This week, I sat down with Dr. Will Grundy, an astronomer and planetary scientist with the Lowell Observatory—the place where Dr. Tombaugh discovered Pluto in 1930! Who better to speak to about Pluto and the “Great Planet Debate”? The debate began in the early 2000s with the discovery of many new objects orbiting beyond Pluto. It was “resolved” with the IAU decision of 2006 that defined a planet as a celestial body that:
“(a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.”
As you’ll see in this episode, the debate is far from over and can get pretty contentious. Planetary scientists, astrogeologists, and people from other disciplines have criticized the IAU’s rather narrow definition and believe that planethood should be extended to all the major bodies of our Solar System. Follow the links below to learn why…
Stories by Williams 
I’m a fan of the geophysical definition of a planet. Dictionaries define words based on the actual usage of those words. It’s a descriptive, not prescriptive, process. What the I.A.U. did was ignore the actual usage of the word planet and prescribe a totally new definition. The geophysical definition, on the other hand, is a pretty good description of what I think most people already understand the word planet to mean.
I concur! I used to push back against people who asked that I write about the planets differently, that I make room for the whole debate thing. However, I came to learn more about the geophysical side of things and concluded that they were absolutely right.
Oddly enough, I just saw this was the topic of today’s XKCD. I kind of like the “spiteful” definition now, too.
https://xkcd.com/3063/