3 - It must have cleared the neighborhood around its orbit....
"Pluto fails to meet the third condition, because its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit). The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified."
In other words, clearing it´s neighborhood means that the particular object must be by far the largest mass in it's orbit.
You might argue that Pluto is not a planet because it is in a resonant orbit with Neptune, but you could reverse that argument and say that Neptune is not a planet.
no, you cannot, because Neptune is by far, VERY FAR, the largest mass in it´s orbit.
I am sure that when we are confronted with such a case, we will create a definition.
Fact is, we need a threshold, otherwise we would have hundreds (not 30) plants on the solar system. It seems you people are not searching for a good scientific definition of a planet... the definition you guys want is "what I studied at school".
Balderdash. The basis of your arguement is that Neptune is fare more massive than Pluto, true, but when it is pointed out that Neptune does not perturb Plutos orbit enough to make Plutos orbit enough to make it unstable through the mechanism of orbital resonance does not mean that the two "planets" have the same orbit. It just implies that in the course of their separate orbits, they never come close enough for close gravitational encounters.The orbits cross, but they are not the same. A simple glance may support your reasoning about mass differences, after all without the special condition of orbital resonance, surely Neptune would "clear" Pluto from it's orbit. But this qualifier is not included in the definition. A scientific definition must be con sidtant irregardless of the tests you submit it to. Some examples- A Earth or Mars type body in a resonant orbit with a super Jupiter, or even a binary star. (if you want to push the mass differential). Is the Earth twin now considered a dwarf planet?. And how about a body in exactly the same orbit as Earth but on the opposite side of the sun. Can only one be considered a planet? A better comparison might be a Mercury size body orbiting opposite Jupiter. They are both in the same orbit and one is much more massive than the other. Does this demote the Mercury body to dwarf status?
Definitions of planets should be applicable to any stellar system, not just the Sol system.
The only definitions that I can think of would be that the body is in a stable orbit around a star, is not fusing hydrogen to any meaningful extent (Jupeter may be fusing the occasional deuterium- deuterium in it's core, but it is trivial). And, the body is spherical. These criteria can be applied universally. You can add additional qualifiers like mass limits (it could elminate the need for sphericity considerations as the mass chosen would incorporate this, so long as you give enough margin so that mass vs volume does not contribute. A larger volume may be needed for a young gas dominate planet vs a rocky planet for sphericity considerations, then again maby not as it doesnt require much gravity for a gas to assume a spherical distribution. The same might be said for a liquid planet versus a rocky planet. Arbitrary radii or masses might then be applied for subclassing planets- dwarf, regular, giant, super giant... All of this is at least partially arbitrary, but at least it could be applied universally.
As I said, you could also eliminate trojans based of some shortfall in mass ratio, but again this does not apply to Pluto. It is not a trojan of Neptune., as it is not in the same orbit of Neptune. If you use the argument that Pluto crosses Neptunes orbit and thus must be grouped into Neptunes orbital influence and Neptune is far more massive than Pluto and so it alone deserves the title of planet, there is again a flaw. If Neptune is defined as a planet, then it must have cleared it's orbit, at least of those objects which are otherwise in the same orbit. Yet, pesky Pulto remains in the defined Neptune orbit, ergo Neptune is not a planet. Resonance must be accounted for, one way or the other. You can define a mass difference and assign the title to the dominate partner, and that is fine for the solar system. But it could break down is other star systems. It is bnot consistant and reasonable. You could have a super Jupeter in a resonant crossing orbit with a Earth size body. Should you demote the Earth body?
Stable orbit and minimum size/ mass/ sphericity are the only criteria that I think are reasonable measures across all star systems. This is not what is applied in the IPS definitions, and it is thus the nonscientific villain.
Whether you set limits that allows only Jupiter to be a planet, or admit the potential dozens of bodies that are round and orbit the Sun is irrelavent, just be consistent. The question of shared orbits or crossing orbits introduce complexities that are open to interpretation. If the orbit is stable for at least a billion years , for example, then it is good enough. Clearing an orbit is a dynamic and never ending process. Jupiter , or at least many examples of large to small planets orbiting stars may entirely eject other quite reasonable planets from the star system given enough time. Even a small planet might perturb a larger planet enough that it interacts with the parent star and gets ejected from the system. Heck, even a moon might do that. So, even the orbital stability is relative. You can only say the body is in a stable orbit within some time/ computed limit. Then there are the rare but possible encounters with extrasolar passing stars or orphan planets (is it still a planet?) that can perturb orbits.
To error is human... and I'm very human.