Jupiter, the largest planet in the Solar System, has a greater mass than all the other planets combined. It has a thin ring around it, over sixty moons and its swirling cloud patterns will leave you in awe (or feeling nauseous). Jupiter, along with Saturn, Uranus and Neptune is a Gas Giant, meaning that it is almost entirely composed of gas surrounding a small rocky core. Mainly hydrogen and some helium, Jupiter's component gasses are similar to those found in stars. Indeed, if Jupiter was a few times larger it too would have been a star. The internal structure of the planet is as follows: There is a gaseous atmosphere beneath which is a mantle of compressed hydrogen. Deeper still, the hydrogen is at such pressure that it actually becomes metallic.

As well as being named after the king of the Roman gods, the planet is also famous for its Great Red Spot, which everybody knows is a gigantic storm system about twice the size of the Earth. The spot has existed for at least three hundred years, and was first observed by Giovanni Cassini (not Galileo as some people think). Galileo did however discover the four Galilean satellites (obviously, as they are named after him).

Io as imaged by the Galileo Probe.

These main moons are Io, Europa, Ganymede and Callisto, all bar Europa are larger than the Earth’s moon. Io with its giant volcanoes, Europa with its surface of cracked ice which may conceal an ocean beneath, Ganymede which is larger than the planet Mercury and Callisto with its heavily cratered surface have all been described as worlds in their own right. Furthermore, the three innermost of these satellites, Io, Europa and Ganymede experience tidal heating due to the gravity of Jupiter.

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