Planetary properties leading to liquid water and through it to life

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Though alternative chemistries for life can not be totally excluded, the large cosmic abundances of hydrogen, oxygen, and carbon, their strong chemical affinities, the ability of carbon to form the complex chemical compounds required by the high organizational needs of life, and the excellent properties of water as a solvent and medium for chemistry, strongly suggest that life based on carbon and water would be by far the most likely form of life in the Universe. Liquid water, however, cannot be a common cosmic commodity because it can exist only in specific ranges of temperature and pressure. Low gas pressures and tempera-tures, e.g., prevent liquid water from existing in free space. Planets near their star lack water due to the high temperatures of their formation, while planets at large distances would have water as ice. Subterranean pools of liquid water in cold, solid planets, such as a larger Mars, are possible and could even harbor life in these underground pools that would use chemical rather than solar energy, as do organisms on Earth around deep-sea vents. Solar energy, however, is by far the most abundant and hence the most logical form of energy for life, but requires liquid water on the surface of the planet, which presupposes a planetary atmosphere of considerable pressure and therefore a planet of considerable gravity and hence, mass. It would also require other conditions, such as an effective Cold Trap, a reasonable Ozone Layer, a modest Greenhouse Effect, and the avoidance of temperature extremes which could lead to an irreversible run-away glaciation or run-away greenhouse effect. All these planetary requirements are directly related to the mass of the planet, the mass of the star, the distance of the planet from the star,and the different orbital parameters of the planet (spinning rate, inclination of the axis, eccentricity of the orbit, precession, etc.). The complexity of these requirements might make planets like ours, that are able to retain liquid water on their surfaces for billions of years, a rare occurence in the Cosmos, w~th obvious implications on the frequency of life-harboring solar systems in the Galaxy.