Rocky Planet
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A rocky planet (or terrestrial planet) is a planet whose body is composed mainly of rock (silicate) and metal, with a solid surface. They differ from gas giants such as Jupiter and Saturn, which are mostly composed of hydrogen and helium.
The Solar System contains four rocky planets: Mercury, Venus, Earth, and Mars. All are located in the inner region closest to the Sun, and they have higher density than gas giants.
Internal Structure
Rocky planets typically have three layers: a core (usually iron and nickel), a mantle of silicate rock, and a crust on the surface. This layering arose when the planet was molten, with heavier iron materials sinking inward and lighter materials rising outward. If part of the iron core remains liquid and moving, the planet may generate a magnetic field, as Earth does.
Surface and Atmosphere
The surface of rocky planets shows impact craters, mountains, valleys, and ancient geological features. Mercury has many craters and an extremely thin atmosphere. Venus has a thick atmosphere of carbon dioxide, resulting in extreme heat. Earth has an atmosphere of nitrogen and oxygen and liquid water on the surface. Mars has a thin atmosphere predominantly of carbon dioxide, with ice at both poles.
Formation
The standard explanation holds that rocky planets formed from dust and rock in the disk orbiting the Sun at its formation. The region near the Sun was hot, so gases and ice could not remain stable there; only rock and metal accumulated and grew into large bodies.
Similar Objects
The term is sometimes extended to large rocky moons such as Earth's Moon and Io (a moon of Jupiter), since they share a similar internal structure. Many planets discovered around other stars are similarly classified as rocky based on their mass and size.