Pluto is NOT a planet

I was startled to see so many people disappointed that Pluto isn’t considered a planet anymore. To anyone that knows much about Pluto, it makes perfect sense. Considering Pluto a planet is like considering Greenland a continent.

Pluto is one of the largest of the trans-Neptunian Kuiper Belt (a second asteroid belt outside the boundaries of the Solar System, over 20 times larger and 200 times more massive than the one between Mars and Jupiter), but is one-fifth the size of our moon. Pluto’s composition of ices means that if Pluto was brought into the inner solar system, it would acquire a tail like a comet and eventually burn off.

What’s even more embarrassing is that another Kuiper Belt object, Eris, was discovered in 2005 that is actually even bigger than Pluto! Pluto isn’t even the biggest of the objects out there! What’s more, they are constantly discovering huge objects on the Kuiper Belt. Most of them have the same general look as Pluto: rocky balls of frozen ices. It’s also commonly believed some moons in the solar system, some many times larger than Pluto, are originally Kuiper Belt objects.

In other words, Pluto isn’t that special, as astronomers are now finding out. Does an object this dubious really deserve the honor of being called a planet, in the same category as Earth and Jupiter?

Lots of people get sentimental about Pluto, but what about poor Ceres? Now there’s a world left off from our childhood nursery rhymes! Ceres, as you might know, is the largest of the asteroids and composes at least a third of the mass in the Mars-Jupiter asteroid belt, a much greater percentage than Pluto has in the Kuiper Belt. Ceres is smaller than Pluto but rock all the way through and is, in many ways, a much better candidate than Pluto for planethood.

The specific definition of what constitutes a planet is a definition that is long overdue – this is one of the many occasions where words mean things, and that definition should be specific as possible. The solar system has a lot of junk in it and not all of it is a planet. In fact, I once heard a great way an alien space probe would define our solar system: the Sun, Jupiter, and some debris!