Russia is Bigger than Pluto
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.
Is Pluto a planet?
from CGP Grey
Key Facts In This Video
Pluto is smaller than 9 moons in the Milky Way galaxy. (1:32)
Pluto was discovered in 1930. (2:55)
Pluto was re-categorized as an object in the Kuiper Belt, alongside other icy orbs. (3:33)