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What makes a planet a planet?

Last year, the debate over Pluto’s planet status temporarily resurfaced when NASA’s New Horizons spacecraft flew by the tiny world at the edge of the Solar System. Once considered the ninth planet, Pluto was famously demoted a decade ago to a dwarf planet — a new kind of classification for objects in our cosmic neighborhood. It was a change that some didn’t take very well, and 10 years later, a few still believe that Pluto’s status should never have been altered.

So what caused Pluto to start running with a new crowd of objects? The major catalyst for the reclassification began with the discovery of another world at the Solar System’s edge in 2005. At the time, many astronomers thought this new space rock was the 10th planet, since it looked like it might have been bigger than Pluto. The discovery ultimately led to a huge discussion about what defines a planet in our Solar System, one that resulted in Pluto’s new title.

Check out what happened in the video above.