Planetarycore

Planetarycore is the central region of a planet, typically distinguished by high density and distinct chemical or physical properties from the surrounding mantle or envelope. In terrestrial planets, cores are usually metallic and differentiated from overlying rocky layers; in gas and ice giants, the core is a more ambiguous region of heavy elements overlain by extreme-pressure layers such as metallic hydrogen or high-pressure ices. Core formation occurs during early planetary differentiation when dense materials sink toward the center under gravity.

In rocky planets, the core is commonly envisioned as a solid inner core and a surrounding liquid outer core, predominantly iron and nickel with lighter elements mixed in. Earth’s inner core is a solid sphere roughly 1,200 kilometers in radius, while the outer core is a liquid iron-nickel layer about 2,000 to 2,300 kilometers thick; convection in this liquid layer drives the planet’s magnetic field. The precise structure of cores on other terrestrial worlds can vary with composition and thermal history, but a solid–liquid dichotomy is a common theme.

Giant planets present a more complex picture. Their cores are thought to be enriched in heavy elements and shielded by deep, high-pressure layers, such as metallic hydrogen in Jupiter and Saturn or thick ices/rock in ice giants. The size, composition, and even existence of distinct cores in such planets remain subjects of active research and debate, with estimates spanning a range of possible core masses.

Study methods include seismology, gravity and magnetic field measurements, and space missions that probe internal structure through gravity harmonics and planetary dynamics. Exoplanet observations use mass-radius relations to infer core properties indirectly. Core characteristics influence a planet’s geodynamics, magnetic field, and evolution.