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!