Terrestrial planet

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The terrestrial planets. From left to right: Mercury, Venus, Earth, and Mars

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 the bulk structure and composition but not the surface conditions or the atmosphere. A terrestrial planet may be much hotter or colder than Earth and may, or may not, have much 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.

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.