rockyplanet

Rocky planets, also known as terrestrial planets, are celestial bodies with solid surface layers composed primarily of silicate rocks and metals. They form in the inner regions of protoplanetary disks where temperatures are high enough to prevent volatile compounds such as water and methane from remaining in gaseous form. The four rocky planets in the Solar System—Mercury, Venus, Earth, and Mars—represent the primary examples. Their masses range from about 0.055 Earth masses (Mercury) to one Earth mass (Earth itself), and their radii are between one‑half and one Earth radius. Rocky planets are usually less than 1.5 times the radius of Earth and possess surface gravities sufficient to retain a dense, rocky surface.

Occasionally, the boundary between rocky planets and larger ice giants is blurred. Planets with radii up to roughly 1.8 Earth radii and densities above about 5 g cm⁻³ are generally classified as rocky, whereas larger bodies tend to contain significant volatile layers. The discovery of potentially rocky exoplanets in the habitable zones of nearby stars has focused research on atmospheric composition, geologic activity, and prospects for life. Detection methods such as transit photometry and radial‑velocity measurements allow estimates of radius and mass, from which density and internal composition can be inferred. Current models of planet formation posit that rocky planets result from collisions between planetary embryos in dynamic, crowded early systems. Studies of exoplanetary systems continue to refine the definition of rocky planets and explore the diversity of their physical characteristics.