The availability of liquid water is the most important factor that makes a planet habitable, because water is a very effective polar molecule and hence an excellent solvent and facilitator for the complex chemistry of life. Its presence presupposes a planet with a significant mass that guarantees the presence of a substantial atmosphere, and a reasonable spinning rate to avoid overheating. It also implies that the planet is at moderate distances from its central star, a range that is called the Ecosphere or the Habitable Zone. Since the evolution of life to high intelligence seems to take billions of years, it requires also that the central star must be neither too massive, that will produce a lot of lethal UV radiation and will have too short a life-span to allow life to evolve, nor of very small mass which will be producing too feeble a radiation to sustain life. The detection of free Oxygen in the atmosphere of a planet is a very strong evidence for the presence of life, because Oxygen is highly reactive and would rapidly disappear by combining with other elements, unless it is continuously replenished by life as the by-product of the process of photosynthesis that builds food for life (sugars) from CO2 and H2O.