Ice giant in mass. In 1952, science fiction writer James Blish coined the term "gas giant" and it was used to refer to the large non-terrestrial planets of the Solar System. However, in the 1990s, the compositions of Uranus and Neptune were discovered to be significantly different from those of Jupiter and Saturn. They are primarily composed of elements heavier than hydrogen and helium, constituting a separate type of giant planet altogether. Because during their formation Uranus and Neptune incorporated their material as either ices or gas trapped in water ice, the term "ice giant" came into use. Modelling the formation of
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,