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Solar system planets. Computer artwork of the nine planets of the solar system. Their order from the Sun (not shown) is: Mercury, Venus, Earth, Mars (the four rocky inner planets), Jupiter, Saturn, Uranus, Neptune (the four outer gas giant planets) and Pluto (a Dwarf planet on the outer edge of the solar system). Jupiter's vivid colours arise from trace chemicals in its clouds. Saturn's clouds are similar, but less turbulent due to less internal heat compared to Jupiter. Uranus's blue colour is due to a layer of methane absorbing red light. The colour of Neptune is similarly due to methane plus an unidentified component. Not to scale. (KEYSTONE/SCIENCE PHOTO LIBRARY/VICTOR HABBICK VISIONS)