On Aug. 24, 2006, the International Astronomical Union (IAU) decided that Pluto wasn't a planet anymore.
Since that date, Pluto has been officially classified as a dwarf planet along with four other confirmed dwarf planets in the outskirts of the solar system.
This decision sparked outrage around the world, and the controversy is still very much alive today. Astronomers in favor of Pluto's demotion argued that a planet by definition should have cleared its orbit around the sun. Pluto and the more distant dwarf planets fail to meet this criterion.
Why it mattered
When Pluto was reclassified as a dwarf planet, many astronomers and scientists weren't happy with the result.
"I'm embarrassed for astronomy," said Alan Stern, leader of NASA's New Horizon's mission to Pluto and a scientist at the Southwest Research Institute. "Less than 5 percent of the world's astronomers voted."
Stern argued that according to the IAU's new definition, many other planets in our solar system would fail to meet the same criterion. Earth, Mars, Jupiter and Neptune, for example, all share their orbits with asteroids.
But if Pluto were to keep its planetary status, many of the solar system's dwarf planets would also need to be classified as planets — and there could be hundreds in our solar system.
While astronomers continue to debate the definition of planet and the implications thereof, the public has come together multiple times to defend Pluto's title.
For now, Pluto will just have to be the king of the dwarf planets. And that's pretty cool, too.
