http://solarsystem.nasa.gov/planets/

The Search for Life in the Solar System

The current search for exoplanets is being conducted with the ultimate goal of finding and studying habitable worlds (and potentially life) outside of the Solar System. However, the search for other worlds by humans was not always so far-reaching; there was a time when humans were unable to breach the boundaries of the Solar System, when our neighbouring planets were mysterious wandering points of light that hid their secrets. From that time until now, the study of planets within our solar system has continued (in fact the field has grown tremendously, and even includes the study of moons), and has contributed to the development of the very tools that are used today to search for exoplanets elsewhere in the Cosmos.

The minimum requirements for life are generally agreed to be liquid water and a source of energy. The current understanding is that none of the planets in the solar system (apart from Earth) are capable of supporting liquid water, and therefore by this criteria cannot support life as we know it. However, there is evidence that Mars was once home to large bodies of water, and even rivers; some believe that subterranean life may still exist on Mars. If the search is extended to moons, several moons can be found orbiting Jupiter and Saturn that show evidence of vast subsurface oceans. One moon of these moons, Titan, has lakes of liquid methane on the surface. Can life exist with methane as a substitute for water?

The study of the bodies within the solar system and the relationship to the search for life is detailed further in the following sections:

Mars

Moons of Jupiter and Saturn

Life on Earth: Extreme Environments