Terrestrial planet

A terrestrial planet is mostly made of rock (silicate). Earth is the "original" terrestrial planet. When astronomers started understanding the kinds of planet they extended the term to include our nearest rocky neighbours: Mercury, Venus and Mars.

It is often said that they are similar to Earth. This is true of most of the structure and composition, but not the surface or the atmosphere. A terrestrial planet may be much hotter or colder than Earth, and may have much more or much less atmosphere.

With the discovery of planets orbiting other stars (exoplanets), the term terrestrial planet has been extended again to any rocky (silicate) planet orbiting any star.