Jupiter. Artwork of the gas giant planet Jupiter and two of its moons. Jupiter is the largest planet in the Solar System, with a diameter over 11 times that of Earth. Two of its Galilean moons are seen here, Io (centre left) and Europa (upper right). The Great Red Spot, an enormous storm system, is seen at centre right. At centre left, on the night side of Jupiter, red flashes of lightning can be seen. Jupiter mainly comprises gaseous hydrogen and helium, with traces of ammonia, water vapour and methane. These trace gases are thought to contribute to the colours of its cloud bands.