The earliest definition of the word "planet" considered the moon and the sun planets, and not Earth. But the definition evolved over time, eventually including Earth and cutting Pluto out of the Solar System's planet lineup.
Russia is Bigger than Pluto
Is Pluto a planet?
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Key Facts In This Video
Pluto is smaller than 9 moons in the Milky Way galaxy. 01:32
Pluto was discovered in 1930. 02:55
Pluto was re-categorized as an object in the Kuiper Belt, alongside other icy orbs. 03:33
Written By Curiosity Staff July 31, 2014
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