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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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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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.

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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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