Quote Originally Posted by crayzz View Post
On the note of neighbourhood clearing:

There are a couple of metrics for this. It's not a binary thing, there are calculations you can do to see how well a planet has cleared it's neighbourhood. Jupiter consistently tops the list based on these metrics, whereas the Pluto, Eris, Ceres et al are always on the bottom. Now exactly how well these metrics actually describe what we mean by "cleared neighbourhood" is the subject of some debate, and of course whether a cleared neighbourhood is actually an important factor is also up for debate, but there are measureable relationships between a planet and it's orbital neighbourhood, and there is a large difference between the relationship the 8 uncontroversial planets have with their neighbourhood and the relationship the rest of the planetary candidates have. The fact that the math backs this up to some extent makes me a lot more sympathetic to the more intuitive objections to including Pluto et al as a planet from 100+ years ago: I think early astronomers were grasping at something when they said "wait no, Ceres seems pretty different from the other planets" when more asteroids were found, and later when the gradual shift from "Pluto is a planet" to "maybe it isn't" occurred as more and more Kuiper belt objects were discovered.
The only planets whose planethood I am strongly commited to are Earth and Venus, on the basis of their similar masses. Mercury and Mars are too small, the others have too much pressure.

I am very serious about having a problem with planethood changing due to the distance of the object from the Sun. What about a Jupiter+ mass object with a very eliptical orbit like a long period comet? it couldn't clear its orbit because it would precess, but it would clearly (every million years or so) strongly interact gravitationally with any lighter objects in circular orbits near the star.