It’s rare you come across passionate arguments in astronomy, but whether Pluto is a planet has turned out to be a touchy subject. The New Horizons probe mission, which has sent back some of the best photos and information we have about the once and future planet, has revived the debate — especially because Pluto may have yet another argument for being a planet hidden in those photos.
It’s worth noting that the definition of a planet is relatively arbitrary: Pluto doesn’t count because it hasn’t cleared out the Kuiper Belt. But there’s evidence in the New Horizons data that Pluto might have a weather system. Photos may show clouds drifting above Pluto’s surface, and there are other signs that it has an atmosphere, like a low-altitude haze in the photos.
That Pluto may have an atmosphere and weather system sets off a whole new set of questions. What’s it made of? How does it affect the other systems of Pluto? If there are clouds, does that mean it has some sort of precipitation cycle? But to the minds of many, it also means that Pluto’s argument for being a planet has just been reinforced; there are plenty of large bodies that lack atmosphere.
This won’t change the mind of the International Astronomical Union, which doesn’t consider atmosphere as a factor when designating a body a planet. But it does lead to another question: If Pluto is a large body, with all the stuff the other planets have, why is it not a planet as well?