A Mystery of Space: S T A R S- [Pluto]

Pluto, the ninth and farthest planet from the sun, is the planet that we know the least about. It is too far away (3.6 billion miles from the sun) for a telescope to be able to study it, and there haven't been any spacecrafts that have traveled by it.
In 1905, American astronomer Percival Lowell discovered that Uranus and Neptune had slightly erratic orbits, leading him to believe there was a ninth planet. Pluto was then discovered in 1930, 25 years after the search for the planet first began.

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Pluto Fact Sheet