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.
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