Early literature regarding the Solar System, following scientific speculations dating back to the 17th century, assumed that every planet hosted its own native life forms—often assumed to be human in form, if not in attitudes. Later literature began to accept that there were limits set by temperature, gravity, atmospheric pressure and composition, or the presence of liquids that would set bounds on the possibility of life as we know it existing on other planets. By the 19th century the Moon was given up as an airless desert, incapable of supporting life on its surface (hopes for subsurface life continued until later). Jupiter and the planets beyond were too large, too cold, and had atmospheres composed of poisonous chemicals. Mercury was too close to the Sun and its surface was exposed to extremes of temperature. The asteroids were too tiny and airless. By the early 20th century, prospects for life in the Solar System focused on Venus, the larger moons of Jupiter and Saturn, and especially Mars.
With the onset of the Space Age, planetary probes cast increasing doubt on the likelihood of extraterrestrial life in the Solar System, at least life of any magnitude greater than organisms such as bacteria. By the mid-1960s, it was firmly established that life could have no foothold on the hostile surfaces of Mercury or Venus, and that Mars could hardly support any macroscopic life forms on its surface, much less an advanced civilization. In the 1980s it was shown that the surfaces of Jupiter's moons were just as hostile to life. More recent fiction focused on the Solar System has thus tended to address its exploration for purposes such as terraforming, the engineering of planets for human habitation, than the possibility of any existing life.