Planetary core
The planetary core consists of the innermost layer(s) of a planet; which may be composed of solid and liquid layers. Cores of specific planets may be entirely solid or entirely liquid. In the Solar System, core size can range from about 20% (Moon) to 85% of a planet's radius (Mercury).
Gas giants also have cores, though the composition of these are still a matter of debate and range in possible composition from traditional stony/iron, to ice or to fluid metallic hydrogen. Gas giant cores are proportionally much smaller than those of terrestrial planets, though theirs can be considerably larger than the Earth's nevertheless; Jupiter has one 10–30 times heavier than Earth, and exoplanet HD149026 b has a core 67 times the mass of the Earth.
In 1798, Henry Cavendish calculated the average density of the earth to be 5.48 times the density of water (later refined to 5.53), this led to the accepted belief that the Earth was much denser in its interior. Following the discovery of iron meteorites, Wiechert in 1898 postulated that the Earth had a similar bulk composition to iron meteorites, but the iron had settled to the interior of the Earth, and later represented this by integrating the bulk density of the Earth with the missing iron and nickel as a core. The first detection of Earth's core occurred in 1906 by Richard Dixon Oldham upon discovery of the P-wave shadow zone; the liquid outer core. By 1936 seismologists had determined the size of the overall core as well as the boundary between the fluid outer core and the solid inner core.
Planetary systems form from a flattened disk of dust and gas that accrete rapidly (within thousands of years) into planetesimals around 10 km in diameter. From here gravity takes over to produce Moon to Mars sized planetary embryos (105 – 106 years) and these develop into planetary bodies over an additional 10–100 million years.
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