The fifth planet from the Sun, and the innermost of the giant outer planets. It contains two-thirds of the matter in the Solar System, apart from the Sun. It has been observed in close-up by five space probes: Pioneer 10 and 11, Voyagers 1 and 2, and Galileo. Its basic characteristics are: mass 1·90 × 1027 kg; equatorial radius 71 492 km/44 423 mi; polar radius 66 854 km/41 541 mi; mean density 1·33 g/cm3; rotational period 9 h 55 min 41 s; orbital period 11·9 years; inclination of equator to orbit 3·1°; mean distance from the Sun 5·203 AU. It is made primarily of hydrogen (82%) and helium (17%), and believed to have an innermost core of terrestrial composition of 5–10 Earth masses, a large outer core of hydrogen and helium in a metallic phase, a liquid hydrogen/helium mantle, and a deep gaseous atmosphere. The planet has a significant internal source of heat, and radiates twice as much heat from inside as it receives from the Sun.
The face of the planet is covered by clouds, organized into bands, called belts and zones. Zones are light, and cold (−130°C) because they are high in the atmosphere; belts are darker and warmer clouds (−40°C) at a lower elevation; and a third, warmer level of clouds has also been observed (20°C). The uppermost clouds are inferred to be solid ammonia, the middle clouds ammonium hydrosulphide, and the lowest clouds water. The rotation period in the equatorial region is 5 min faster than that of the rest of the planet - a differential which contributes to the formation of a richly coloured banded structure in the cloudy atmosphere. Complex currents and vortices are observed within the bands, including a long-lived atmospheric storm called the Great Red Spot. The known Jovian moons (Mar 2004) number 63, including the four large satellites discovered by Galileo (1610), which are distinct worlds in themselves and lie in near-circular orbits in an equatorial plane. A dark ring of dust around the planet was discovered by Voyager, between 100 000 and 215 000 km from the centre of the planet, consisting of material from the surface of Jupiter’s four innermost moons.