[Phantom Hoover]

Neptune has 'cleared' Pluto in that Pluto is locked to its orbit at a 3:2 resonance. It's a very sharp criterion, really. Orbital mass fractions show a similarly dramatic division between the planets and the dwarfs.

[Bobboau]

ok, well in that case pluto has cleared neptune.

how about this, if neptune wasn't there but everything else was the same, would pluto then be considered a planet? If we somehow found another solarsystem and it had nothing in orbit around it's star but a few pluto-like objects in distinct orbits, would they be considered planets?

[Phantom Hoover]

Pluto hasn't cleared Neptune because Neptune's mass is 4 orders of magnitude larger. If Neptune wasn't there Pluto still wouldn't count as a planet because its orbit is crossed by countless asteroids and a few other dwarf planets, some of larger mass, which are in unrelated non-resonant orbits because Pluto doesn't have the gravitational influence over them.

[Bobboau]

whatever, I reject that criteria so I don't know why I'm arguing about it.

[Phantom Hoover]

If you don't even understand the criteria why are you bothering to get worked up about them?

[Klaustrophobia]

I refuse to release my deeply ingrained elementary school education of the solar system.  All thorough my formative years of wonder and discovery of space and science and stuff, there were 9 planets.  Pluto was one of them.  Then they took away part of my childhood.  This is my reason for rejecting Pluto's non-planet status.

[Bobboau]

If you don't even understand the criteria why are you bothering to get worked up about them?

I understand and disagree. you could easily have an earth sized body in a similar configuration as pluto is, I don't think the question of if something is a planet or not should have anything to do with anything outside of itself and the star it's orbiting.

[Dragon]

Neptune has 'cleared' Pluto in that Pluto is locked to its orbit at a 3:2 resonance. It's a very sharp criterion, really. Orbital mass fractions show a similarly dramatic division between the planets and the dwarfs.

What does "cleared" mean anyway? No bodies in the same orbit at all, no bodies in the same orbit except ones locked in resonance, no bodies in the same orbit that are not locked in a resonance or in Lagrange points...? Most planets have "trojan asteroids" in their L4 and L5 points, if their presence meant an object didn't "clear" its orbit, then the only planet in the Solar System would be Saturn (probably because Jupiter keeps messing with its trojans, though perhaps we just can't find them). IAUs definition was chosen in a rather lousy way.

Orbital mass fractions could be a good way of differentiating dwarf planets from "full" ones. Another would be designating the most massive object in an orbital neighborhood as a planet (that would make Ceres a planet, but still exclude Pluto).

[Phantom Hoover]

What does "cleared" mean anyway? No bodies in the same orbit at all, no bodies in the same orbit except ones locked in resonance, no bodies in the same orbit that are not locked in a resonance or in Lagrange points...?

idk maybe you could use one of these three different well-defined metrics that all show a massive quantitative difference between planets and dwarfs

[Bobboau]

using your source, mass the difference between Pluto and Mercury is about one order of magnitude, which is about the same as the difference between Saturn and Jupiter, or between neptune and Saturn, or between Earth and Mars, or between Mars and Mercury. so it looks like there is a fairly smooth gradient there.

also I find it entertaining that one of the guy's who's name is in one of those metrics agrees with me. (and I also find it entertaining that he is named after the ****ing Sun)

[Phantom Hoover]

using your source, mass the difference between Pluto and Mercury is about one order of magnitude, which is about the same as the difference between Saturn and Jupiter, or between neptune and Saturn, or between Earth and Mars, or between Mars and Mercury. so it looks like there is a fairly smooth gradient there.

Yes but Mercury has cleared its orbit. It's not about the mass, it's about the parameters given, all of which agree that there's a 3 or 4 order of magnitude difference between the true planets and the dwarfs. Stern's quoted objections to the redefinition are extremely weak, as noted in the article, and I think it is extremely relevant that at the time of the redefinition he was heading the New Horizons mission and had a vested interest in ensuring that it would still arrive at a planet.

[Bobboau]

ok, so if Mercury somehow had it's orbit shifted to the outer solar system and ended up in a 1:1 resonance with neptune it would stop being a planet?

Look, I can invent some sort of metric that shows a >5 order of magnitude difference between earth and all other objects in the universe, the question is if that metric should be used to determine if something is a planet. I don't think any of those should.

[Phantom Hoover]

If Mercury got shifted into orbit around Jupiter it would magically stop being a planet as well.

[Bobboau]

so by that "as well", I can assume the answer to my question is "yes" then?

(you would have an easier time convincing me that moons should be classified as a subtype of planet than a planet should stop being a planet because it's further away, BTW)

[Scotty]

Whenever I hear somebody tell me "Pluto isn't a planet, it's a dwarf planet" I hear "this isn't a car, it's a sub-compact car" which is still a ****ing car.

[Phantom Hoover]

I think a better analogy here would be to a mobility scooter.

[Scotty]

It would be if we called cars "automatic scooters".  The term "dwarf planet" literally has planet in it, but Pluto isn't a planet.  Of course.

[NGTM-1R]

so the plan 9 from outer space was to have a planet 9 from outer space?

I dunno, haven't watched it.

You should.

Being drunk, high, or full of snark is required, though.

[Bryan See]

There's one from Scott Manley:

[watsisname]

It would be if we called cars "automatic scooters".  The term "dwarf planet" literally has planet in it, but Pluto isn't a planet.  Of course.

Guinea pigs are not pigs, nor are they from Guinea.  Neutron stars are not stars.  We drive on parkways and we park in driveways.  Names are names.

There seems to be a lot of confusion in this thread about why astronomers decided to no longer call Pluto a planet, and instead introduce this new category of "dwarf planets".  Id' like to try to give some insight as to our motivation for doing this, and what these names mean.

We have discovered that there are a lot of objects in our solar system that are big enough to be round.  We could, if we like, choose to call all of them planets.  Or we could try to narrow it down.  Why would we do that?  Well, we notice that only a few of these objects are really unique.  They are unique from one another, and they also really stand out from everything else in their orbital space.   This is often described [in my opinion very poorly] as "they've cleared their orbit".  Obviously, no planet has cleared its orbit completely.  Not even Earth.  What we really mean is that they have cleared their orbits of other bodies that are comparable to itself in mass.  In other words, the object itself makes up the overwhelming majority of the total mass in its orbital space.

We can quantify this with such measures as the Stern-Levison parameter, or planetary discriminant, as Phantom Hoover mentioned earlier.  We notice that "planets" and "dwarf planets" are separated by several orders of magnitude by these measures.  This is huge!  Dwarf planets are not simply "smaller planets".  They are a completely different class of object!  They have failed to meet one of the criteria we choose to define a planet -- they do not dynamically rule their orbital space.  They are instead remnants of incomplete planet formation.  They did not accrete into a single body to dominate that region.

So it is important to note here that we do not simply define planets by size or by mass.  Those are important, because they determine when bodies shift into a more spherical shape by hydrostatic equilibrium, but it's not the whole story.  The other part of the definition is a statement of dynamics.  If you put Earth into the same orbit as Venus, neither would be planets.  If that seems silly, ask yourself how stable such a configuration would be.

tl;dr -- physical properties are important, but dynamical ones are, also.