Being a planet is not really a state of being. It is instead a human construct, a categorical designation, and a slippery one at that. Which means that whatever Pluto actually is, its essential Plutoness, is not reliant on whether some humans who live billions of miles away from it decide to call it a planet.
Does it even matter what we call Pluto? Well, yes and no.
Pluto’s planetary status has prompted much debate in the past decade. In 2006, the International Astronomical Union reclassified it as a dwarf planet—a move that lots of people described as a demotion. Pluto had left the ranks of then-nine classical planets and joined the peanut gallery of dwarf planets, only a handful of which have names.
But what’s so great about being a planet anyway? In our solar system, planetary status is pretty elite. There are eight planets (Mars, Venus, Earth, Mercury, Jupiter, Saturn, Uranus, and Neptune) and hundreds or maybe thousands of dwarf planets (Pluto, Eris, Haumea, Makemake, Ceres, &c).
Pluto is tiny compared with Earth, but it's not just its size that makes it a dwarf. Here's how Vox explained it:
What finally led the International Astronomical Union to reconsider Pluto's status was [the Caltech astronomer Mike] Brown's discovery of... Eris, [a dwarf planet] that was actually a bit more massive than Pluto. Aware that this would probably just be the first of many, the IAU voted to approve a new definition that would eliminate all of these objects from the list of planets — rather than continue to add more and more planets in future years.
Gail Oxton, who built the software for New Horizons and has been on the mission since 2002, says she and her colleagues were “devastated” when Pluto got demoted. “And it still stings a little,” she told me, “especially when we hear yet another joke about poor Pluto.” (Oxton says she did find some of those jokes to be funny, however, and she still keeps this comic taped to her wall.)