All of the planets in the solar system would be rotating in the same direction if the solar system formed the way science says it did.
The accepted explanation for why some planets rotate in an anomalous direction is that their directions of rotation were changed by large cosmic impacts. There were periods of tremendous bombardment throughout the history of the solar system, especially in the first stages of its formation, when there were still large amounts of loose matter in the solar nebula, some of which condensed into the planets and other planet-sized objects. Even the moon is now believed (because of its composition) to have been formed from material ejected from the Earth due to a tremendous collision very early in its history. This helps to give us some perspective on how violent such impacts could be; certainly some could be enough to set a planet spinning in a different direction than it would have were it left alone, especially if the collision occurred early in its formation.
Last updated: 21 Mar 2008
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