Please tell me if someone comes up with something!

Have a look at drplokta's comment further down this page.

Possibly (from a non-proper astonomer perspective) because Pluto's orbit is highly inclined it does not actually 'cross' that of Neptune: There is a 6AU gap between them (six times the Earth-Sun distance). So if Neptune was not classified as a planet because of its proximity to Pluto, then Earth is not a planet due to its proximity to Mars and Venus.

Oh, That is trying to answer the question of why Neptune isn't disqualified rather than why Pluto is.

NASA says that Pluto orbits "among the icy wrecks of the Kuiper Belt", which is how it gets demoted as a planet. Although the four planets of the inner solar system (especially Mars) aren't exactly clear of 'obstructions' [link originally from apod] in their orbits.

Good point. Pluto and Charon are linked gravitationally to each other in the same way as our moon is to Earth, i.e. they both present the same face to each other all the time (can't remember the exact term, tidal orbit? trapped orbit? My mind has betrayed me!) Some astronomers have probably reasoned that in order for Pluto to qualify as a planet, we would also have to qualify Charon as a planet (making Pluto/Charon the first classified twin planet in this solar system), and what would then happen if we had a planet as small as Charon (about the same diameter as the distance from Lands End to John O'Groats)? There are a number of asteroids in this size bracket (Ceres, Vesta etc), and even our own Moon and several moons of the gas giants, which would possibly result in the classification of Jupiter and Saturn as 4- or 5-planet systems.

The idea of Pluto being disqualified because it crosses Neptune's orbit is absurd. The closest distance between the two is far more than the closest distance between Earth and Mars - probably closer to the distance between the asteroid belt and Jupiter, which would under the same ruling thus declassify Jupiter asa planet. They haven't thought this one through much.

Personally, I reckon the classification is still flawed. It should be made very simple:
A Planet is a celestial body that (a) is as large or larger than as Earth's moon, (b) is in orbit around the Sun, and (c) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. Although it will still relegate Pluto, and probably also Xena, it will give a clear deliniation above which we can confidently say "Yes, it is a planet".

Here from useless_facts;

So basically . . . having Pluto disqualified from Planethood makes no sense?

|Meduza|

"tidally locked"

Aah, that'll be it! :D

we would also have to qualify Charon as a planet (making Pluto/Charon the first classified twin planet in this solar system)

Wouldn't that be cool! I wrote a story about something like that in eight grade.

*eighth

of the orbit that you're looking at. Pluto and Neptune stay well out of each other's way, so it's not the failure to clear the perihelion end of its orbit that Pluto fails at. But I don't know what objects Pluto is sharing the outer solar system with well enough to say what, if anything, Pluto is being charged with not having pushed aside. Several hundred trans-neptunian bodies have been found in the last few years, some with moons of their own, some nearly as big as Pluto, one of them bigger. But the outer system is so big that a mere few hundred needn't get in each other's way.

Ceres is another world that loses out by this rule, again because it shares orbits with thousands of other bodies, when a proper planet would have flung them aside, the way Jupiter and even tiny Mars does. It's an interesting criterion, and I'm not sure where it came from; I never heard of it before today. I suspect they're looking towards the day when they have to decide when the objects in the protoplanetary disk of another star become planets: the answer becomes, when they've ejected or collected the other objects they share an orbit with.

That is an interesting definition, and part C makes more sense than any definition I've heard up to now. It would exclude asteroids and Kuiper Belt objects, but I don't think just crossing the orbit is enough to eliminate a body - there are hundreds of asteroids and comets that cross Earth's orbit, but that doesn't exclude us.

I still think the definition of planet is fatally flawed, because it attempts to define a number of radically different objects. Terrestrial planets like Earth and Mars ane very different beasts from gas giants like Jupiter and Saturn.

There have also been several "extra-solar" planets discovered. Planets that don't orbit any star, whether ejected from a star or they formed in isolation, they fail under rule A.

The best thing might be to leave a certain vagueness about the term "planet" (though I like the "clear it's neighbourhood" rule), and then come with more rigid definitions of the different classes of planet.

"Clearing the neighbourhood" means that there are no other objects in the vicinity of its orbit except for smaller objects that are governed by its gravitational influence. Pluto is in a 3:2 orbital resonance with Neptune, so it counts as being under its gravitational influence.

That makes sense. Do you have a source I can cite?
(about the definition, not about the resonance)

That's an excellent paper, and looks like it addresses all the relevant questions. Figures 1 and 3 at the end are particularly helpful. Thank you.

Interesting, sensible arguments. Thanks :)

Ooo.

He's just cited a paper supporting his comment, and it's just what we need. The journalists correctly noted that Pluto's orbit crosses Neptune's, but crucially omitted any reference to the resonance -- which leaves Pluto bound to Neptune, and not a planet in its own right. Hurrah for knowledgable commenters!