Terrestrial Planet
Terrestrial planets (from Latin earth, soil), as opposed to gaseous planets , are planets composed of rock and metal, that is to say, they generally have three concentric shells (core, mantle and crust). Their surface is solid and are composed mainly of non-volatile, usually rock silicate and metal, usually iron. Their density is relatively high and between 4 and 5.5.
In the solar system , terrestrial planets are the four inner planets are located between the Sun and the asteroid belt : Mercury , Venus , Earth and March. The Moon and Io , the first of four large moons of Jupiter, has a similar structure and could therefore be classified as terrestrial.
The terrestrial planets are much smaller than the gaseous planets but have a much higher density because they are composed of iron and silicates.
It actively seeks planets of this kind among planetary systems other than ours, but their detection is difficult due to their low mass (and their proximity to the star) compared with that of gas giants and stars. A terrestrial exoplanet was discovered and this discovery was published in April 2007: Gliese 581 c (another appeared to have been discovered, OGLE-2005-BLG-390Lb , January 26, 2006).
Process of Differentiation
The heaviest materials ( nickel , iron ) converge toward the center of the planet to form its nucleus. Under pressure, the central core may be solid, but the accumulated heat can leave some of this fluid. Other materials, density slightly lower, find themselves caught between the core and the surface, forming a cloak whose fluidity is a function of the residual temperature of the planet. The surface is a crust of lower density materials. The less dense materials ( water , gas ) are expelled to the surface and can be a atmosphere where the gravity of the planet can retain them despite their lightness and the tendency of the solar wind to carry them.
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