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Encyclopedic overview
The internal structure of the inner planets. The internal structure of the outer planets. The color of Neptune is exaggerated to differentiate it from Uranus.
A planetary core consists of the innermost layers of a planet. Cores may be entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. In the Solar System, core sizes range from about 20% (the Moon) to 85% of a planet's radius (Mercury).
Excerpted from Wikipedia’s “planetary core” article, available under the CC BY-SA 4.0 licence.