Earth's dust ring. Computer simulation of a ring of asteroidal dust orbiting the Sun with the Earth. The Sun and Earth are shown respectively as yellow and green spots. The colours in the ring represent dust density, from blue (lowest) to white (highest). It is thought that the dust came from the zodiacal cloud which envelops the inner Solar System, and has become locked in a gravitational resonance with the Earth. The dust ring orbits with the Earth, but forms a peak density in the Earth's 'wake' (large white area, right). This model may explain the asymmetry of the zodiacal cloud observed by the IRAS satellite.