Alternatively, life may have formed less frequently, then spread—by meteoroidsfor example—between habitable planets in a process called panspermia. Numerous discoveries in such zones since have generated numerical estimates of Earth-like planets —in terms of composition—of many billions.
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. 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.
Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth.
It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbonsuch as methaneethane or propane. These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts.
The carbon atom has the unique ability to make four strong chemical bonds with other atoms, including other carbon atoms. These covalent bonds have a direction in space, so that carbon atoms can form the skeletons of complex 3-dimensional structures with definite architectures such as nucleic acids and proteins.
Carbon forms more compounds than all other elements combined. The great versatility of the carbon atom makes it the element most likely to provide the bases—even exotic ones—for the chemical composition of life on other planets.
Planetary habitabilityHabitability of natural satellitesand Exobiology Some bodies in the Solar System have the potential for an environment in which extraterrestrial life can exist, particularly those with possible subsurface oceans.
According to NASA's Astrobiology Strategy, "Life on other worlds is most likely to include microbes, and any complex living system elsewhere is likely to have arisen from and be founded upon microbial life. Important insights on the limits of microbial life can be gleaned from studies of microbes on modern Earth, as well as their ubiquity and ancestral characteristics.
If extraterrestrial life was found on another body in the Solar Systemit could have originated from Earth just as life on Earth could have been seeded from elsewhere exogenesis. The first known mention of the term 'panspermia' was in the writings of the 5th century BC Greek philosopher Anaxagoras.
The Nobel prize winner Francis Crickalong with Leslie Orgel proposed that seeds of life may have been purposely spread by an advanced extraterrestrial civilization,  but considering an early " RNA world " Crick noted later that life may have originated on Earth.
Life on Venus In the early 20th century, Venus was often thought to be similar to Earth in terms of habitability, but observations since the beginning of the Space Age have revealed that Venus's surface is inhospitable to Earth-like life. However, between an altitude of 50 and 65 kilometers, the pressure and temperature are Earth-like, and it has been speculated that thermoacidophilic extremophile microorganisms might exist in the acidic upper layers of the Venusian atmosphere.
Life on Mars Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and now can occasionally be found as low-volume liquid brines in shallow Martian soil. Nevertheless, present conditions on Mars' subsurface may support life.
Scientists have indications that heated subsurface oceans of liquid water may exist deep under the crusts of the three outer Galilean moons —Europa,    Ganymede     and Callisto. Life on Europa Internal structure of Europa.Extraterrestrial life, also called alien life (or, if it is a sentient or relatively complex individual, an "extraterrestrial" or "alien"), is life that occurs outside of Earth .
If extraterrestrials ever landed on Earth, they would find us extremely strange. Their first intimation of our existence might well be a presidential speech or the Olympic Games, a mud wrestling match or perhaps a T.V. show like Ally McBeal or 3rd Rock From the Sun.
World Mysteries Blog. Humanity is totally visible to extraterrestrials According to him, he saw debris from the crashed flying saucer and was told by a friend about corpses of smallish extraterrestrials. On the evening that the alien bodies were recovered, he “blundered” into the Roswell Army Hospital.
Modern World, Science. Life on Earth was created by extraterrestrials who were mistaken for gods by our primitive ancestors. Help us to build an embassy for them!
News › Science World UFO Day How close have we actually got to finding aliens, and what would they be like? Exciting data has emerged from Mars and Enceladus, but extra-terrestrials will.
If There Are Aliens Out There, Where Are They? and Philip Morrison said at the end of their seminal Nature article on searching for extraterrestrials Discover world-changing science.