New Horizons probe successfully arrives at Pluto

[Batman]

A really huge dwarf planet would be self-contradictory. I mean that's essentially an ordinary planet.

[Borgholio]

A really huge dwarf planet would be self-contradictory. I mean that's essentially an ordinary planet.

Essentially...except for the part about clearing it's orbital path of other objects. That's literally part of the definition of planet.

[Batman]

,,,Which a really huge dwarf planet would sort of inevitably do? On account of being huge (well, for a dwarf planet)?

[Jub]

,,,Which a really huge dwarf planet would sort of inevitably do? On account of being huge (well, for a dwarf planet)?

Yes, but say we found one that hadn't, as yet, done so. Do we classify it based on what we expect that it may do?

[Borgholio]

,,,Which a really huge dwarf planet would sort of inevitably do? On account of being huge (well, for a dwarf planet)?

Well yeah if it got large enough. It's not likely there's a planet that sized in the Kuiper belt though, Neptune shakes things up too much. We'd have to look beyond for something about the size of the Earth. If it were a gas giant, our infra-red maps of the sky would have found it. So if such a thing exists, it'd likely be outside the belt and of a very specific size.

[Simon_Jester]

Mercury is about ten times bigger (as in more voluminous, not wider in diameter) than Pluto. I suppose a Mercury-sized Kuiper Belt object might well clear its orbital track; but I am genuinely not sure.

[SpottedKitty]

A really huge dwarf planet would be self-contradictory. I mean that's essentially an ordinary planet.

Essentially...except for the part about clearing it's orbital path of other objects. That's literally part of the definition of planet.

Wouldn't there also be a question about the definition of "orbital path"? Look at the orbits of pretty much anything beyond Neptune; some are more or less close to the ecliptic, and some are moderately eccentric or inclined at quite an angle, e.g. Pluto, Makemake, Eris. How close a match does the orbit of those other objects have to be before they'd be considered "in the orbital path" of a maybe-planet? I don't think I've seen any discussion of this at dedicated astronomy websites.

[Borgholio]

Wouldn't there also be a question about the definition of "orbital path"?

Not necessarily. If you look at the orbital paths of many Kuiper objects, they pass through a dense field of debris at least twice during their orbits. The major planets don't do that.

[InsaneTD]

Why was the definition of a planet decided? And when was it last redefined?

[Borgholio]

Why was the definition of a planet decided? And when was it last redefined?

Because of the discovery of Eris. Eris and other large Kuiper belt objects are nearly as big (maybe a bit bigger) than Pluto, and is was feared that if too many more of these new objects were discovered, we'd have a solar system with dozens or even hundreds of planets. That'd be a bit of a pain to keep track of. So in 2006 they defined a planet as something that easily includes the 8 original planets but excluded Pluto and other Kuiper belt objects.

[Eternal_Freedom]

It also excluded Ceres and Vesta, which would otherwise be planets (they're certainly bigger than Pluto).

[Borgholio]

It also excluded Ceres and Vesta, which would otherwise be planets (they're certainly bigger than Pluto).

Eh, what? Pluto is nearly three times bigger than either of them. :-/

[Eternal_Freedom]

Sorry, brain fart.

Ceres is considered a dwarf planet though, it meets the criteria for it; like Pluto it only misses out on major planet status because it hasn't cleared it's orbit.

[Borgholio]

Yeah Ceres is big enough to be spherical...it contains most of the mass of the asteroid belt. But the fact it's in the middle of the belt kinda blows it's chances of being planet 5. Poor Ceres.

[jwl]

Although honestly, if we found a random rock the size of Mercury in the outer system, would we de-planetize Mercury, too?

I doubt it. One of the established criteria for a planet is that it has cleared out it's orbital path of other large objects. Pluto is part of the Kuiper belt so that's why it fell out of that designation. Mercury has a clear orbital path so it would still be a planet. If we found an object way out there in the Kuiper belt as large as one of the 8 known planets, it would have had to clear all other objects out of it's area for it to achieve planet status itself. Otherwise it's just a really huge dwarf planet (how's that for a contradiction?).

Now if a really huge dwarf planet were in fact discovered that far out, the public and the IAU are going to go apeshit all over again...

Doesn't it have to be other similarly sized objects? Otherwise Neptune would be a dwarf planet.

Meaning you'd not have to just discover one really huge dwarf planet, but several.

I honestly don't see the problem if that were to happen. All I'd do is rename dwarf planets or something else, like coplanets or something. After all, rouge planets can be huge and Ganymede is bigger than mercury, and no-one seems to care about that.

[Jub]

Doesn't it have to be other similarly sized objects? Otherwise Neptune would be a dwarf planet.

Meaning you'd not have to just discover one really huge dwarf planet, but several.

I honestly don't see the problem if that were to happen. All I'd do is rename dwarf planets or something else, like coplanets or something. After all, rouge planets can be huge and Ganymede is bigger than mercury, and no-one seems to care about that.

I think capturing an object as a moon counts as clearing it from your orbital path, thus if the only objects in your orbital path are also orbiting you then you've met the criteria.

[Borgholio]

I think capturing an object as a moon counts as clearing it from your orbital path, thus if the only objects in your orbital path are also orbiting you then you've met the criteria.

That's one of the problems some astronomers have with the "clearing the path" definition. Jupiter for instance has a couple sets of asteroids in front and behind it in the same orbit, so the path isn't completely cleared. But Jupiter is most definitely a planet. So....yeah.

[SpottedKitty]

That's one of the problems some astronomers have with the "clearing the path" definition. Jupiter for instance has a couple sets of asteroids in front and behind it in the same orbit, so the path isn't completely cleared.

That's odd, I thought "concentrating the bits and pieces in the L4/L5 Lagrange points" did count as "cleared". They're certainly not going to be wandering off anywhere in a hurry once they've been perturbed into L4/L5.

BTW, it's not just Jupiter; the Sun/Earth points definitely have some dust and minor junk in them, the Earth/Moon point might have dust (not definitely proved one way or the other yet), and at least a couple of the other gas giants definitely have sizeable asteroids in their points, just like Jupiter.

[Borgholio]

That's odd, I thought "concentrating the bits and pieces in the L4/L5 Lagrange points" did count as "cleared". They're certainly not going to be wandering off anywhere in a hurry once they've been perturbed into L4/L5.

And you would not be wrong. Thing is, there is no strict definition of what "cleared" means, so it could be argued that Neptune isn't a planet because Pluto crosses it's orbital path once in awhile. It's a bit vague for some people, hence the controversy. That's why if they find a planet as large as Mercury or one of the Jovian moons in the Kuiper belt, that's basically going to cause a shitstorm.

[SpottedKitty]

Thing is, there is no strict definition of what "cleared" means, so it could be argued that Neptune isn't a planet because Pluto crosses it's orbital path once in awhile.

I see what's happened, the definition of "cleared" is having an armwrestling match with the definition of "resonance". I thought that had all been settled when it was discovered just how crazy-complicated the orbits of moons/rings/assorted junk could get.

[Borgholio]

I see what's happened, the definition of "cleared" is having an armwrestling match with the definition of "resonance". I thought that had all been settled when it was discovered just how crazy-complicated the orbits of moons/rings/assorted junk could get.

If only...

Yeah, the discovery of the Kuiper belt really threw a wrench in the things. I personally think that it would make things very simple to say that a planet is a single spherical object above a certain mass. Then they can set the mass to some arbitrary level that keeps the original 8 planets while excluding the dozens of Pluto-sized objects farther out.

[jwl]

I think capturing an object as a moon counts as clearing it from your orbital path, thus if the only objects in your orbital path are also orbiting you then you've met the criteria.

What I meant was that there has already been an object bigger than one of the 8 planets which has been discovered and isn't a planet: Ganymede, and no-one wants to give it planetary status. If there are moons bigger than planets, and all known rouge planets are bigger than Jupiter; why can't some "dwarf" planets also be bigger than planets?

[Borgholio]

Ganymede clearly orbits a planet so it's not a planet in itself, it's a moon. The barycenter of Jupiter and Ganymede is within Jupiter's mass, so it's not a double-planet system. That part is fairly simple. The part about the "rogue" planets you mentioned depends on scale. If all the "rogue" planets are smaller than Mercury (which so far, they are), then there's no reason to upgrade them from dwarf planet status. If we somehow manged to find a massive planet (unlikely, since it would have shown up on our infrared sky surveys), then we would probably consider it a whole new planet class if it was the size of Jupiter but hadn't cleared it's orbital path of other large objects.

[Eternal_Freedom]

In regards tot he discussion of whether lumping stuff into the L4 and L5 points counts as "clearing it's orbit" perhaps a distinction of "the plant has removed anything in it's orbital path it might otherwise have collided with at some point" since the Trojan asteroids aren't going to hit Jupiter anytime soon.

[jwl]

Ganymede clearly orbits a planet so it's not a planet in itself, it's a moon. The barycenter of Jupiter and Ganymede is within Jupiter's mass, so it's not a double-planet system. That part is fairly simple. The part about the "rogue" planets you mentioned depends on scale. If all the "rogue" planets are smaller than Mercury (which so far, they are), then there's no reason to upgrade them from dwarf planet status. If we somehow manged to find a massive planet (unlikely, since it would have shown up on our infrared sky surveys), then we would probably consider it a whole new planet class if it was the size of Jupiter but hadn't cleared it's orbital path of other large objects.

No, all rouge planets found are bigger than Jupiter, not smaller than Mercury. They aren't planets because they don't orbit a star, they wander around the galaxy as "rouge" objects. Basically, there are three main non-planet objects which still have enough mass to be spherical: rouge planets, dwarf planets, and moons. There are moons bigger than one of our eight planets. All known rouge planets are bigger than all eight of our planets. So if there is a dwarf planet bigger than one of our eight planets, what is the problem?