Sedna

So there has been quite a bit of discussion of the recently discovered object 2003 VB12, unofficially known as Sedna. Some people want to call it the 10th planet. Personally, I think this is silly. Yes, it is large enough to become spherical, but it is smaller than Pluto, which is barely considered a major planet. The problem is that there is no really good definition of what exactly a major planet is. Thinking about this I have come up with my own list of requirnments to call something a major planet.

1) Non-self luminous object orbiting a star. (Smaller than a star or a brown dwarf)
2) Large enough to be spherical under its own gravity. (Larger than an asteroid or a comet)

So far this lets in all of the traditional planets, plus Ceres, Sedna, Quaoar and a half dozen other known objects. On a side note it also kicks out black holes orbiting a star.

3) Gravitationally dominant in its orbit. This gets rid of Ceres, Sedna, Quaoar, Pluto and those other similar pesky objects.
Ceres isn't dominant in the main asteroid belt, Jupiter is. Pluto is pulled around Sol by Neptune, giving them a 3-2 harmonic orbit. This does possibly leave in Sedna and Quaoar though.

4) Has a average orbital radius that is close to that predicted by the Titus-Bode rule. This throws out Sedna, Quaoar and all those other small objects that are occasionally greatly perturbed by major planets. It does come close to getting rid of Neptune, but not quite.

So there we have it, a 4 point checklist to determine if something is a major planet or something else.

More stuff later.