Pluto is the last of the nine familiar planets to be visited by a spacecraft and the program made a major discovery even before leaving the ground.
NPR: NASA Sends Spacecraft to Pluto
First, its orbit appears to run in the opposite direction from those of the nine known planets.
ECONOMIST: X marks the spot
"Make no mistake about it, " said Geoffrey Marcy of San Francisco State University, who has helped find nine of the 12 planets discovered since 1995.
CNN: Astronomers spy new neighbors outside solar system
If the International Astronomical Union approves the proposal, it will change not just that old saying but textbooks and toys and all the other things that rely on nine planets.
NPR: A Down-to-Earth View of the Planet Debate
So there are eight planets in the solar system, not nine and not 10.
NPR: Pluto, the Un-Planet?
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