Aliens refer to extraterrestrial life which is not originated here in the earth. Many questions have been raised such as are we alone in the universe? Or are there any intelligent creatures beyond us? Many scientists are working hardly to get knowledge about extraterrestrial life. Finding life beyond the Earth is one of the great challenge to modern science. The possibility that virus might exist extra terrestrially also has been proposed.
Many scientists consider possibility of existence of extraterrestrial life to be plausible but there is no direct evidence of its existence. There has been an ongoing search for signs of extraterrestrial life, from radios used to detect possible extraterrestrial signals, to telescopes used to search for potentially habitable extra solar planets. It has also played a major role in works of science fiction. Alien life, such as bacteria, has been hypothesized to exist in the Solar System and throughout the universe. Planet Venus and mars, Jupiter’s moon Europa and Saturn’s moon “Titan” and “Enceladus” are likely to be believed for existence for extraterrestrial life. In May 2011, NASA scientists reported that “Enceladus” is emerging as the most habitable spot beyond Earth in the Solar System for life. 2013 November 4, based on Kepler Space Mission astronomers stated more than 40 billion earth sized planets are orbiting around sun like stars and red dwarfs in the milky way galaxy.
Life on earth is based on the 26 elements. However, 90% life is built upon these six elements Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus and Sulphur. These elements are building blocks of life formation on earth. The planets Venus and Mars along with several natural satellites orbiting Jupiter and Saturn and even comets are suspected to possess nice environments in which life might exist. A sub-surface marine environment on Jupiter’s moon – Europa might be the most suitable habitat in the Solar System outside Earth for organisms. Liquid water is widely thought to have existed on Mars in the past and there may still be liquid water beneath the surface. In July 2008, laboratory tests aboard NASA’s Phoenix Mars Lander had identified water in a soil sample. The Presence of Hydrogen and Methane in the mars atmosphere supports the possible existence of life on mars. Jupiter’s moon Europa is also major speculation for extra-terrestrial life support as it is covered with thick icy layer. Hypothermal vents on the bottom of the ocean, if they exist, may warm up the ice and could be capable of supporting organisms. Titan – the largest moon of Saturn has been demonstrated with the existence of liquid hydrocarbon lakes in the Polar Regions, the first stable bodies of surface liquid discovered outside Earth. Enceladus – a moon of Saturn, has some of the conditions for life including geothermal activity and water vapor as well as possible under-ice oceans heated by tidal effects. A trace of Carbon, Hydrogen, Nitrogen and Oxygen has been detected on the atmosphere of Enceladus. All these research and reports have raised questions that is there really existence of extraterrestrial life?