"cleared out" does not mean "created a surrounding vacuum"

Neptune has definitely "cleared out" Pluto, whose orbit is completely controlled by Neptune -- much like a moon orbiting a planet.

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What does it mean? We've discussed this before. Pluto and the plutinos do not appear to have been sheparded into place, but rather are the remnants of the previous population after Neptune has cleared out the rest.

Not only Pluto is insignificant in size compared to Neptune, but Pluto's orbit is under Neptune's dominance (in 2:3 orbital resonance). In addition, being a member of the Kuiper Belt, Pluto does not control its neighborhood, i.e. hasn't "cleared its neighborhood".

That of course does not mean that Pluto itself is insignificant, no way. It is a very intriguing object, however not a planet.

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Clearing happens very slowly out there! It was a stitch-up!

"and (c) dominates the neighborhood around its orbit clearing it of comparable objects."


Rather Pluto, Orcus, Ixion, and Huya where born or forced into their orbit, they never come close to Neptune because of the 3:2 resonance period. Pluto gets closer to Uranus than Neptune.


Pluto was announced in the New York Times on March 14th, 1930, in an article titled: "Ninth Planet Discovered on Edge of Solar System; First Found in 84 Years" No. 26,347. The article starts: "The Sphere, Possibly Larger than Jupiter, Meets Predictions." Lowell Observatory thought they had found Planet X (6.6x more massive than Earth, 8,000+ miles in diameter)

In 1959 a lot of astronomers still thought Pluto was 14,500km (8,700 miles) in diameter. That was back when Pluto was truly considered a Planet! We now know that Pluto is 1/500 Earth's mass and only 2300km in diameter.

(click here to see Pluto starting to shrink in 1959)
Source: Planets 'Other Worlds of Our Solar System' Golden Library of Knowledge (1959)

-- Kevin Heider

Lets say, for example, that there were still alot of objects in Neptune's orbit. Under the new definition, would it not be a planet?

This definition seems to mean at some point, a planet like Neptune, Mars, etc would look the same as they do now but not be considered a planet because there is too much "debris" in it's orbit. In 400 years, if Pluto's orbit were cleared (not that this is realistic) would it be promoted to Planet status again?

I do not think this was as scientifically thought out as some of you seem to think. But that is just my opinion.

They need to define a size, period. If we find a Jupiter size planet orbiting a different star, and find out it is surrounded by a large asteroid field, what will we call it? We will call it a planet without even a second thought.

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Pluto T-shirts?? I'm way more excited about Europa than Pluto. Why can't I have a Europa t-shirt? Europa can beat Pluto anytime, and I bet it's larger, too!!!

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In an evolved stellar system such as ours, that scenario is highly improbable...Neptune's gravity would clear out or dominate any significant objects. The only type of system where you'll see a gas giant that hasn't cleared its orbit will be in a protoplanetary disk...but then IMO we're dealing with protoplanets/planetoids, not true planets.

Yeah a planet and planetoid could easily look the same. So what do Titan and Io look like?

If Pluto's orbit was cleared in 400 years it would be promoted, but again... you'll only find objects in the process of clearing their orbits in young, evolving stellar systems.

A planet, since I don't think even a large asteroid belt will come close to rivaling the mass of the giant planet.

I used to favor calling everything gravitationally-round and not a star a planet with different sub-classes. In fact, I still think this type of scheme will be the most efficient at some point in the future...but a big flaw in this idea is that nobody is going to accept or call a moon a "secondary planet." The public actually understands the concept of moons, so there's no reason to change that and make things (in the public's eye) confusing. I should note I think a moon should still be round...non-spherical satellites can be called moonlets.

So since we're already accepting a certain class of spheroids as non-planets based on orbit...we might as well eliminate objects with another significant orbital trait: shared/non-dominance. For these objects (including objects in a young stellar system) I prefer planetoid instead of dwarf planet simply because it's not an oxymoron. So our solar system contain planets, planetoids, and moons...plus asteroids, comets, moonlets, and debris. Still plenty of sub-categories from there based on physical traits or location relative to the Sun.

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It depends on the mass and orbital properties of those bodies. There are BILLIONS of objects in Saturn's orbit, but Saturn is still a planet because it completely dominates those billions of bodies.

I do not think that your opinion is as scientifically thought out as you seem to think. To honestly think that there would be any difference between Pluto's orbit now and in 400 years is absurd. Pluto was formed and settled into its orbit billions of years ago. Planets are not bouncing around the solar system as if it were a pinball machine.

No, not "period". "Period" is used when you want to establish your opinion without discussion. That is not going to happen on an internet forum!

OK, let's be clear about something. If there is a Jupiter-size planet surrounded by a large asteroid field, then we are surely witnessing the result of some catastrophic and recent event. The very reason that you DON'T see Jupiter-size planets in asteroid fields is because their immense gravity either ejects or absorbs any asteroids in their immediate vicinity rather quickly.

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I agree that dwarf planets is an awkward and misleading name because it implies that the distinction is based on size rather than orbital properties. However, I think planetoid has the same problem. We have 3 known types of shared/non-dominant orbits: moons, trojans, and the unnamed category of resonant bodies like Pluto.

There has to be some apt mythological term for these "escort" planets.

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Quick question: Percentage wise, what is the amount of KBO's closer to the sun than Pluto vs. the amount of KBO's farther out than Pluto?

Everything I have seen so far on my search shows Pluto, then the Kuiper Belt, not Pluto in the Kuiper Belt.

If you have a good sight that shows what I am asking, please Linky it.

Thank you for the help.

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That is why I stated "(not that this is realistic)". I am not really concerned about Pluto itself, more about how the definition will hold up to other objects that will be discovered in the near future. Let's say we find a planet the size of Mercury in our asteriod field ( I know it is not possible in our solar system, but it is not impossible to see elsewhere, I just use ours for the sake of clarity), how would we classify it?


I meant "Period" as in to use nothing else but size to considered to determine planet status, not as in "Period, end of discussion". I wouldn't be on a board asking so many questions if I didn't expect different answers.




Considering all the different things scientist discover each year, I don't think a "what if" scenario for discussion is very absurd in relation to what the IAU has set forth as requirements to be a planet.

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I'm assuming you're talking about an object the size of Mercury orbiting another star within an asteroid belt around that star?

If so, then under the IAU's current definition in relation to extrasolar planets, it would be a planet.

Under the IAU's definition of planet in our solar system, it would be a dwarf planet.

And this underscores the silliness of the situation. Basically, we have a different definition in our own solar system simply because people wanted to keep the number of "planets" down to a small number when in our own solar system. That's it, that's the whole reason, and a poor reason at that. It would be like defining stars in the normal way in other galaxies, but only stars with solar systems like ours should count as stars in our own galaxy-- so as to avoid having too many stars in our own back yard! The definition of planet should just be based on the internal physics of the object, and that pretty much comes down to size. Who cares how many end up being in our solar system, that's not our concern. If we're going to have "dwarf planets", then it already means we have subclasses of planets, so why not also "major planets", and "satellite planets"? Then you can keep the "major planets" down to 8, that's fine. That would certainly seem like the rational thing to me, it would not devalue investigations like New Horizons, it would extend seamlessly to other star systems, and it would be fully in keeping with the way other objects are classified in astronomy. I am at a loss why there's any other alternative that makes sense.

There are very few objects that orbit outside Neptune's and inside Pluto's orbit. However a large chunk of the Kuiper belt's population (about 15-20%, over 200 known objects) share their orbit with Pluto. More precisely they have a semi-major axis very close to Pluto's.

Here is a site with plenty of diagrams.

No. The reason why the definition was restricted to our solar system was to avoid arguments like whether all planets must orbit stars etc. It was just a simpler problem. The IAU now has a committee looking at extending the definition to extrasolar planets.

The current definition relating to extrasolar planets is incomplete (because there was strong disagreement), and was designed as a working definition to tide things over until a more formal definition could be agreed to. It's not surprising there was some contradiction between it and the new definition.

The reason why the 'clearing the neighbourhood of its orbit' was inserted was because the orbital dynamicists and other people interested in the formation of solar systems (as opposed to the planetary astronomers who are primarily interested in the objects themselves) argue that there is a fundamental, demonstrable difference based on this concept of orbital dominance between the 'big 8' and the rest. I think they would agree that the actual wording leaves a little to be desired (but they only had a few days to come up with it), and I wouldn't be surprised if this wasn't tidied up in the future.

An arguable point of view, but not one everyone shares. Ganymede is bigger than Mercury in dimension, but not in mass, but even if it were very few astronomers would accept it as a planet, because it is orbiting Jupiter and not the Sun - a criteria based on where it is.

No, it doesn't. Unless you're going to argue that a minor planet (like Eros) is a type of planet? Or the Pennsylvania Dutch are Dutchmen living in Pennsylvania? Or a golden parachute is a type of parachute? 'Dwarf' in 'dwarf planet' is not an adjective. 'Dwarf planet' is a compound noun.

Perhaps this says something about you? You're at liberty to argue what you wish, but just because other people disagree with you doesn't make them irrational.

But there are apparently orbital dynamicists who disagree strongly with that concept. So the argument continues on all fronts, not just between planetary astronomers and orbital dynamicists.