Geology of solar terrestrial planets Their size, radius, and density are all similar. Terrestrial planets have numerous similarities to plutoids (objects like Pluto), which also have a solid surface, but are primarily composed of icy materials. During the formation of the Solar System, there were probably many more (planetesimals), but they have all merged with or been destroyed by the four remaining worlds in the solar nebula. The terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. The Moon is similar, but lacks a substantial iron core. Three of the four solar terrestrial planets (Venus,
History of Solar System formation and evolution hypotheses itself. Most of the mass collected in the centre, forming the Sun; the rest of the mass flattened into a protoplanetary disc, out of which the planets and other bodies in the Solar System formed. There are, however, arguments against this hypothesis. French philosopher and mathematician René Descartes was the first to propose a model for the origin of the Solar System in his "Le Monde" (ou Traité de lumière) which he wrote in 1632 and 1633 and for which he delayed publication because of the Inquisition and it was published only after his death in 1664. In his view,