by Sarah Todd

Created on: August 10, 2008

Jupiter is contracting. Scientists estimate the largest planet in the solar system shrinks by about two centimetres every year.

The Sun and all its planets formed at the same time - about 4.6 billion years ago. All the planets were a lot hotter then, and Jupiter's size means it is taking a long time to cool down.

Named for the Roman king of the gods, Jupiter is two and a half times larger than all the other planets in our solar system combined. If Jupiter was hollow more than one thousand Earth-sized planets would fit inside it.

Ancient astronomers often referred to Jupiter as a bright "wandering star". The planet is associated with the mythology and religions of many different cultures. It is also, depending on Mars' orbit, the third or fourth brightest object in our night sky after the Moon and the planet Venus. When the sun is low in the sky Jupiter can sometimes be seen with the naked eye.

Jupiter is classified as one of the gas giants, together with Saturn, Uranus and Neptune. Collectively all four planets are known as the Jovian planets the word "Jovian" is the adjectival form of Jove, used by astrologers during the Middle Ages. The planet's astrological influence is responsible for joy and happiness, and over time the older adjectival word "jovial" has become a synonym for the same emotions.

About ninety percent of Jupiter's upper atmosphere is made up of hydrogen. Helium comprises most of the remaining gas, together with a small amount of different elements. The outer atmosphere is clearly marked by different bands at different latitudes, which indicates turbulence and possibly storms along their borders.

The gas and elements of the planet's interior are denser, with traces of ammonia, silicon-based compounds, methane and water vapour joining hydrogen and helium. Traces of ethane, carbon, neon, phosphine, sulphur, hydrogen sulphide and oxygen have also been discovered.

Jupiter's atmosphere is thought to extend a long way into the planet, which scientists believe consists of a small rocky central core. Similar in composition to Earth's core, it is estimated to have a diameter of around 24,000 kilometres and a mass 10 to 15 times that of Earth. Temperatures on Jupiter range from -130 degrees Celsius at the top of the clouds, increasing to about 30 degrees Celsius 70 kilometres under the clouds and reaching perhaps 20,000 K at its centre.

Jupiter has at least 63 moons. The four largest were identified and named by astronomer Galileo Galilei in 1610. The quartet,

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