Pluto is the most mysterious planet. We know very little about it. It is too far away to see in detail with a telescope, and no spacecraft have ever seen it up close. Because Uranus and Neptune have slightly erratic orbits, a 9th planet was suspected. The pull of gravity from this 9th planet pulled Uranus and Neptune out of a perfect circular orbit. This led to Pluto's discovery in 1930 during a search throughout the sky for the 9th planet.
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