<P> The Solar System has four terrestrial planets: Mercury, Venus, Earth, and Mars . Only one terrestrial planet, Earth, is known to have an active hydrosphere . </P> <P> 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 . </P> <P> 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, 1.87 and 2.52 g cm, respectively, and Haumea's density is similar to Pallas's 2.8 g cm). The Earth's Moon has a density of 3.4 g cm and Jupiter's satellites, Io, 3.528 and Europa, 3.013 g cm; other satellites typically have densities less than 2 g cm . </P> <P> The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure . Because a greater uncompressed density indicates greater metal content . Uncompressed density differs from the true average density (also often called "bulk" density) because compression within planet cores increases their density; the average density depends on planet size, temperature distribution and material stiffness as well as composition . </P>

The uncompressed density of a planet in our solar system