Jupiter is more like a star than a planet in its make up. It appears to consist largely of gas and liquid, with a solid core. In fact, many astronomers believe that if Jupiter had been just 100 times larger, the Solar System would have become a two star system.
Although we think of hydrogen and helium as gases, they can take other forms when vast masses of them are combined. At the top of Jupiter's atmosphere, hydrogen exists as a gas. However, moving downwards, the pressure increases, and the gas becomes more and more liquid-like.
At 10,000 km (6,000 miles) beneath the top of the clouds, the atmospheric pressure is one million bar (one bar is the pressure on Earth at sea level), and the temperature is 6,000 kelvin. Under these conditions, hydrogen forms a liquid metal (similar to the state in which we find the metal mercury on Earth). So most of Jupiter consists of a shell of metallic hydrogen, almost 60,000 km (37,000 miles) thick, that sits over a core of rock and ice.
