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Planetary formation. Conceptual computer artwork of planetesimals during the early history of the solar system, showing the Sun at the centre of the concentric circles. Planetesimals were irregularly shaped pieces of rocky debris, which were gravitationally attracted to each other and collided, eventually forming protoplanets. In the history of the solar system, planets formed soon after the Sun, about 4.6 billion years ago. (KEYSTONE/SCIENCE PHOTO LIBRARY/JEAN-FRANCOIS PODEVIN/SCIENCE PHOTO LIBRARY)