In the fourth episode, Prof. Brian Cox looks at the jewel of the solar system – the ringed planet Saturn.
However, for most of its life, Saturn has not had rings. Cox explains how the rings were most likely formed when an ice moon orbiting the gas giant broke up to create the rings we are now so familiar with. For that matter, Saturn was originally a rock planet like Earth, before it developed a thick atmosphere of hydrogen and helium.
As well as examining Saturn, we also examine one particular moon of this world: Enceladus, which was discovered to have hydrothermal vents firing saltwater into space.
The Planets is on BBC Two at 21.00.