Planetary core

For the Earth's core, see Inner core and Structure of the Earth § Core.

The planetary core consists of the innermost layers(s) of a planet.

The core may be a solid or a liquid layer, as is Mercury's, whilst the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% (the Moon) to 75% of a planet's radius (Mercury).

Gas giants also have iron-rich cores. Although these cores are proportionately much smaller than those of terrestrial planets, gas giants are so large that their cores can actually be larger than Earth. Jupiter's core is thought to be approximately 12 times the mass of Earth (3% of Jupiter's total mass), and the exoplanet HD 149026 b is thought to have a core approximately 70 times the mass of Earth.

It is thought that some gas giants orbiting very close to their primaries may have their atmospheres stripped away, leaving only their core behind. This as-yet hypothetical class of planets are called "Chthonian."

Some moons, asteroids and other minor planets may also have well-differentiated cores depending on their size and history. Jupiter's moons Io and Europa are in many ways sisters of the terrestrial planets and have very substantial cores comprising about a third of their radii. The large asteroid 4 Vesta is likewise believed to have a differentiated structure with a distinct core.

Power source of the Earth's core

The core of the Earth is still hot because it contains radioactive uranium which was derived from a local supernova explosion before the earth formed. It is the heat produced by radioactive decay that has prevented the Earth's core from cooling and solidifying. Heat sources include gravitational energy released by the compression of the core, gravitational energy released by the rejection of light elements (probably S, O, or Si) at the inner core boundary as it grows, latent heat of crystallization at the inner core boundary, and radioactivity of K or U.