The emergence of life on Earth provides the obvious prerequisites for the assumption that the same conditions could arise on other planets. You can more or less definitely talk only about the evolution of life, which resembles the earth.
In past centuries, the existence of life on the planets of the solar system is considered highly probable. This is especially linked to the discovery of methods of astronomy seasons (seasons), the possible sea and land, and the so-called canals on Mars. Even there the abstract assumption of the existence of the Selenites, Martians, etc. Some scientists in the early XX century considered the presence of Martian vegetation proven and Venus - possible.
Astrobiologists continue to lead the search for even elementary forms (bacteria, protozoa) on Mars, Venus. Considered promising to find some satellites of the gas giants Jupiter and Saturn with subsurface oceans, water ice and the atmosphere (Europa, Callisto, Ganymede, Enceladus, Titan).
Extraterrestrial life - one of the most important attributes and are also actors in the works of many authors. References to the existence of life beyond Earth are more to the ancient and medieval literature. Modern science fiction tradition clashes with extraterrestrial life laid works of Roni Joseph Sr. and HG Wells.
Topics related to extraterrestrial life are diverse. It attempts to identify difficult to distinguish the line between reasonable and unreasonable life (Strugatsky brothers "Noon, XXII Century."
In addition to the broad disclosure in the literary and cinematographic works of science fiction, the possible appearance of extraterrestrial life in Earth's biosphere, paleocontacts modern alien visits and personal contacts with them also appear in studies of UFO enthusiasts, as well as some conspiracy theories.
