Well, it’s official. Pluto is no longer a planet. It is now a dwarf planet. While this is probably a reasonable result in and of itself, the new definitions have a few problems. I won’t bother duplicating the definitions nor point out all the various news coverage. Google is your friend. Instead, I’m going to argue with the definition. (Hey, it’s my blog after all.)
The new definition for planet will include the eight classical planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. It will include nothing else. It will also exclude absolutely every object orbiting around any star other than Sol. Yes, folks, that’s right. The new definition explicitly states "in orbit around the sun". it also excludes any object that does not dominate its neighborhood which is what excludes Pluto since Pluto crosses Neptune’s orbit.
I have one major issue with the definition. It only applies to the Solar System. What’s so special about our stellar system? (Yes, I know, it’s our home, but that doesn’t make it special.) Even if our system is special, why can we not have a generic definition of just what a planet is?
Another problem is that this definition, even were it to be generalized to other stars, precludes binary or multiple planets. If two equal mass objects, say the size of Earth, were in mutual orbit and also in orbit around Sol, neither one would be considered a planet. That’s right, folks. Since neither one would have dominated the neighborhood and neither one is clearly a satellite of the other, neither object would be a planet. Wonderful, eh?
I can see that the science fiction world is going to look at the astronomers and then thumb its collective noses at them. I can see the science fiction community adopting something much closer to the original proposal. I can see something as follows being used as the definition for a planet in science fiction:
- in orbit around a star
- is not a star
- is sufficiently massive to overcome rigid body forces to become approximately round (allowing for a bulge at the equator for spinning objects)
- is not a satellite of another object
There would also likely be different classes of planet. One such set of classifications might be:
- gas giant: like Jupiter or Saturn
- rocky: balls of rock like Earth or Mars
One can also define a binary planet as two planets that orbit a point in space between them. This can be generalized to more than two planets as well. Thus, in the Solar system, Pluto and Charon would be considered a binary planet while Luna would simply be a satellite of Earth.
Obviously, there is much room for debate. I just can’t see the scifi people accepting such a short sighted view as the astronomers have taken. Not to mention, none of us has to agree with the astronomers.