The planetary core consists of the innermost layer(s) of a planet.
The core may be composed of solid and liquid layers. The cores of Mars and Venus may 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 85% 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 planets".
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.
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