The planet Jupiter's more massive than all the other planets in the Solar System put together. It's a stormy mass of raging gas and metallic hydrogen.

The planet Jupiter is the largest planet in the solar system. It has a diameter 11 times that of the Earth and a mass (more than 300 times that of the Earth) which is greater than twice the sum of all the other planets. Nevertheless its mass is less than one thousandth that of the Sun.

It orbits the Sun at a distance of about 500 million miles (780 million km) in just under 12 years.

Internal structure

The structure of Jupiter is very unlike that of the Earth. The visible 'surface' of Jupiter is, in fact, the uppermost layers of clouds of methane and ammonia. The interior of Jupiter is still something of a mystery, which NASA's current Juno mission hopes to shed more light on. Some scientists think that it's likely to be made up of a solid core of material similar to that of the Earth.

Surrounding this is thought to be a dense mixture of metallic hydrogen and helium. On the Earth we know these two as gases which at very low temperatures can be liquefied; in Jupiter's interior the pressure is so high that the hydrogen takes up a state in which it behaves like a metal.

Outside this metallic hydrogen zone is a shell of liquid molecules, mainly hydrogen and helium, with the cloudy atmosphere, about 1000km deep, above.

The planet Jupiter's atmosphere

The temperatures in Jupiter's atmosphere are very cold ranging from -130°C at the top of the clouds to 30°C about 70km below.

Appearance

From the Earth Jupiter can be seen, even in a small telescope, to show a disc with polar flattening. Across the disc several bands of dark and light clouds can be seen with a giant feature called 'The Great Red Spot' being visible during each rotation. Jupiter has tremendous thunderstorms in its atmosphere and also aurorae.

The planet Jupiter is one of the brightest objects in the night sky. A combination of its orbital period, 11.9 years, and that of the Earth means that we see Jupiter at opposition (its closest to the Earth) every 13 months. It is then very bright.

Satellites

With a pair of binoculars, you can see the four moons discovered by Galileo. These are the four largest of Jupiter's family of 16 satellites which all lie within the magnetsphere extending 650km into space. The diameters of the biggest, Io, Europa, Ganymede and Callisto range between 3000 and 5000 km whereas the smallest, Leda has a diameter of only 10 km.

The names of all of Jupiter's satellites come from mythological lovers of Jupiter, except Amalthea, who was his nurse.

Jupiter's third moon lo

Io, the closest of the four large moons to Jupiter, is the most fantastic. Because of tidal forces of Jupiter and the other moons the surface moves in and out by some 100 metres. This generates a lot of heat which causes a peculiar form of vulcanism in which volcanoes emit fountains of sulphur compounds from subsurface liquid sulphur magma. Several of these volcanoes were most recently seen in eruption by the NASA New Horizons spacecraft on its way to Pluto.

Voyager I discovered a faint ring on Jupiter by chance. The rocky particles in this ring may have originated on Io or from meteoritic or cometary debris. The ring is not visible from the Earth.

Magnetic field

Jupiter has a magnetic field more than 10 times that of the Earth’s. The interaction of this field with the solar wind causes an enormous doughnut-shaped system rather like the van Allen belts around the Earth. The moon, Io, lies within this field structure and is responsible for the bursts of radio waves which are observed to come from Jupiter.

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