Structure of Jupiter. Cutaway computer illustration showing the interior structure of the planet Jupiter. Jupiter is the fifth planet from the Sun and the largest in the solar system, with a mass two and a half times that of all the other planets put together. At the planet's centre is rocky core (orange, centre) about the size of Earth, which is composed of solid rock, iron and frozen water. Surrounding this is a fluid layer of liquid hydrogen and helium under such high pressure that it acts like molten metal. Around this is the inner atmosphere, consisting of liquid hydrogen and helium under less pressure. The outer atmosphere (outermost layer) consists of hydrogen and helium gas, with traces of methane, ammonia and water vapour.