Jupiter is the planetary giant, 143,000 kilometres across, over 300 times the mass, and a thousand times the volume, of the Earth. It is, like the Sun, mainly made of hydrogen and helium. If it were much bigger, the gravity of the planet would pull the gases together and turn it into a star.

Jupiter radiates twice as much heat as it gets from the Sun. It is 780 million kilometres from the Sun, and rotates once every 10 hours. However, it orbits the Sun only once every 12 years.

Jupiter began as a small rocky planet two or three times the mass of the Earth. This was enough to hold a huge atmosphere, and to compress the gases near the surface into a liquid. Today, therefore, the planet still has a small, rocky core, surrounded by thick layers of metallic and liquid hydrogen and helium, which merge into an 'atmosphere', also of hydrogen and helium. Jupiter also has an enormously powerful magnetic field.

The atmosphere has winds and clouds that run in streams parallel to the Equator; these correspond with the broad coloured bands that can be seen with a telescope.