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The outer planets are usually defined as Jupiter, Saturn, Uranus and Neptune. The innermost of those, Jupiter, moves around the sun the fastest. The speed of planet is a function the size of its orbit only -- mass doesn't matter, as figured out by Kepler about 500 years ago. Thus Mercury, the innermost of all planets, has the greatest orbital speed (87.96 Earth days for an orbit), and Neptune the slowest (164.81 Earth years). If you want to include Pluto, then it has the greatest orbital period (247.7 earth years)

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∙ 17y ago
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