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,
Gustav Holst likens to "unresolved timelessness ... never ending, since space does not end, but drifting away into eternal silence". Apart from his concession with "I Vow to Thee..."', Holst insisted on the unity of the whole work, and opposed the performance of individual movements. Nevertheless, Imogen wrote that the piece had "suffered from being quoted in snippets as background music". During and after the composition of "The Planets", Holst wrote or arranged numerous vocal and choral works, many of them for the wartime Thaxted Whitsun Festivals, 1916–18. They include the "Six Choral Folksongs" of 1916, based on West Country tunes, of