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The largest planet in the Solar System and the fifth from the Sun, lying at a mean
distance of about 5.2 astronomical units from the Sun. Jupiter is a gas giant, mostly
hydrogen and helium, with a mass of 1.898 x 10^27 kg, or about 0.001 solar masses,
or 318 times Earth masses. It is more than twice as massive as all the other solar
system planets combined. Jupiter's diameter measures 11 times that of Earth. Its rotation
period, 9.93 hours (Jupiter/Earth ratio = 0.41), is the shortest of all the solar
system planets. Its orbital period is 11.857 Earth years. Jupiter has an extensive
family of satellites (79 known) and a faint ring system; Jupiter's ring. Jupiter probably
has a core of rocky material amounting to something like 10 to 15 Earth masses. Above
the core lies the main bulk of the planet in the form of liquid metallic hydrogen.
This exotic form of the most common of elements is possible only at pressures about
3 million bars, as is the case in the interior of Jupiter (and Saturn). Under the
extreme pressure found deep inside Jupiter, the electrons are released from the hydrogen
molecules and are free to move about the interior. This causes hydrogen to behave
as a metal; it becomes conducting for both heat and electricity.
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