Terrestrial planet
From Aetilc
A terrestrial planet, telluric planet, or rocky planet is a planet that is composed primarily of silicate rocks or metals. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth (Terra and Tellus), as these planets are, in terms of structure, "Earth-like".
Terrestrial planets have a solid planetary surface, making them substantially different from the larger giant planets, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.
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[edit] Structure
All terrestrial planets have the same basic type of structure, such as a central metallic core, mostly iron, with a surrounding silicate mantle. Some satellites also 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, generated through volcanism or comet impacts, in contrast to the giant planets, whose atmospheres are primary, captured directly from the original solar nebula.
[edit] Density trends
The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure. Because a greater uncompressed density indicates greater metal content. Uncompressed density differs from the true average density (also often called "bulk" density) because compression within planet cores increases their density; the average density depends on planet size, temperature distribution and material stiffness as well as composition.
The uncompressed density of terrestrial planets trends towards lower values as the distance from the star increases. The rocky minor planet Vesta orbiting outside of Mars is less dense than Mars still at, 3.4 g·cm−3.
Calculations to estimate uncompressed density inherently require a model of the planet's structure. Where there have been landers or multiple orbiting spacecraft, these models are constrained by seismological data and also moment of inertia data derived from the spacecraft orbits. Where such data is not available, uncertainties are inevitably higher. It is unknown, whether extrasolar terrestrial planets in general will show to follow this trend.
[edit] Types
[edit] Silicate planet
The standard type of terrestrial planet seen in the most planetary systems, made primarily of silicon-based rocky mantle with a metallic (iron) core.
[edit] Carbon planet (also called "diamond planet")
A theoretical class of planets, composed of a metal core surrounded by primarily carbon-based minerals. They may be considered a type of terrestrial planet if the metal content dominates.
[edit] Iron planet
A theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass. Mercury in the Solar System has a metallic core equal to 60–70% of its planetary mass. Iron planets are thought to form in the high-temperature regions close to a star and if the protoplanetary disk is rich in iron.
[edit] Coreless planet
A theoretical type of terrestrial planet that consists of silicate rock but has no metallic core, i.e. the opposite of an iron planet. Coreless planets are thought to form farther from the star where volatile oxidizing material is more common.
