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Early solar system planets. Artwork of the early solar system, with a possible fifth outer planet, similar in size to Neptune and Uranus. This theory postulates that this planet was ejected from the solar system due to gravitational interactions with Saturn and Jupiter. The presence of this fifth outer planet helps explain why the early Earth did not collide with an early Venus. (KEYSTONE/SCIENCE PHOTO LIBRARY/NICOLLE R. FULLER/SCIENCE PHOTO LIBRARY)