Extraterrestrial life, also called alien life, is life that does not originate from Earth. These hypothetical life forms may range from simple single-celled organisms to beings with civilizations far more advanced than humanity, although many scientists expect extraterrestrial life to exist in some form, there is no evidence for its existence to date. The Drake equation speculates about the existence of intelligent life elsewhere in the universe, the science of extraterrestrial life in all its forms is known as exobiology. The science of astrobiology considers life on Earth as well, in 2015, remains of biotic life were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old. According to one of the researchers, If life arose relatively quickly on Earth and it has also played a major role in works of science fiction. Over the years, science fiction works have increased the publics interest in the possibility of extraterrestrial life, some encourage aggressive methods to try for contact with intelligent extraterrestrial life, whereas others argue that it might be dangerous to actively call attention to Earth. Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and this hypothesis relies on the vast size and consistent physical laws of the observable universe. The chemistry of life may have begun shortly after the Big Bang,13.8 billion years ago, Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread—by meteoroids, in any case, complex organic molecules 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, since the 1950s, scientists have argued the idea that habitable zones around stars are the most likely places to find life. 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. Life on Earth requires water as its solvent in which biochemical reactions take place, More generally, life based on ammonia has been suggested, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, about 29 chemical elements play an active positive role in living organisms on Earth. About 95% of this matter is built upon only six elements, carbon, hydrogen, nitrogen, oxygen, phosphorus. These six elements form the building blocks of virtually all life on Earth. The unique characteristics of carbon make it unlikely that it could be replaced, even on another planet, 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, carbon forms more compounds than all other elements combined. The great versatility of the atom makes it the element most likely to provide the bases—even exotic ones—for the chemical composition of life on other planets
Image: Telescope Kepler NASA
Electron micrograph of martian meteorite ALH84001 showing structures that some scientists think could be fossilized bacteria-like life forms.