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Solar system. Computer illustration showing relative sizes of planetary orbits. The inset shows the inner Solar System magnified from the main image. The mean distance from the Earth to the Sun (approximately 150 million kilometres) is termed an astronomical unit (AU). While the inner planets are relatively close together, the outer solar system is increasingly spread out. For example, the mean distance between the gas giants Uranus and Neptune is over 10 AU. The small rocky planet Pluto is the furthest from the Sun on average, orbiting at a distance of around 40 AU. (KEYSTONE/SCIENCE PHOTO LIBRARY/MARK GARLICK)