Located 778 330 000 kilometres of 5.2 AU from the Sun, Jupiter is the fifth planet and is undoubtedly the largest, with a diameter of 142 984 kilometres. It is equivalent to twice all the other planets combined or approximately 318 times the size of Earth. Weighing 1.900e27, it is the heaviest planet.
Jupiter is a gas planet. Gas planets do not have solid surfaces, but rather, their gaseous material gets denser with depth. Therefore when the diameters and radii quoted for such planets are for level that correspond to a pressure of 1 atmosphere. What we see when we looked at the gaseous planets, is in fact the tops of clouds high in their atmosphere. The coloured bands which can be seen in Jupiter's clouds are believed to be the result of subtle chemicals reactions in it's atmosphere but exact details are unknown. These colour correlate with the altitude of the cloud. Blue is the lowest in the atmosphere, followed by browns and whites, with red being the highest in the atmosphere.
Scientists have discovered that Jupiter is about 75% hydrogen and 25% helium with traces of methane, water, ammonia and "rock". Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.
The amount of information known about the interior of Jupiter and the other gas planets is limited and will probably remain this way for some time. This is because we have not yet invented atmospheric probes that will survive the conditions on these planets. The data from Galileo's atmospheric probe goes down only about 150 km below the cloud tops.) So far, Jupiter has been visited or by-passed by Pioneer 10 (1973), Pioneer 11, Voyager 1, Voyager 2 and Ulysses. Currently, the spacecraft Galileo is in orbit around Jupiter and for the next two years or so, we will be receiving more data about this giant planet.
It is believed that Jupiter has a core composed of rocky material that amounts to something equivalent to the weight of 10 to 15 Earths. Above such a core would lie the main bulk of the planet in the form of liquid metallic hydrogen. It is an electrical conductor and the source of Jupiter's magnetic field. This layer probably contains some helium also. The outermost layer is made up of hydrogen and helium gases, as opposed to the same elements found in liquid form in the interior. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts.
It is known that Jupiter radiates more energy into space than it received from the Sun. Jupiter's interior is extremely hot, with the core perhaps measuring 20 000 Kelvin or so. This heat is created by the slow gravitational compression of the planet.
Jupiter's huge magnetic field which extends more than 650 million kilometres. Inside this large magnetic field lie Jupiter's moons and this huge magnetic field may partially explain some of the activity on Io. A concern to the designers of the Voyager and Galileo spacecraft, and any space travellers, is that the environment near Jupiter contains high levels of energetic particles trapped by Jupiter's magnetic field. This radiation is intense and would be fatal to an unprotected human being.
Like Saturn, Jupiter also has rings, but they are much fainter and thinner. They were only discovered when two of the Voyager 1 scientists insisted that after travelling 1 billion km it was at least worth a quick look to see if any rings might be present. They were not expected to be there, but surprisingly were. They're primarily composed of very small grains of rocky material but unlike Saturn's rings, seem to contain no ice.
Jupiter has 16 satellites, with others being discovered, but which have not yet been confirmed. Four of these are the Galilean moons, and the are 12 smaller ones. But due to the tidal drag produced by the Galilean satellites, Jupiter is gradually slowing down. At the same time, these forces are changing the orbits of the moons, forcing them farther and farther from Jupiter.