Historical Background

The consideration of whether humankind is alone in the universe or shares it with other creatures has long been part of intellectual inquiry into the nature of the universe. The early Greek philosophers, for instance, speculated about the existence of other inhabited planets. With the recognition that the other planets of the solar system are bodies like Earth came both scientific and popular speculation about life on other planets. Much of the modern effort of exploration in the solar system has been driven implicitly, if not explicitly, by this search for extraterrestrial life. Those who investigate the possibility of extraterrestrial life and the effects of extraterrestrial environments on living organisms from Earth are called exobiologists.

In the late 1800s, the Italian astronomer Schiappareli thought he saw linear features crossing the surface of the planet Mars. (These features are actually just the attempt of the eye and brain to find a pattern in random features seen at the edge of detectability). His “channels” became, in English translation, “canals” and became the inspiration for Percival Lowell to not only found his own observatory for their study, but to also widely lecture to audiences about intelligent life on a dying Mars. Lowell's viewpoint was the inspiration behind H. G. Wells's fictional novel War of the Worlds, which was dramatized in the infamous Orson Wells radio broadcast of October 30, 1938.

The recurrent fascination with Mars was a major factor behind the 1976 NASA Viking mission, which landed a spacecraft on the planet. Part of its task was to test Martian soil for evidence of the chemicals produced by living organisms, an experiment that produced a negative result. With Viking removing doubt about the possibility of major life forms on Mars and Venus proven to be too hot to be habitable, scientific speculation turned to the outer planet, Jupiter, and its moons, as well as the cloud‐enshrouded moon Titan, which orbits Saturn. The outer atmosphere of Jupiter is now known to be too turbulent to support a stable environment for life. Additionally, Jupiter's moons are probably too ice‐enshrouded for life to be found in the oceans lying beneath the ice, and Titan has been shown to be too cold to support life. A 1996 report of the discovery of microscopic bacterial fossils in a meteorite that originated on Mars caused great excitement, but continued study suggests that the structures that were found are likely not organic in nature. Nevertheless, the announcement in early 2000 that high resolution photography of small areas of the Martian surface reveals extensive patterns of erosion features, apparently identical with drainage features on Earth due to water, has rekindled interest in the planet. If water does exist somewhere on the planet, then maybe the evidence for living organisms also is present.

Even if life does not exist elsewhere in the solar system, there are billions of stars and potential solar systems that could provide hospitable environments. With scientific proof yet to be found, nevertheless the most popular modern genre of written fiction, television, and movie entertainment is science fiction, with its worlds ( Star Trek, Star Wars, and so on) populated by a vast variety of other creatures.