Terrestrial planet

A terrestrial planet is mostly made of rock (silicate) and may, or may not, have much atmosphere. Earth is the "original" terrestrial planet and we extended the term to include our nearest rocky neighbours: Mercury, Venus and Mars. It is often said that they are similar to Earth. However this similarity is not meant to include the planets' surface conditions. It is the bulk structure and composition which is similar, not the atmosphere.

With the discovery of planets orbiting other stars (exoplanets), the term terrestrial planet has been extended again to any planet which is similar to Earth orbiting any star.

Structure

All have a core, a mantle, and a crust. These structures can be compared to a boiled egg; the central yolk is the core; the white albumin is the mantle; and the shell is the crust. The crust of a terrestrial planet is comparatively thin, with the core and the mantle taking up the vast bulk, sometimes with a very large core, sometimes much smaller.

Our terrestrial planets have metallic cores of mostly iron, with rocky mantles and crusts.