Okay, I don't really have a dog in this fight, but there are those who do: Apparently the head of the New Horizons Pluto mission thinks the IAU's new definition of "planet" -- the one that drops Pluto from the list -- isn't all that it's cracked up to be.
Here's the new IAU definition:
The IAU therefore resolves that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A "planet" is 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.
(2) A "dwarf planet" is 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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
My thoughts: It's all very arbitrary, especially considering that just a week ago the IAU was considering a definition of "planet" that would have kept Pluto and added three more pluto-like objects.
(The (c) criterion was a last minute addition). What's wrong with just saying that the nine planets are planets and anything else is not a planet? Or, that anything above an arbitrary size that orbits the earth is a planet?
Also, do we really even need an "authorized scientific" definition? I mean, honestly. We don't have a "scientific" definition of "continent," yet geologists, geographers, and the laity all generally refer to Europe as a separate continent even though it really makes no sense to do so. (And why isn't Australia just a big island?). What would happen if we all just ignored the IAU definition?
I suppose that a definition becomes useful when referring to objects orbiting other stars, but with those, you also get into the brown dwarf problem...
Also, thanks, everyone, for the birthday wishes.