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,
Upsilon Andromedae from the star. Both of these planets are in more eccentric orbits than any of the planets in the Solar System (including Pluto). Upsilon Andromedae d resides in the system's habitable zone. The orbital parameters of this three planet system have been fully determined. The system is not coplanar, with each other or with the stellar rotation, as in our solar system. Simulations shows that the measured configuration of the planets produces indeed stable orbits for at least 100 million years, where planets "b" and "d" remain roughly coplanar. General relativity is expected to have strong effects on planet "b",