Terrestrial planet from the original solar nebula. The Solar System has four terrestrial planets: Mercury, Venus, Earth, and Mars. Only one terrestrial planet, Earth, is known to have an active hydrosphere. During the formation of the Solar System, there were probably many more terrestrial planetesimals, but most merged with or were ejected by the four terrestrial planets. Dwarf planets, such as Ceres, Pluto and Eris, and small Solar System bodies are similar to terrestrial planets in the fact that they do have a solid surface, but are, on average, composed of more icy materials (Ceres, Pluto and Eris have densities of 2.17,
Water atm, water boils at . At very low pressures (below about 0.006 atm), water cannot exist in the liquid state and passes directly from solid to gas by sublimation—a phenomenon exploited in the freeze drying of food. At very high pressures (above 221 atm), the liquid and gas states are no longer distinguishable, a state called supercritical steam. Water also differs from most liquids in that it becomes less dense as it freezes. The maximum density of water in its liquid form (at 1 atm) is ; that occurs at . The density of ice is . Thus, water expands