Terrestrial planet

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 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.

Structure

All terrestrial planets have a core, a mantle, and a crust. They are a bit like 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 thin, with the core and the mantle taking up the vast bulk, sometimes with a very large core, sometimes much smaller. Terrestrial planets have metallic cores of mostly iron, with rocky mantles and crusts.

All terrestrial planets have the same type of structure: a central metallic core, mostly iron, with a surrounding silicate mantle.

The Moon is similar, but has a much smaller iron core. Io and Europa are also satellites that have internal structures similar to that of terrestrial planets. Terrestrial planets can have canyons, craters, mountains, volcanoes, and other surface structures, depending on the presence of water and tectonic activity. Terrestrial planets have secondary atmospheres, got from volcanism, meteorites, and photosynthesis. In the giant planets, their atmospheres are primary, captured directly from the original solar nebula.

Terrestrial Planet Media

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  The terrestrial planets of the Solar System: Mercury, Venus, Earth and Mars, sized to scale

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  Relative masses of the terrestrial planets of the Solar System, and the Moon (shown here as Luna)

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  The inner planets (sizes to scale). From left to right: Earth, Mars, Venus and Mercury.

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  Sizes of Kepler planet candidates based on 2,740 candidates orbiting 2,036 stars as of 4 November 2013 (NASA)

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  Artist's impression of a carbon planet

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  Geysers erupting on Enceladus