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The Outer Solar System

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On Tuesday the 13th of March, between ten and eleven in the evening, while I was examining the small stars in the neighborhood of H Geminorum, I perceived one that appeared visibly larger than the rest: being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet.

William Herschell, 1781

Questions to Explore:
  • How did astronomers discover Uranus, Neptune, and Pluto?
  • Why are Uranus and Neptune so different in appearance?
  • Why is Neptune self-luminous whereas Uranus is not?
  • What do we know about the ring systems of Uranus and Neptune?
  • What kind of object is Pluto?
  • How have astronomers tried to find planets beyond Pluto?

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