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Explanation:

Reason 1: Some planets, like Uranus are farther from the sun, and thus have a longer orbit, or planets closer to the sun, like mercury, have such a short year because they are so close to the sun.

Reason 2: The sun's gravitational pull is stronger on planets closer to it then farther, which is why Mercury has such a short year in comparison to Earth or Uranus.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.