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Terrestrial planet facts for kids

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Terrestrial planet size comparisons
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.


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 planets within the Solar System

Masses of terrestrial planets
Relative masses of the terrestrial planets of the Solar System, and the Moon (shown here as Luna)
Telluric planets size comparison
The inner planets (sizes to scale). From left to right: Earth, Mars, Venus and Mercury.

The Solar System has four terrestrial planets under the dynamical definition: Mercury, Venus, Earth and Mars. The Earth's Moon as well as Jupiter's moons Io and Europa would also count geophysically, as well as perhaps the large protoplanet-asteroids Pallas and Vesta (though those are borderline cases). Among these bodies, only the Earth has an active surface hydrosphere. Europa is believed to have an active hydrosphere under its ice layer.

During the formation of the Solar System, there were many terrestrial planetesimals and proto-planets, but most merged with or were ejected by the four terrestrial planets, leaving only Pallas and Vesta to survive more or less intact. These two were likely both dwarf planets in the past, but have been battered out of equilibrium shapes by impacts. Some other protoplanets began to accrete and differentiate, but suffered catastrophic collisions that left only a metallic or rocky core, like 16 Psyche or 8 Flora respectively. Many S-type and M-type asteroids may be such fragments.

The other round bodies from the asteroid belt outward are geophysically icy planets. They are similar to terrestrial planets in that they have a solid surface, but are composed of ice and rock rather than of rock and metal. These include the dwarf planets, such as Ceres, Pluto and Eris, which are found today only in the regions beyond the formation snow line where water ice was stable under direct sunlight in the early Solar System. It also includes the other round moons, which are ice-rock (e.g. Ganymede, Callisto, Titan, and Triton) or even primarily ice (e.g. Mimas, Tethys, and Iapetus). Some of these bodies are known to have subsurface hydrospheres (Ganymede, Callisto, Enceladus, and Titan), like Europa, and it is also possible for some others (e.g. Ceres, Dione, Miranda, Ariel, Triton, and Pluto). Titan even has surface bodies of liquid, albeit liquid methane rather than water. Jupiter's Ganymede, though icy, does have a metallic core like the Moon, Io, Europa, and the terrestrial planets.

The name Terran world has been suggested to define all solid worlds (bodies assuming a rounded shape), without regard to their composition. It would thus include both terrestrial and icy planets.

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See also

Kids robot.svg In Spanish: Planeta terrestre para niños

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