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.