"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.

Do orbital dynamicists not also work on multiple star systems, and on galaxies? Why is there no need for such complicating issues involved in defining "star" and "galaxy"? For example, many stars have system center of masses the lie inside other stars, this does not disallow their classification. Also, we have "satellite galaxies" (like the Magellanic clouds) but they are still galaxies. Why, is that just an arbitrary distinction? No-- it's because we could then not extend all we know about stars to those objects. That's the key issue from the astronomical point of view-- a classification is a means of applying inductive logic that is mostly lost in the current arrangement.

Yes, there is a widespread (and quite inadvisable) predisposition against calling satellites planets. In addition to the arguments I've already given, consider these issues: if or when we discover life on Europa, it will seem mighty strange to not be able to even call it a planet. Also, I'll bet the astronauts who walked on the Moon felt like they were walking on a planet, and to disallow that view on a fairly useless technicality seems quite odd to me, yes. Finally, we have the New Horizons mission to Pluto, and the investigators there are certainly going to think they are exploring a planet (and I guarantee you they will speak of it in that terms when they discuss their findings with each other). All for a very simple reason-- it has the physics of a planet.

Those are poor examples, because one comes from the very topic that I claim is poorly handled at present, and the other is not from astronomy or even science (and they were really the Pennsylvania Deutsch-- they were Germans!). I grant you that it is being used as a compound noun, but I would argue that such usage is to be avoided in science, where clear word usage is quite important (notwithstanding the misnomers we currently accept, but note planet classification gives us an opportunity to correct misnomers, not introduce them.)

I never said they were irrational, I said they had lousy arguments, and I've explained why. Perhaps I overstate my objections for effect, but I do feel the arguments in favor of an inclusive definition of planet are vastly better. Not only should scientific terminology be accurate and clear, it should also aspire to help people see the connections between things, not the exclusions. Above all, I feel an inclusive definition of planet will also help motivate the expanded study of these interesting objects, not restrict our special attention to a subgroup that we have known about for, in some cases, milennia.

Ken G:

Why does not referring to Pluto as a planet somehow devalue the New Horizons mission? Pluto remains the same facinating object it has always been; it is still the largest KBO (If you consider the Scattered Disc separate) and it will doubless reveal a great deal about the early history of our Solar System, whatever word we decide to call it.

My main problem with relying purely on internal physics in defining planets is that it ignores (and distorts) the overall structure of our solar system. The Kuiper Belt is a facinating structure in its own right, and the various objects within it are best seen, in my view, as components of a larger whole. To arbitrarily describe some (hundreds?) of those components as planets while ignoring the rest creates a lopsided view of our system, where nine planets exist between 0 and 40 AU, but 200 exist between 40 and 50 AU. Isn't it a bit misleading to say that they are planets, when in fact they are components of the Kuiper belt? After all, there aren't a lot of people campaigning to have all the rocks that make up Saturn's rings classified as moons.

I also don't understand why Titan and Europa aren't considered planets, but then I'm having trouble understanding why Jupiter and Saturn aren't considered stars.

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I may be wrong, but it is my impression that there are many more people saying that the rocks of Saturn's rigns are moons, than are saying that Jupiter is a star.

And I never claimed otherwise.

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There is a growing tendancy to think of Man as a rational, thinking being, which is absurd.- Marvin the Martian.

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

I agree that it shouldn't, but of course it does. NASA does missions to comets and asteroids all the time, but you barely hear about them unless they do something dramatic like ram it, or if the comet happens to be Halley. So the public does care about what objects get the "hype", and "planet" is hype. An inclusive definition would use this hype to the advantage of scientific discovery-- an exclusive definition accomplishes the opposite.

You are practically making my point here. Go out on the street and ask somebody, anybody, what a "Kuiper belt object" is. Then ask them what a planet is. See what I mean?

So call them "Kuiper Belt planets", those that conform to the definition. How does that detract from studying the Kuiper belt? Just the opposite-- if it contains planets, it makes it an even more interesting population than if it merely includes "dwarf planets" (which, by the way, I'm fine with saying as long as they are still planets-- if they have the physics of planets).

Why wouldn't you make the same argument for "stars"? We have some hundred billion of them in our galaxy, but only a tiny fraction are in star-forming regions and gobs of them are in globular clusters in the halo. Does this distort any views about stars? A star is defined by its internal physics so that what we learn about one star can be applied to another, to a large degree. That's the whole point of astronomical classification, not just "taxonomy". Orbital dynamics has not a lot to say about that, I'll bet even the formation is not all that different.

A moon has its own definition, and it is OK to base that on its orbit because I would argue that "moon" is not a primary characteristic of the object, it's kind of a minor detail like what constellation a star is in, or if it has a binary companion or not. For example, a star in a globular cluster probably has a much different history and environment than a star in an HII pocket in a giant molecular cloud. Why are they both stars? Because they share so many important characteristics and knowledge from one extends to the other, pure and simple.
Jupiter and Saturn are not considered stars because their internal physics, i.e. what they "are", is wholly different from stars. Can you tell me the huge differences in the internal physics of Titan compared to Mercury that relates to Titan being a moon?

Sub-brown dwarfs are still considered stars, and their internal phyiscs aren't that different from those of Saturn and Jupiter.

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There is a growing tendancy to think of Man as a rational, thinking being, which is absurd.- Marvin the Martian.

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

That is a valid point, but it is not the issue here. Unfortunately, whenever you draw a line, there will be some object right on the edge of that line, challenging the validity of any such line. This is pretty inescapable, and will be true no matter what definition you choose. It is the fallacy of lines, but still they are necessary. We do the best we can. The issue is not where to draw the line, it is what the line should be based on. Physical (and astronomical) classifications should be based on the internal physics, so that you can try to extend knowledge across the class of objects. If sub-brown dwarfs were thought to have internal physics closer to Jupiter than the Sun, they would not be classified as stars (and perhaps they shouldn't be, that's another issue). It is felt that the fact that they have a significant nuclear energy source of some kind, and so have a structure that must readjust to reach equlibrium in a self-consistent way with that energy source, makes them more like a star than like Jupiter. At no point will you find the orbit of an object, or the effect it has on its surroundings, entering into the definition of a star, of a nebula, of a cluster, of a black hole, of an asteroid, of a dust particle, of an atom, of a universe. So what gives with planets?

There are people who disagree with anything! I never said the argument wasn't continuing, though my impression is that the position put forward in the IAU definition is gaining ground and acceptance within the astronomical community is solidifying, but my point was that it was not put in just to keep the number of planets in the solar system low.

Because stars and galaxies do not demonstrate such fundamental, demonstrable differences between themselves.

No, because they are orbiting objects like themselves.

We can extend what we know about one object to another regardless of its classification.

Classification schemes have lots of different attributes and purposes. Some highlight similarities, some highlight differences. Whichever scheme is chosen often depends on historical accident, and might not be the one any particular individual would have chosen themselves. But it doesn't make it irrational.

Inadvisable from your point of view. But there is a difference between satellites and objects that are not satellites. That's why they got the term applied to them in the first place! Whether you include this difference in any classification scheme, and how much weight you give it is a question of judgment (or taste!).

Why strange? Since when did the status of planet have anything to do with whether there was any life there? You seem to be saying that planets are important, objects with life are important, therefore the two should be covered by the same term!

They felt they were walking on a world (and said so), but none of them indicated they thought the Moon was a planet.

It is not a technicality. It is a clear fundamental, demonstrable difference. A difference that is not useless, as it clearly discriminates between two types of object. The difference is that its not an important difference to you, but it was a very important difference to those who insisted on its inclusion in the definition.

Guarantee? You know them that well? Though I suppose it will be true when discussing things with the boss... But all this demonstrates is that there is a group of astronomers who don't like the new definition. What a shock!

It has the physics of a planemo, which seems to be the term gaining currency to describe the overall class of object, with planets and dwarf planets being subcategories (and perhaps not the only ones). Things could have gone the other way, with planets being the overarching term, and different terms used for the sub-categories, But it didn't. Neither system is intrinsically superior scientifically. That's just the historical accident I was talking about earlier.

I thought that made it a good example. No one confuses minor planets with planets. No one will confuse dwarf planets with planets either once the term has gained usage and practice.

As you say, arguably. But note that this is just an argument for replacing 'dwarf planet' with another term. It's not an argument for not making the distinction between the two class of objects in the first place.

If they were vastly better, then a majority would accept them, which is not the case. The reason why the argument went on (may even still be going on) so long is the fact that they are closely balanced. If people are maintaining a stance based on lousy arguments, surely they must be being irrational!

It should also help people see the differences.

Well, that's just a hunch. There's no evidence of such an effect, other than that NASA may have sent a probe to Pluto to 'complete the set' - but they would've thought of another argument (the first probe to an entirely new class of objects, perhaps?).

I'm not talking about them.I'm not convinced.

I think if the original proposal had not also included the moon of Pluto, it probably would have passed. The barycenter thing is what the BA latched onto in his objection (the moon of Earth moving out to become a planet), whereas even a baseball/sun barycenter could be outside the sun if the baseball were thrown far enough away.

On this Site http://www.gps.caltech.edu/~mbrown/w...anplanets.html there is a list of all known objects in our solar system big enough to be round by by gravitation. (And a grafik of their orbits.)

You can clearly see that even in this subset of KBOs Pluto is not the innermost.

The most prominent KBO nearer then Pluto (39.53AE) would be Orcus (39.34AE) with 1100km diameter. And the nearest of the "planetlike" KBOs would be 2004TY364 (38.72AE) with 540km diameter (this is 0.81AE nearer then Pluto - more then the distance between Sun and Venus).

Nice link, Thanks Eckelston!

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What fundamental, demonstrable differences? An irregular galaxy that has just undergone a collision does not demonstrate fundamental demonstrable differences? The point is, more is the same than is different, as with planets.

And by what meaning do you say "like themselves"? Internal physics, perchance? Because if you are going on the center of mass, then they are not orbiting objects like themselves-- the center of mass is in the other guy! (But I agree with you-- where the center of mass is does not change whether or not the other object is "like itself", this is my point.)

Indeed, and yet herein we find the whole point of classification.

I agree that "irrational" is too strong of an aspersion, I would merely say that the current scheme is not the best one and will become more and more of a liability as we explore more "planets". In fact, here is my prediction: dwarf planet will eventually be considered a subclass of planet, first in practice, and ultimately in the books.

Indeed, and that is entirely what we are discussing.

Yes, that's pretty much correct. Planets are the most important classification of object outside of stars, and life on them is the most important subtopic. This is as useful a convergence of facts as the fact that stars are important for making surfaces livable.

"World" is not a scientific classification, but if it were to become so, I would have no objection to using that term instead of "planet" when we discuss these objects.

I'm still waiting to hear about all these fundamental differences. Shall we now classify Mars and Earth differently since Mars has a "demonstrably" different shaped orbit? Clearly it's had a different history in some important way. But then, so has a supergiant compared to the Sun.

No, I don't know them at all, but I do know a definition that is a liability rather than an asset, and is likely to be treated that way in practice.

Well, at least it makes sense to have a term that includes the objects I'm talking about, I just think it is appallingly awkward to use "planemo, planet, and dwarf planet" when "planet, major planet, and dwarf planet" works just fine and is far more straightforward and logical.

That I agree with, scientifically it doesn't matter what words are used, as long as they can be mapped into the same scheme. But some systems are much "cleaner" and logical than others. In my opinion, 'planemo' is simply awful-- what is the justification for inventing a new word when we already have 'planet'? Again, I see nothing beyond the desire to keep 'planets' to a minimum in our solar system, to elevate their status at the expense of others (witness the OP title).

I would paraphrase that, no one confuses minor planets with major planets-- don't those words just make more sense?

That doesn't follow, the majority can be wrong, especially when historical accidents get brought along. To me, the opportunity here was to break from historical accidents that constantly hamper astronomical terminology (witness the appalling stellar classification system), but they blew the chance. It may not be too late for another opportunity, but the highest need is to have an agreed on set of words, even if poorly selected.

Granted, I really don't mean to suggest that anyone who doesn't agree with me is a fool, despite my polemic tone.

That is so-- and that's why classification schemes are important, because if done poorly they tend to magnify less important differences and obfuscate more important ones. That's the reason we feel this discussion is important to have.
A new class of objects does have some appeal, but I think it is viewed as a very technical class that the general public really doesn't "get". What I'm saying is, why not simply call it a new class of planet?! Wouldn't that stimulate more interest? Indeed, my money says that's exactly what TKOs are going to end up being treated as, IAU despite.

But stars in globular clusters, as far as I understand it, aren't accompanied by thousands of other objects that are smaller, irregular or almost spherical versions of themselves. If they were, the structure and dynamics of a globular cluster would be too complex to arbitrarily single out a few scattered random examples as "stars", just because they happen to be slightly more round than the dozens around them that aren't. The same applies to planets in the Kuiper belt. Artificially labelling a random sample of them as "planets" because they cross some arbitrary line of being "round enough" ignores the vast majority of the matter within the belt, effectively distorting its structure.

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There is a growing tendancy to think of Man as a rational, thinking being, which is absurd.- Marvin the Martian.

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.