After much lively discussion on the "What is a Planet" thread, I'd like to cool things down somewhat with a detailed explanation of how you would classify bodies by "orbital dominance".

This idea did not originate with me but, after much consideration and examination of data, I believe it is the simplest and most practical classification scheme. Several people in the "What is a Planet" thread contributed to the refinement of this classification system.

Definitions
First of all, some definitions, beginning with what I consider to be a cultural maxim:

"Sol is a star. Earth is a planet. Luna is a moon."

In my opinion, those statements carry a broad consensus in the modern astronomical community and, as importantly, in the world. This is important because we should not create confusion in the non-astronomical community by changing the accepted meanings of words without good reason. The "orbital dominance" scheme succeeds in this regard.

Planet - a large body that orbits a star and has attained "orbital dominance"
Planetoid - a large body that orbits a star but has not attained "orbital dominance"
Asteroid - a small body that orbits a star
Moon - a large body that orbits a planet (or planetoid)
Moonlet - a small body that orbits a planet (or planetoid, or asteroid)

"large" vs "small" - a size cutoff for a body to draw a line between what is a "small world" and what is a "big rock". A measure of sphericity (roundness) is the most logical place to draw this line but it's not really a compelling issue outside the realm of astronomy -- meaning that changes to this line will not generate the same public outcry as with redefining 'planet'.

Orbital dominance - the idea that a body "dominates", in terms of mass and gravity, all of the other bodies that stably exist in the same orbital region.

Orbital region - for any particular body, the range of distance from its perihelion (closest to sun) to its aphelion (furthest from sun).

Resonant orbit - an orbit of a smaller body that allows it cross into or exist within a larger body's orbit, because of timing, without becoming gravitationally unstable.

Trojan - a body that orbits at the L4/L5 Lagrange points of a larger body (60 degrees ahead or behind in the orbit). Trojans can be classified as either planetoids or asteroids, dependent on size. This is an existing term for such bodies.

The Solar System
So how does this work? Let's start with the obvious by classifying our Solar System.

First things first.... Defining the orbital regions:
*objects in this orbital region only approach 30 AU by maintaining an orbital resonance with Neptune. Non-resonant objects are at least 35 AU away.

** the aggregate mass of the trans-Neptunian region is unknown, but it has been estimated at perhaps 10% of the mass of the Earth. If only the estimated mass of the 10 largest known TNOs were used in this calculation, the dominance of 2003 UB313 would be 40-50%.

Making sense of the data
As can be seen, 8 of the 10 orbits have clearly dominant members (98% to 100%). The Asteroid belt and the Kuiper Belt are the two exceptions. The mass of the asteroid belt is well known, but keep in mind that the 12 largest asteroids make up 75% of the total mass of the belt. This is important because it underscores that you do not have to catalogue the mass of every object in a region to determine if the largest body has orbital dominance.

Given the dominance of the 8 traditional planets (98% to 100%), it is clear that the 40% figure for Ceres is dissimilar and justifies the historical demotion of this body from its former planetary status. Astronomer Mike Brown, discoverer of many large Trans-Neptunian Objects, has suggested planetary status of any object with greater than 50% of the mass in its orbit region. (http://solarsystem.nasa.gov/scitech/....cfm?ST_ID=105)

Promotions would be rare
When a large body is discovered in a new orbital region (such as Ceres or Pluto), it certainly appears to be planet since it holds 100% of the mass in that orbital region. But if similar-sized objects are later discovered, it may be necessary to demote the object from planetary status (as with Ceres).

It's important to note that it would be very rare to promote an object to planetary status because later discoveries can only decrease the orbital dominance of a body. Promotion is possible only if we discover that we have grossly erred in measuring the mass of bodies in an orbital region.

The Kuiper Belt and beyond
What drives this debate, however, are the large bodies now being discovered with regularity beyond Neptune.

Currently, the largest known objects are:
* mass & diameter are estimated

As you can see, there is no clear cutoff in size for Trans-Neptunian objects. The transition from planetoid (large) to asteroid (small) is going to be contentious no matter where the "line" is drawn. One consolation, however, is that all of these bodies are far less massive than the smallest planet, Mercury, so it doesn't really infringe upon the categorization of planets. And since crossing this line makes a body a 'planetoid', not a 'planet', no one outside of the astronomical community will really care about how the location of that line is determined. "Gravitational roundness" is a well-understood and obvious concept, that's where I personally think it should be drawn.

Is there an outer edge to the Kuiper Belt?
Based on the orbital characteristics of many Trans-Neptunian objects, there seems to be a "cliff" of sorts in the frequency of objects beyond 50 AU. This has led to speculation of a large body in the 50 AU area clearing out the region much in the same way that moonlets create the divisions in Saturn's belts. However, many larger objects are being discovered that approach Neptune but also stray out far beyond 50 AU. The jury is still out on whether 50 AU is actually a firm limit. It currently doesn't seem to be an obstacle for 6 of the largest TNOs in the list above.


Extra-Solar Systems
How well would this classification scheme work with solar systems other than our own? Considering our lack of knowledge of other systems, it's hard to say. But I will point out that categorizing by "orbital dominance" fits nicely with the traditional observation limits of astronomers.

The first extra-solar planets we find will typically be the most massive (with the strongest gravitational effect) or the largest (causing the greatest occultation). As a result, we can say with relative certainty that the first objects found are going to be planets. If we later discover that an object is part of a larger belt, it can be demoted then. However, this will be an exceptional condition since mass drops much more quickly than size -- meaning that you'll quickly reach a point where the object will remain dominant in its orbit no matter how many more tiny objects you find. In addition, the aggregrate occultation and gravitational effects of belts may be more apparent from a distance, identifying belts, as a whole, long before individual members can be resolved.


Call for Comments
If anyone feels that parts of this classification scheme needs clarification, please post your concerns and I will edit the post to address them. Also remember that this thread is specifically intended to deal with this classification scheme; I don't want to get into arguments about comparisons with other systems.

If you have any objections about shortcomings to this scheme, please post them and I will edit the post to add an "objections" section and hopefully provide an adequate response to it.

And obviously, I will be occasionally editing this post to fix typos and formatting errors!

Thanks for everyone's input and ideas in the earlier discussions!

Comments
I have considered that in the past, but it seems unnecessary at the current time. For obvious planet-moon systems, only the "collision" genesis for moons seems to create a moon relatively large enough to affect the percentage. Earth and Pluto, of course, are the only examples we have so that's a small sample size.

With regards to large, co-orbiting planetoids, I would defer addressing that possibility until the need arises. One important consideration, I think, is to always remember to not make decisions that are more appropriate for the future. If and when co-orbiting planetoids are discovered, we can then address that issue based on the observed data.

However, I'm not as keen as before on using the "barycenter lies between the two bodies" as the qualification for dual-planet status. This is because that approach comes perilously close to violating our original (and important) maxim that "Sol is a star. Earth is a planet. Luna is a moon"

The Earth is 80 times more massive than the moon, and its radius is just 1/60th of the distance between the gravitational centers of both bodies. In other words, their common barycenter is 3/4ths (60/80) of the way to the Earth's surface. If the moon were just 30% or so more massive or further away, Earth-Luna would qualify as a double planet using this approach.

Now I realize that this 30% does not exist so it's a moot point, but that is still a little closer than I'd like to be to violating a centuries-old maxim. I've said this before: Luna is a moon, by definition, and we are not allowed to rewrite the definition for such a basic term. Any classification scheme that meddles with that will be DOA.


I did not count the mass of Plutinos in the aggregate mass of the Neptune orbit. Their orbital resonance is an exceptional situation most likely caused by the fact that there is no massive object beyond Neptune (at least within 100 AU) to disrupt the resonance. If there were, I think we'd see a Kuiper Belt more like the asteroid belt, shepherded into a narrower range by large bodies inside and outside.

But keep in mind that a Xena-sized body is already in Neptune's orbit -- Triton. If the mass of the entire belt were 10% of Earth, as I've seen estimated, Neptune's orbital dominance would still be 99.3%. In fact, Neptune's dominance would be a 94% even if the Earth itself were orbiting around it!

Jupiter is almost 1,000,000 times as massive as the entire asteroid belt, and the 4 Galilean moons are also each more massive that the entire belt. Jupiter's dominance might drop to 99.978%, depending on how the numbers round.


Sadly... I am out of room for adding comments to the original post (15K text limit). Please review the thread to see answers to other questions!

On a first pass, I really like the idea. It is simple, concise and more importantly, it "draws the line" in broad strokes. I will need to let it seep and decant for a bit and throw ideas at it but so far, I really find it to offer at last some promise on the debate.

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First, I'd like to say "that's great!, Thanks, this is a nice system".

Second, it looks to me as though you are not including the mass of the moon as part of the mass of the Earth, thus the Earth and Jupiter do not round off to 100%. I suggest that for this system to work well, that a planet's dominance should be counted including itself and all of its moons, moonlets, and rings. This will prevent some rare system around another star where an orbit is occupied by two large co-orbiting bodies from having no dominant body in the orbit.

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I have considered that in the past, but it seems unnecessary at the current time. For obvious planet-moon systems, only the "collision" genesis for moon's seems to create a moon relatively large enough to affect the percentage. Earth and Pluto, of course, are the only examples we have so that's a small sample size.

With regards to large, co-orbiting planetoids, I would defer addressing that possibility until the need arises. One important consideration, I think, it is always remember to not make decisions that are more appropriate for the future. If and when co-orbiting planetoids are discovered, we can then address that issue based on the observed data.

(Note: added to comment section of original post)

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

Yes, one question I would raise would be the possibility of a double-planet system. Something like Pluto-Charon, though in an orbit bereft of other objects.

It is well-known that they orbit a barycentre - at a point somewhere between the two. In a different orbit, they could IMO well be a double-planet system.

Though I agree that that is hardly an over-riding concern at the moment.

In regards to Neptune's orbital dominance and % of mass in that orbital area, are you including in that figure for Neptune the mass of the plutinos and even perhaps twotinos - that are orbitally dominated by Neptune - even if they do not strictly cross Neptune's orbit?

And what would the figure be if the mass of all the objects in the Kuiper Belt - up to 50AU was included as well as that of those in the exact orbital region?

Another thought, what if the mass of the asteroid belt was included in calculating the figure for Jupiter - as its widely accepted that the presence of Jupiter close to that region prevented the formation of a larger object in that region - so its Jupiter's orbital dominance that creates and maintains the Asteroid Belt.

Would those figures approach the 98% mass dominance enjoyed by the Earth? Or at least under 99%?

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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident." Arthur Schopenhauer - renowned 19th Century German philosopher.

Since orbital dynamics are a key to classification, there are many bodies that could be reclassified if they were in a different orbit. If Mercury and Luna were swapped, their planetary and moon status would change.

However, I'm not as keen as before on using the "barycenter lies between the two bodies" as the qualification for dual-planet status. This is because that approach comes perilously close to violating our original (and important) maxim that "Sol is a star. Earth is a planet. Luna is a moon"

The Earth is 80 times more massive than the moon, and its radius is just 1/60th of the distance between the gravitational centers of both bodies. In other words, their common barycenter is 3/4ths (60/80) of the way to the Earth's surface. If the moon were just 30% or so more massive or further away, Earth-Luna would qualify as a double planet using this approach.

Now I realize that this 30% does not exist so it's a moot point, but that is still a little closer than I'd like to be to violating a centuries-old maxim. I've said this before: Luna is a moon, by definition, and we are not allowed to rewrite the definition for such a basic term. Any classification scheme that meddles with that will be DOA.

I did not count the mass of Plutinos in the aggregate mass of the Neptune orbit. Their orbital resonance is an exceptional situation most likely caused by the fact that there is no massive object beyond Neptune (at least within 100 AU) to disrupt the resonance. If there were, I think we'd see a Kuiper Belt more like the asteroid belt, shepherded into a narrower range by large bodies inside and outside.

But keep in mind that a Xena-sized body is already in Neptune's orbit -- Triton. If the mass of the entire belt were 10% of Earth, as I've seen estimated, Neptune's orbital dominance would still be 99.3%. In fact, Neptune's dominance would be a 94% even if the Earth itself were orbiting around it!

Jupiter is almost 1,000,000 times as massive as the entire asteroid belt, and the 4 Galilean moons are each more massive that the entire belt. Jupiter's dominance might drop to 99.978%, depending on how the numbers round.

nope. Good questions, though.

(note: questions added to comments section of original post)

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

Well, there are a few things I don't like about it, but over all it's an OK definition.

Double Planets. Obital Dominace would not classify them as planets at all, even if they were the only two bodies in a particular orbit. The issue posted with the Bary Center didn't use the inverse square law of gravity but a simple proportion. It would actually take a body almost two times the moons mass at it's distance to have a bary center at earths surface, to get it out of the atmosphere, its almost 3.8 times the moons mass.

I think the Bary center for defining double planets is still a valid aproach, when you account gravity work on an inverse square.

The other issue.

I'm not fond of anything the declassifies the Pluto/Charon system as a planet(double). For this reason the Size classaifaction makes more sense then redefining.

Although it's not likely it's entirely -posiible- to have a asteroid like belt of mars to earth sized objects in the habitable zone of a star, if they had a Gas Giant close enough on each size of the plantary cloud to keep the cloud stablized. In this case there could be easily a dozen or more habitable earths all sharing the same orbit.

And they would still be planets.

I think orbitable domince, and size should both make a case for planet hood, not one over the over.

In the case of Pluto/Charon, a classification as Double Planet fits, I see no valid reson to demote them, scietifically or other wise from that status.

Not pointing fingers at any -one- person, but lately I've come to the conclusion that for some reason classifing 2003 UB313 as a planets seems to actually offend a few people. I'm not exactly sure why that is.

I think any classification that excludes both Pluto/Charon and UB313 from a planatary classification is a mistake. 90% of the folk out there would be irritated to say the least if suddenly they had to back petal on calling Pluto a planet. How would science be precieved if suddenly they came out and said 'well, it's not really a planet' especially when that perception could be carried over into other issues where it's far more import for science to make a stand on.

Over all were stuck with pluto as a Planet, and hence, UB313 as one too.

But this is all moot until the IAU nails it down officially, one way or the other.

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There is no problem that cannot be solved by a suitable application of high explosives - US Army Demolitions School

I just saw Hayley's comet, she waved, Said "why you always running in place? Even the man in the moon disappeared, Somewhere in the stratosphere" - Shinedown

http://worldsofothersuns.home.comcast.net/

Fair enough. I wasn't sure if it worked like that for barycenters; I got my "simple proportion" information from another website and a second that corroborated it. I'll follow up on this again and make any necessary corrections to the original post. Thanks for bringing this up!

While I appreciate the "coolness" factor of double-planet systems, let's not lose sight of the fact that Pluto makes up 90% of the mass of the Pluto-Charon system. I don't know how low that number should be before we call something a "double planet", but let's not pretend that the size difference between Pluto and Charon is negligible.

I dunno. I think that the size of an orbital region and the maximum size of multiple large bodies in that region will be related. Do you really think that a handful of Earth-sized bodies could co-orbit in an orbital region the size of our asteroid belt? They would have to be in a Trojan-like resonance at their L3/L4 Lagrange points.

While this is theoretically possible, it seems impossible given our current understanding of planetary formation. That may be why we see only collections of very small bodies (i.e. glorified rocks) at the Lagrange points of large bodies like Jupiter. Hektor is the largest, and it is far too small to be gravitationally round.

I believe that Pluto-Charon are still officially considered a planet-moon system, but someone correct me if that is wrong!

I think the primary objection is that 2003 UB313 is just one member of thousands of belt objects, much like Ceres is just one member of the asteroid belt. That's the most common explanation and that is what excludes it from planetary status in this scheme.

Perception is important, but a classification scheme still needs to be consistent. It is just as easy to claim that perceptions would be bad if the IAU decided on a system that swelled the Solar System to 25 planets over the next decade.

Whether UB313 becomes a planet does depend on how the IAU resolves this issue. But, in an orbital dominance scheme, neither Pluto nor UB313 would be considered planets.

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

It's a extrapolation based on known ring systems like saturn, where two gas giants in a resonance orbit about 2 AU's apart, might shepard the gas/dust between them similar to the shepard moons of saturn, the dust would collese into something resembling asteroids, eventually up to planatary sized bodies.

If i remember from when I read about this there is a upper limit as to the number and mass of large bodies that could form and be stable enough not to colide into each other. It was around 10 earth masses, but can't remember the max number of them.

The supposition did mention this could only happen in a system where the planets formed at thier orbits, and didn;t migrate, which does make it very unlikely. But a remote posibity.


It's just as likely that we'll only find 4 or 5 more of that size, and the rest will be asteroid sized snow balls. I don't think declassifing a planet to prevent a -possible- large quantity of them form being called the same is all that valid of a reason for an approach either.

Having 25 Belt Planets wouldn't really bother me, as we could differentiate them by the term Belt. Would work for both Kuiper objects and sheparded ones.

Something akin to 8 Dominant Planets, and 25 Belt Planets, makes more sense to me then 8 Dominant Planets and 25 Large KBO's.

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There is no problem that cannot be solved by a suitable application of high explosives - US Army Demolitions School

I just saw Hayley's comet, she waved, Said "why you always running in place? Even the man in the moon disappeared, Somewhere in the stratosphere" - Shinedown

http://worldsofothersuns.home.comcast.net/

I know that you are going off of memory, but if you ever find an online reference to that I'd love to read it!

You may be right, but I am doubtful. I remember when Sedna was discovered, a big deal was made about it being a fluke that Sedna was at one of its closest points on a highly erratic orbit and that there could be hundreds of bodies like that in the outer solar system.

We are just now discovering the bodies just beyond the edge of the Solar System (out to 50 AU), and that is only a small fraction of the total space in the Kuiper and Oort regions. I think it's premature to assume we won't find more Pluto-sized bodies, at least until the new discoveries start slowing down.

I agree. If we think that the Trans-Neptunian bodies are not that much different than the 8 inner planets, then it certainly does make sense to classify them together, no matter how many planets that would leave us with.

That said, I believe that there is actually very little similarity between the 8 inner planets and all of the bodies outside of Neptune outside of the fact that they are gravitationally round.

Their orbital characteristics are dramatically different (eccentricity, inclination, dominance and distance from the sun). In addition, there are huge differences in mass -- the largest KBO is perhaps just 1/10th the mass of the smallest planet. There are far more dissimilarities than similarities, which supports the idea of putting them in separate classifications.

There is no functional difference between those two alternatives. You have still separated the KBOs from the Inner planets according to their orbital dominance -- all you have really done is changed the labels. I fail to see how this is any different than the scheme I posted except for your desire to incorporate the word 'planet' for KBOs. But doing that has required you to add a new qualifier (Belt & Dominant) that is not really necessary.

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

One other point I would raise in regards to a possible double-planet system is that someday we might well find a double planet system somewhere in the heavens. 2 objects of similar sizes, maybe even 3 in interlocking and similar orbits - depending on the gravitational effects and the stability offered by the LaGrange points etc.

I'm not entirely familiar with how that all works - but I've read here that it is theoretically possible. So, we may well find this eventuality someday - but one would have to assume that such a system has an inherent susceptibility to being unstable - to a higher degree than a more traditional planet - moon, or planet - moons system. The gravitational effects of passing objects would surely be multiplied to some degree by the twin gravity wells in such a system - and produce more unpredictable results.

So while there's nothing to stop such a system theoretically being discovered, its also possible that these type of systems might not endure for the same length of time as a traditional system - and its in fact likely that there will be some event over the tremendous time periods we acknowledge is relevant to discussion of this sort of thing that will likely disrupt such a double-planet system.

So what I'm getting at is that we may discover a double-planet system and then over a period of thousands, tens of thousands, millions of years - length of time not being important to this particular argument, it may evolve into something else. Perhaps the 2 bodies could end up in differing orbits that no longer leaves them as double-planets - perhaps planets in their own right, or perhaps 1 could end up in orbit around another, larger world (like the Earth around Jupiter for instance - if that ever happened the Earth would become a moon).

The point I'm trying to make is that in classifying heavenly bodies you have to accept that things are not set in stone and that the Universe is a dynamic place that is subject to change on grand unimaginable scales and that given that, how we describe an object also needs to recognise this dynamic element and can't be set in stone. I think, baric, that you adhere to this understanding, but that there are some that don't. What particularly irks me are those who call for all objects that have gravitationally rounded to be called `planets.' And because of this, they call for all the large moons in the Solar System to be referred to as planets - because if they orbited the Sun in a different orbit they would be planets!

A spurious argument that ignores the understanding built up over centuries, millennia perhaps, about how we differentiate between planets and moons - and also a recipe for inviting confusion and ridicule with the general public.

One has to recognise the importance of orbit and gravitational/orbital dominance in how we've defined heavenly bodies in the past - and differentiated between planets and moons for instance. With any classification system that relies as heavily on empirical data as our understanding of what a planet is - there are going to need to be tweaks and small changes made to how that definition stacks up as we discover more information and gain a better understanding of the vast Universe out there - and that is truly something we are only just beginning to really do.

I say this because I think the orbital dominance classification system baric champions here adheres to the principles and understandings we've had in the past of how to define a planet and just tweaks things slightly at the edges given the new discoveries we've been making in the past 15 or so years - and which is why I also believe its the best way to go. But at the end of the day, that's just IMO.

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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident." Arthur Schopenhauer - renowned 19th Century German philosopher.

Baric?

I really really tried to find a reference to that Gas Giant's sheparding a planatry belt between them theroy(extrapolation) last night.

The original reference I read was around 1996 and I didn't remember the name of the source, but I seem to recall somehting about it being simulated a few times, so my guess is it was a on a univerities site.

Seraching for a similar write up I couldn't find anything that matched exactly what I had read about, expect one close one, that talked about planets forming in gass/dust rings, compressed by a new sun's periodic outbursts.

I did find plenty talking about two or three planets sharing orbits, in a horseshoe formation (?) and about saturns rings forming more moons over time.

I'll keep hunting for it, as I want to reread that thing myself.

__________________
There is no problem that cannot be solved by a suitable application of high explosives - US Army Demolitions School

I just saw Hayley's comet, she waved, Said "why you always running in place? Even the man in the moon disappeared, Somewhere in the stratosphere" - Shinedown

http://worldsofothersuns.home.comcast.net/

Just wanted to remind everyone that the Moon does in fact orbit the Sun in its own right; it's annual orbit round the Sun is effectively circular.

Oh, and I like yur definition too; of the "IAU three" it would have been the one I voted for, had I been given the chance. Maybe you could gatecrash their party in Prague this August and start campaigning to make it official.

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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.

Unfortunately, by deferring that, it leaves open the possibility that two co-orbiting earth size bodies would each not be classified as a planet, right? Or an earth-twin in some sort of hybrid orbit around a Jupiter. I would think that that would make your scheme, as you say, DOA too.

Well, it technically wouldn't be DOA. It would be DODOTCOESB (Dead on discovery of two co-orbiting Earth-sized bodies).

What would make it DOA is trying to redefine a word that the entire world has already settled on a definition for. The best example would be reclassifying our moon as a planet (the moon would no longer be a moon!)

As far as an Earth-twins around a Jupiter, those would just be two Earth-sized moons. There's nothing in this classification scheme that says that a moon cannot be as large as a planet (since it primarily classifies by orbital characteristics, not size.

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

Right, but this is just a function of the orbital distance between the Earth and Moon. If the moon were closer, it would orbit the Earth more quickly and form clear loops in its orbit around the Sun.

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

No, I meant DOA. No governing body would ever adopt it, in other words.I understood that example, in your OP. But there are a lot of reasons that a scheme would be rejected out of hand. I think that's one of them.Not Earth-twins around Jupiter, an Earth-twin in a hybrid orbit near Jupiter. Something clearly not a moon (in our usual idea of moon) but in an orbit dominated by another planet. Or, even shared--that would be the Earth-twins idea, where they might be in high eccentricity orbit so they don't interact directly.

Just because these scenarios don't exist now (hey, we only know about a couple hundred planets anyway--and we don't have the details on them yet), doesn't mean that they shouldn't be considered when we're discussing the definition of planethood. Otherwise, we only really have 8 or 9--and we might as well stick with what we got already.

Do you have a real-world example? After all, it would be easily to speculate on hypothetical situations that would cause problems with any classification scheme.

Exactly. They don't exist now. Not only that, we don't even know if two co-orbiting Earths are possible. Any scenarios that result in co-orbiting bodies currently indicate a large mass differential between the two bodies.

Could an Earth-sized moon naturally form around an Earth-sized planet? Could an Earth-sized planet gravitationally capture another Earth-sized planet? Could a collision between two Earth-sized bodies resulted in the accretion of an Earth-sized moon around an Earth-sized planet?

It's not only that we haven't found an example of this yet. We also haven't found any analogous examples that would indicate that this is even possible.

I think it's folly to try and develop a classification scheme to handle all possibilities when we know so little about what the range of possibilities are. The primary focus should be to address the current classification problems and let future astronomers deal with any problems that might come up in the future.

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

What about an Earth-sized world in the libration point of a Jupiter-sized world? I.E. following the same orbit, but 60 degrees more easterly or westerly.

And what about sixteen worldlets, ranging from Pluto-size to Mars-size, orbiting the same center of gravity, which itself orbits a star?

As I said, we only have 8 or 9 (possibly 10) real-world.Not with high eccentricities, I think. And co-planets are OK too, where they orbit each other.Currently, we don't really have a classification problem. But clearly, it's imminent. That's why all the discussion.

People like to look ahead.
Such systems don't necessarily orbit the center of gravity (or center of mass). For an extreme instance, the artificial earth satellites do not orbit the earth/moon center of gravity.

I thought Newton proved otherwise. But of course the artificial satelite itself (and all its colleagues) must be included to find the center of mass.

Thank you for starting this thread. I'm told that imitation is the sincerest form of flattery.

I have several questions for you to answer. I'll enumerate them in bold so that they won't get lost in the shuffle without being answered. I'll introduce them as I think of them, and will add more as I think of them.

Question #1: You say that this is a maxim: "Sol is a star. Earth is a planet. Luna is a moon." Can you detail the history of this maxim including citations, historic promotions or identifications of this concept as being axiomatic?

Question #2: Dead On Arrival: You claim that any definition that violates your maxim is DOA because of historical and cultural reasons. How does violating the historical and cultural acceptance of Pluto as a planet not make your own definition DOA?

Question #3: Orbital Dominance: What is the empirically derived point of delination between dominance and non-dominance and why?

Question #4: Orbital Dominance Region: What is the reason for defining the orbital dominance region as existing between perihelion and aphelion?

Question #5: Orbital Dominance Region and orbital plane: What is the maximum vertical extent of the orbital dominance region above (north) and below (south) of the orbital plane of an object or planet?

Question #6: Orbital Dominance Region and Eccentricities: Why do you consider the orbital dominance region of an object to be circular/annular in the case of objects with marked eccentricities?

Question #7: Orbital Dominance Region and Ceres: There are 3 large and populous asteroid belts defined by the kirkwood gaps. The orbit of Ceres does not pass into the first and largest belt. Do you exclude that first belt because it is not within the orbital dominance region of Ceres? If so, what is the new mass percentage?

Question #8: Orbital Dominance Region and small solitary objects. Is there any lower limit for size or mass for objects that are in solitary orbits?

Question #9: Conic Section Orbit types: The orbits you have currently used are ellipsical, how do parabolic and hyperbolic orbits fit into your scheme?

Question #10: Kuiper Belt Objects: How many objects are in the Kuiper belt? You stated in a response to dgavin that is was thousands yet wikipedia claims 800. Can you explain the discrepancy?

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I think question nine can be answered right away: If something is on a parabolic or hyperbolic orbit, it won't be along for more than a fleeting time, so we need not bother with it.

Just like we give names to rivers and islands, but not to passing clouds.

Those are already called trojans. In the libration points of our own Jupiter, we only see collections of dust and rocks. Hektor is the largest of them, and it's about 1/500,000th the mass of Earth.

And, if an Earth-sized trojan did exist somewhere, that's exactly what it would be called... an Earth-sized trojan. If it directly orbited Jupiter, it would be an Earth-sized moon. And it directly orbited the Sun in a dominant orbit, it would be an Earth-sized planet.

You bring up a good point, though. I should address trojans in the original post for clarity.

If there were no gravitationally-dominant body, then they would all be planetoids.

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No, he proved that a spherically symmetric body can be considered to have an effect as a point mass at its center of gravity, but non-spherically symmetric conditions do not. The example I gave is a good one--artificial satellites orbit the center of the earth pretty much, not a point 3/4 of the way to the surface (the earth/moon barycenter).Their mass has a tiny effect, compared to that of the moon. The center of mass of the earth/sun system is inside the sun!

Thanks, but this thread is not a debate thread about different classification schemes. This thread is an attempt to discuss and elaborate on one particular scheme.

I have been anxiously awaiting your first post because I know that you will try raise many objections. If they cannot be addressed, then that will be a flaw with this particular scheme.

Thanks.

The beauty about axioms is that they do not have to be proven. An axiom is, by definition, "widely accepted on its own merits." If you dispute that "Sol is a star. Earth is a planet. Luna is a moon", then you will not need to trouble yourself with this classification scheme. However, I think that statement is axiomatic, so any other system that does not adhere to it will have trouble gaining broad acceptance.

There is an historical precedent for demoting bodies that were originally considered to be planets, but were later discovered to be large members of a belt.

Ceres is the obvious precedent. However, the case for demoting Pluto is even stronger than that for demoting Ceres. For although Ceres is not a dominant member of the asteroid belt, it is still by far the most massive member of the belt. Pluto, in contrast, can no longer even claim to be the largest member of the Kuiper Belt.

Observation, which is the foundation of empiricism. The measured level of dominance displayed by the 8 planets (100%, 100%, 100%, 99.999%, 99.998%, 99.979%, 99.975%, 98.8%) is markedly dissimilar to that of Ceres (40%) and Xena/Pluto (2-3%). And considering that the least dominant planet, Earth at 98.8%, is defined axiomatically as a planet, then clearly the other 7 bodies are planets as well.

So that just leaves Ceres at 40%, which has already been historically designated as a non-planet (by the widespread use of the labels "asteroid" and "minor planet"). These terms are never used to describe any of the 8 planets (Mercury through Neptune).

Clarity. One could argue that larger planets dominant outside of their direct orbits (as Jupiter does with the asteroid belt), but using perihelion and aphelion seems to be the most straightforward approach. The minimal precision you may lose by not factoring in the mass of the planet (to measure dominance outside of the perihelion/aphelion) is more than made up for in clarity.

It's not defined. Objects with greatly different inclinations will still gravitationally interact when they coincide on the ecliptic.

Because, with the obvious exception of resonant orbits, bodies in eccentric orbits can still gravitationally interact with other bodies within the same perihelion/aphelion range.

The largest asteroid that does not traverse into Ceres' "orbital region" is the 10th largest, Sylvia. For the sake of argument, aggregating the mass of just the first 9 asteroids (Ceres and #2-9), one would arrive at a dominance of 57% for Ceres. Even at that level, one can clearly see that Ceres is not dominant in the same way as the 8 planets. Thus, in the original post, the statement:

"This is important because it underscores that you do not have to catalogue the mass of every object in a region to determine if the largest body has orbital dominance."

Yes. This was addressed in the original post. Planets, planetoids and moons are "large", moonlets and asteroids are "small." Making the distinction between "large" and "small" was discussed in the original post with the acknowledgement that this was a finer point worthy of a separate debate.

I was assuming bodies with stable orbits. Bodies with unstable orbits will not be around long enough to worry about. Comets, for example, are not addressed in this classification scheme.

Yes. The wikipedia article references the number of objects discovered thus far. My 'thousands' reference is based on the estimates of just about every astronomer who has publicly opined on the topic.


Thanks for the great questions. I knew that I could count on you!

__________________
"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

I like this axiom: "While all answers are responses, not all responses are answers."

Question #1: You say that this is a maxim: "Sol is a star. Earth is a planet. Luna is a moon." Can you detail the history of this maxim including citations, historic promotions or identifications of this concept as being axiomatic?

Answer Status: fiat circular

So, you're saying that you don't need any evidence to back up your claim? Wouldn't it be better to say that any classification scheme that does not adhere to your axiom is not going to gain your acceptance, since you refuse to offer evidence that your axiom is, in fact, broadly accepted? Simply claiming that something is axiomatic does not prove that it is. I ask you a second time to give evidence to support your claim.

Question #2: Dead On Arrival: You claim that any definition that violates your maxim is DOA because of historical and cultural reasons. How does violating the historical and cultural acceptance of Pluto as a planet not make your own definition DOA?

Answer Status: unsubstantiated circular

You claim that there is a historical precedent for demoting bodies that were originally considered planets and that it is Ceres. First, can you offer evidence to support the claim that Ceres has been demoted? Second, can you offer evidence supporting your claim that Ceres is not dominant in the asteroid belt? With 40% of the mass of the entire belt, and nearly 60% of the mass in it's orbital dominance region (per you), Ceres would seem to be dominant to some degree. Third, can you answer the original question about Pluto without regards to Ceres, or is the alleged Ceres Precedent, your only evidence to support your claim to not be DOA?

Question #3: Orbital Dominance: What is the empirically derived point of delination between dominance and non-dominance and why?

Answer Status: Dodged, ambiguous

First, your statement of empiricism aside, you have not declared any point of delineation based on empiricism or anything else. I ask again: what percentage represents the point of separation? Second, you continue to claim Ceres is not a planet for historical reasons but you have not offered evidence to support your claim. Third, you claim that the labels of "asteroid" and "minor planet" designate the objects as non-planets... can you offer proof that those labels indeed have that meaning? Fourth, you claim that the terms "asteroid" and "minor planet" were never used to describe Mercury through Neptune. However, this article reports that the Berliner Astronomisches Jahrbuch for 1854 listed Neptune with the asteroids Astrea through Eunomia.

Question #4: Orbital Dominance Region: What is the reason for defining the orbital dominance region as existing between perihelion and aphelion?

Answer Status: Ambiguities remain

Clarity... not empiricism? So you are artificially limiting gravity, despite the theoretical claim that it has infinite reach, for the sake of convenience?

Question #5: Orbital Dominance Region and orbital plane: What is the maximum vertical extent of the orbital dominance region above (north) and below (south) of the orbital plane of an object or planet?

Answer Status: Unanswered

It's not defined! Well is it 2 dimensional or is it an infinitely thick annulus? Don't all massy objects interact gravitationally? Are you saying this only occurs at the ecliptic? And which ecliptic are you referring to: earth's, the objects? Your statement seems ill-conceived, can you make it clearer?

Question #6: Orbital Dominance Region and Eccentricities: Why do you consider the orbital dominance region of an object to be circular/annular in the case of objects with marked eccentricities?

Answer Status: Ambiguities remain

Are you saying that obects with highly-eccentric elliptical orbits do not interact beyond their perihelia and aphelia? Also, are you saying that an object that an object exerts as much dominance in the aphelial sector of the dominance annulus at 180 degrees from its semimajor axis, despite the fact that the object is at perihelion when in that part of its trajectory?

Question #7: Orbital Dominance Region and Ceres: There are 3 large and populous asteroid belts defined by the kirkwood gaps. The orbit of Ceres does not pass into the first and largest belt. Do you exclude that first belt because it is not within the orbital dominance region of Ceres? If so, what is the new mass percentage?

Answer Status: Ambiguities remain

I ask again, what is the percentage of mass dominance required? When you claimed Ceres was 40%, you drew the line there. Now that you are beginning to follow your own rules, you conclude it is almost 50% more dominant than before, yet you still do not include it. Why can't you pick one dividing line and stick to it?

Question #8: Orbital Dominance Region and small solitary objects. Is there any lower limit for size or mass for objects that are in solitary orbits?

Answer Status: Unanswered, misinterpreted

And interesting response, but I'm not sure it answers the question. Are you saying that you do have a size limit for objects that are dominant in their orbit? If a 10km diameter object was orbiting the sun between Earth and Venus without crossing the orbit of either, would it be considered a planet, regardless of size?

Question #9: Conic Section Orbit types: The orbits you have currently used are elliptical, how do parabolic and hyperbolic orbits fit into your scheme?

Answer Status: Unanswered, assumed or unanticipated

Assuming... So what if a body is unstable. I thought you didn't want to answer questions better left to the future? If it is here now, then what is it now? A Mars or Earth sized object was transiting through the solar system at 100AU, at slightly more than solar escape velocity, how long would it be in the system and need to be accounted for in your plan?

Question #10: Kuiper Belt Objects: How many objects are in the Kuiper belt? You stated in a response to dgavin that is was thousands yet wikipedia claims 800. Can you explain the discrepancy?

Answer Status: Dodged, unsubstantiated

Can you provide evidence of these astronomers' statements? Can you also provide evidence that supports the inclusion of estimates as being equal to empirical data in this regard?

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau

It seems that Ara Pacis misses the point that this is an attempt to establish a definition.
Not one to proof a scientific fact.
And an axiom is an axiom if you say it is an axiom.
That's what axiom means.
The question is, is it a useful one.

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Ara, your combative posture is always a welcome change from the dreary civility and amiability I've come to expect from this forum!

That is true, I have not offered evidence that there is a general consensus that "Sol is a star. Earth is a planet. Luna is a moon".

Although to me this statement is plainly obvious, I'll do my best to accommodate your request:

From wiki:
http://en.wikipedia.org/wiki/Sun
first sentence: The Sun is the spectral type G2V yellow star at the center of our solar system.

http://en.wikipedia.org/wiki/Earth
first sentence: Earth is the third planet from the Sun.

http://en.wikipedia.org/wiki/Moon
second sentence: It has no formal name other than "The Moon", although in English it is occasionally called Luna (Latin for moon), or Selene (Greek for moon), to distinguish it from the generic "moon" (natural satellites of other planets are also called moons).

from wiki:
http://en.wikipedia.org/wiki/1_Ceres
first sentence: 1 Ceres ([ˈsi.riz], Latin Cerēs) was the first asteroid to be discovered.

http://en.wikipedia.org/wiki/Planet#Accepted_planets
notice that Ceres is not in the list of "accepted planets", and also that this list mentions the questioned status of Pluto -- but no other planets.

Well, I hate to pick nits, but I said that those terms are never used to describe the 8 planets. If the only contrary reference is an 1854 catalogue that knew Neptune only as a point of light, then I think that makes my point.

With regards to the dominance %, I stated that it lies between 98.8% and 40% based on empirical observation. I used the "9 asteroids only" example to show that Ceres, even with a hypothetical 57% dominance, is still vastly different than the 8 planets.

I fail to see the need, nor do I have the expertise in orbital mechanics, to postulate on where a more precise line may lie between 40% and 98.8%. But it's in there somewhere. Since we may never see a real-world example in our lifetimes, this is probably a thorny issue better dealt with by astronomers with high-powered PCs and gravity simulation software.

Not for convenience, clarity. Gravity does indeed have an infinite reach, but its effect decreases with the square of distance -- making the effects of distant bodies irrelevant compared to nearby bodies.

The gravitational interactions would be greatest at the ecliptic (of either object), regardless of the difference in inclination between the two orbits. Therefore, I saw no need to define a maximum vertical extent for the purposes of this scheme.

No, I am not saying that. But, unless there orbital resonances, there will be gravitational interactions between two bodies in the same orbital region -- perturbing the orbits until stability is achieved.

I did not draw the line at 40%. I stated that the line is somewhere below 98.8% (axiomatic) and above 40% (historical precedent).

You seem to be obsessed with the notion that every detail must be precisely determined in order for a classification scheme to work. That is not true. In fact, it only sets the stage for further conflict as additional data is gathered that may conflict with previously-misguided speculation about where this "orbital dominance" cutoff exists.

Is it not enough to accept that the current precision addresses every known orbital region while still retaining flexibility needed for future observations?

If astronomers decide that the cutoff point between a "large" body and a "small" is less than 10km diameter, then it would be considered a planet. However, the smallest diameters I've ever seen recommended for this cutoff has been 400 km to approximate the transition to gravitational rounding.

When you pose hypotheticals, I will try to fairly answer them based on the limited data you provide. The classification scheme does not need to incorporate that eventuality until it arises, if it ever does.

The fact that you are even questioning this point demonstrates that you are merely being obstinate. I am really trying to raise the level of discussion with you, but you insist on being snarky and combative. If you would spend 2 minutes on google, you would see many, many pages referencing the estimated "thousands" of Kuiper Belt Objects.

That's beside the obvious fact that an exact count of the number of KBOs has absolutely ZERO relevance to the validity of this classification scheme.

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

Because this is someone else's thread - albeit about a definition I happen to agree with, I won't answer all your points, but the question you pose above is plainly ridiculous! You can't seriously be suggesting that we should redefine the Sun as a planet can you? Or that the general public is confused about the status of the Earth, do we live on a planet or something else?

Not sure why you posed that question really, because the logical reason is that you yourself don't subscribe to that maxim - which leads to the obvious question. If you don't support that maxim, how would you define those objects?

This is the other question you continually pose, which I feel baric has provided a concise and clear response to. There is no exact point delineated empirically as yet because we simply don't have enough information to hand to be able to make that decision. That is not a weakness of this definition though, but one of its strengths!

As baric clearly states, on this criterion, the lowest included planet, Earth, comes in at around 98% orbital dominance within its orbital region, whereas the highest non-included planet comes in around 40%, or 57% depending upon how the orbitally dominant region is defined.

Given the lack of empirical data there is absolutely no sense drawing a line somewhere through there to pin this definition down. What is the point? We have no objects that challenge this definition in that area and, as baric states, putting a line through there somewhere given our lack of empirical data to draw on is sure to invite trouble at some point in the future when we discover a system that raises questions about where the line has been drawn. That instituting such rigidity into a system often creates conditions for trouble later on is well known.

There are several real-world examples of a rigidly imposed set of rules storing up trouble for a later date. In the 1990s several countries in East Asia, and other where - including Argentina, had currency controls to prevent appreciation of the value of their currencies - even as the inflows of investment to their economies demanded revaluation. The rigidity of their system though didn't allow for this appreciation - and eventually it reached breaking point! Which led to widespread financial problems in many countries as they struggled to cope with the upheavals that wracked their countries. Widespread job losses, bank accounts shorn of their value, property and share prices plunging amid the volatility.

Another example would be what happens to many countries that have been under the thumb of a dictatorship or authoritarian regime. Yugoslavia for instance - under Tito's dictatorship for decades, when they're suddenly freed and allowed to express themselves free of repression - the build up of tension explodes in all sorts of unpalatable and terrible effects. One might also look to Iraq for a similar example of what happens when a rigidly enforced system is proven to be unsustainable. Pent up emotions come out. A system like democracy - imperfect and imprecise though it may be at times, has a much better record of avoiding this sort of reality than any other political system I've seen.

Even looking at say the economies of Europe v that of the United States for instance. The amount of regulation through all parts of the European economies is far higher than that in the United States, and what does that do? It means that in Europe the accepted level of unemployment hovers around and above 10%, whilst in the US (and here in Australia), it is far lower at around 5%. Not to mention the general dynamism and entrepreneurship that more freedom and flexibility allow.

The point I'm trying to make with these analogies is that introducing a rigid system - particularly when all the possibilities out there can not possibly be known - a situation that applies to all the above examples, as well as to delineating cut-off points to baric's definition - we only have (still incomplete) data from one star system afterall - is not the answer.

Real world realities show us that having an element of flexibility in a system allows it to adapt and evolve over time - rather than (perhaps) be destroyed at a stroke when the assumptions its based upon are shown to be false. So asking baric to draw a line between Ceres (40% or 57%) and Earth (98%) can only serve to weaken this definition - until we have a better understanding it is not a line we should be seeking to draw.

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