Is Pluto a planet? On Tuesday I talked about how 'Algol' isn't technically an Algol-type star system. This raised the interesting case of "is Pluto a planet". To a large degree, that's a non-argument. Pluto hasn't changed. Here's an example I often run for students. All you readers out there, if you have dark hair-- black or brown hair-- raise your hands. Don't worry about your officemate starting, just raise those hands. Got them up? Okay, I want everyone who has dark hair-- just black hair-- to raise your hands, but anyone else with their hand up, lower it. Yes, dark hair is now defined as 'black'. Brown hair is no longer what I'm calling dark. Okay, those of you who first raised your hands for 'dark hair' but then lowered them when I changed the definition-- did your hair color change? Did my changing the term 'dark hair' change you in any way? No? Good! So it is with Pluto. Certainly the New Horizons mission-- whose PI, Alan Stern, has declared he will continue to call Pluto a planet-- didn't change course when the labeling of Pluto shifted. "Abort, abort, Pluto just turned into a not-planet, turn away before we're doomed!" Too Star-Treky, that. The name is not the thing. I've heard others describe Pluto as a 'binary planet' since its moon, Charon, is massive enough that their shared center of mass is above Pluto's surface. And I've heard Pluto called a big comet. Pluto gets called a lot of things. So does the Earth. Did you know the Earth is not just a planet, but a 'water planet' and an 'earthlike planet', and we're looking for similar ones around other stars? It may sound obvious, but each planet is unique. In actual astronomical use, the term 'planet' is fairly meaningless, akin to saying it's an "orbiting around a star thing, that isn't a star itself". But then there are brown dwarfs... neither star nor planet. So that definition breaks down. Everything is a special case. When the IAU voted to state The "dwarf planet" Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects, that did not change what Pluto is. Nor does it fundamentally change how Pluto should be considered. Pluto is still a fascinating object, rich in history, worth studying, becoming more intriging the more we learn about it. Pluto is a planet, a trans-neptunian object, a comet, a binary, a Kuiper Belt object, and an animated dog. If you want a precision, try this oft-used definition: Pluto is the tenth-largest body observed directly orbiting the Sun. The universe is too rich to fall into tidy categories. Pluto is ... Pluto! [Update: Just to keep things silly, Illinois votes to 'restore' Planethood to Pluto.]