Asked by: Ed via email

Full Question: I have a question on Pluto's official designation. Is it a kuiper belt object or a dwarf planet? If it's a dwarf planet, then is it still a planet? My understanding was that it was downgraded, but to what?

Standard Practice

First, the idea of "official" status is over-rated. It does not happen in science that one person or group gets to dictate what others think. While the exact wording of IAU motions can be debated by zealots ad infinitum, what really matters is how day-to-day astronomers treat things.

Pluto, in general astronomy practice, is one of the largest members of the Kuiper belt. The Kuiper belt is a broad band of small icy objects located beyond the orbit of Neptune. More than 1000 Kuiper Belt objects (KBOs) have been discovered, and about 100,000 of reasonable size are thought to exist.

IAU Motions

As for the International Astronomical Union (IAU) designations, one motion calls it a dwarf planet and another motion calls it a plutoid. These designations are almost identical, but the plutoid definition excludes objects from the asteroid belt. Neither definition is useful, as they do not define a category of objects. They merely call out a special set from standard categories. Both designations simply note that there are large objects in the asteroid belt and the Kuiper belt that are spherical. In all other ways, objects that fit these definitions are typical asteroids and KBOs.

Not a Planet

So, you can see that a dwarf planet is not a planet. It is simply a large, spherical member of the asteroid belt or the Kuiper belt. The asteroids are traditionally called "minor planets", and I don't hear anyone arguing that we should now recognize several hundred thousand planets in our solar system. Consider "dwarf planet" to mean the same as "giant asteroid" or "giant KBO".

Yes, this is a downgrade for Pluto. But it happened for the largest asteroids (Ceres, Pallas, Vesta, Juno) during the 1800s as well. Nothing really new or wrong with it. In fact, such changes are inherent in the process of science. It is supposed to self-correct over time. And if we are wrong now, well, wait a hundred years until we learn more.