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.
Famous quotes containing the words planetary and/or core:
“What is a television apparatus to man, who has only to shut his eyes to see the most inaccessible regions of the seen and the never seen, who has only to imagine in order to pierce through walls and cause all the planetary Baghdads of his dreams to rise from the dust.”
—Salvador Dali (1904–1989)
“The ideal of men and women sharing equally in parenting and working is a vision still. What would it be like if women and men were less different from each other, if our worlds were not so foreign? A male friend who shares daily parenting told me that he knows at his very core what his wife’s loving for their daughter feels like, and that this knowing creates a stronger bond between them.”
—Anonymous Mother. Ourselves and Our Children, by Boston Women’s Health Book Collective, ch. 6 (1978)