I know what you mean, but is this literally true? I don't think it is, but nor do I trust my dodgy math. :-k

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The sun is 300,000 times as massive as the earth, but is only 400 times farther away than the moon. So, the_shaggy_one is wrong. The sun's force on the moon is 300,000/(400)^2, or about twice the earth's.

I'm still liking your overall idea, minus including non-spheroids. If we include everything spherical, that'd be 5 rock planets (counting Ceres), 4 gas planets, and probably dozens of icy planets. Anything lopsided can be an asteroid, comet, or "cryotoid" (hey it could catch on ). Plus it smoothly divides our solar system into three categories. It seems pretty simple and not arbitrary to me.

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I was trying to limit the planets to 8, not 9. However, the elimination criteria could be manipulated to allow additional masses so that if we claim Pluto as a planet then it would almost automatically include 2003 UB313 and some of the other planet-sized KBOs. In my current elimination system even the moon would be considered a planet, although that was not my intent. Anyways, my system is supposed to be flexible instead of static. Pluto does have two strikes against it (#3 &4) but I also thought it (and comets and asteroids) had a lower density than water, but I was wrong. That elimination query should probably be removed or revised.

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You're right, I phrased that poorly. But if you delete the error, my point is still valid. The Moon is definately a satelite of the Earth, and should not be considered a planet if we're going to define planets as orbiting only stars.

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Why?
But currently it's nine planets, according to your criteria? Doesn't it fail 3, 6, and 10? Or am I misunderstanding one of them? (BTW, did you mean to say "the sun" in #5? If not, how does it differ from #6)
No, you phrased it perfectly. Otherwise, Eroica might not have caught it.
Why "definately"?

OK, MM. But remember: Gravity. That´s the key to my scheme. The plot of a body´s trajectory will tell us about its nature; Good old Astronomy taken to the center stage. Simple and neat. I would have to include non-spheroids in that scheme, for getting rid of exceptions.

Yes, we´d have lots of planets, but only a few would be significant. We would have fewer asteroids, but we´d still got a whole lot. I guess that even in my scheme there would be more asteroids (planetoids) than planets. Case someone feels unconfortable with the high number of planets my scheme yields, remember that our databases will have to store millions of them in the future, as extrasolar "planets" continue to be discovered.

(*) I liked "cryotoid".

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Why "definately"? Because that's definately the way it is. I mean, unless you want to dispute that the Moon is gravitationally bound to the Earth in such a way that it circles the Earth every 27 or so days.

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That argument seldom works
I know, for a fact, that there are a lot of people that would consider the earth/moon system to be a double planet (I am not one of them), so it must not be so obvious to them. Your error is one of their reasons: the moon is fairly large in relation to the earth.

But the fact remains that the Earth-Moon barycenter is inside the Earth at all times, so they're not co-orbital bodies like a double planet would be. They're a planet and a moon.

Edit: I did a check, the Moon's mass is slightly more than 1% of Earth's.

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Personal Preference. I like Holst. I also don't think Pluto should be a planet, especially if similar bodies are declined for sake of history, culture, popular opinion, or ego. Although that last statement sounds hypocritical, I make a distinction between Holst informing my criteria and being my criteria. The desire for 8 planets harkens back to an earlier post in this thread where I thought we should have a demarcation between Central Planets and Extended Planets. Perhaps we need at least 3 categories for inner terestrial, outer gaseous, extended icy, and maybe even a forth for trans KBO-Sedna types.

I'm OK with Pluto being a planet as long as we also include Charon as a doublet and at least 2003 UB313 with possible consideration given to the other large bodies in that range. I don't see how categorization of an object as a KBO, TNO, or SDO necessarily precludes classification as some form of Planet. The Kuiper Belt was predicted as a source of short period comets, not long period planets, so we should not conflate the two.

According the the criteria. My post on this elimination system was mostly just to promote the elimination system, while the criteria were mostly just made up to serve as examples. At any rate, the criteria should be revised.

Well, I had suggested altering Rule #3 from 5° to 10° off of the ecliptic, and that would remove this elimination point from Luna which is only 5.1° from the ecliptic. I wanted to change it to 10° because 5° eliminated Mercury (7°), which was not my intent, but 10° still eliminates Pluto which is my intent.

Rule #5 was changed to "star" in a later post but not reflected in the original elimination rules post. The difference between this rule and #6 is that it was specifically meant to allow for extra-solar planets and rogue planets (parabolic and hyperbolic orbits are still orbits) and to act as a secondary elimination point against planetary satellites.

Rule #6 was meant to eliminate Luna and all minor and major satellites of any planet (with no comment on double planets). I acknowledge that there may be reasons to claim that Luna orbits the sun, but if we look at it based on the reference frames of Earth, Luna, and the Sun, the moon apears to orbit the earth as primary. The perspectives that present the moon as orbiting the sun, with perturbations by the earth are temporal-polargraphic and exist for POVs external to the 3 bodies in question. Can anyone tell me if the moon orbited back on itself shortly after it was made (assuming giant impact theory) or if it always orbited slowly enough to appear to be orbiting the sun? BTW, this criteria also eliminates an earth-like planet orbiting a brown-dwarf or super-jupiter; it would not be a planet but a satellite. However, a planet orbiting a white dwarf, neutron star, or black hole is still a planet.

Rule #10 was meant to eliminate Cruithne but also seems to eliminate moons. It is also designed specifically to allow double planets, meaning that such systems count as two planets instead of a planet+satellite. However, with the changes in Rule #3 Luna only has two elimination points and 3 are needed to demote an object from planetary status.

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Like I said, there are people who disagree. There's no "definately" about it.

PS: Here's my old post with cites to older articles in Sky and Telescope.

Pluto/Charon is a primary with a rather large satellite. It is bound to its primary and an overhead view would show its orbit looking like a picture made by the Spirograph toy.

Earth/Moon is fundimentally different, being co-planets. The moon's orbit is always concave toward the sun, like any Primary.

So... the distance from the sun affects the definition! The greater the distance, the larger the ratio that still falls under "normal" satellite. And, the absolute size of the primary also matters, not just the ratio.

Tricky. So what does making the distinction buy us?

Well, it looks like we all need to define both a double planet and populations for use in any form of elimination-format planet classification system.

Double Planets
I think this wikipedia article discusses three common defining characteristics of a double planet v a planet+satellite. Allowing for one exception, it is a double planet if:

1- The Secondary object contains 10% or more of the total mass of the two objects OR secondary object has a mass equal to 10% of the primary. Wikipedia lists both and there is a mathematical difference (10% of 100+10 which is 110 (=11) OR 10% of 100 (=10)).

2- The barycenter of the two object system is not within the sphere of the primary. I assume they are referring to the solid material and not the gaseous, but that may need to be refined.

3- The secondary object shows a greater gravitational attraction to the primary mass than to the star resulting in A) a planet instead of stellar related inclination and, B) an orbit that may curve back on itself. It is unclear if these orbital influences are mutually exclusive or if a satellite might demonstrate stellar influenced orbital characteristics by chance.

There is also the issue with two large planets that happen to be co-orbital but whose orbital characteristics are not apparently revolutional with reference to the primary object. These may also be considered double planets, although Companion or Twin Planets might be better.

4- Other criteria we might add to an elimination based criteria for double planet is whether the secondary appears to orbit the primary from the perspective of the A) Primary object, B) Secondary object and the C)Star. We might include Synchronicity as #5.

Earth and Moon: The moon meets all the criteria for satellite except #3A,B which may be debated as coincidental. (When the moon was younger did it orbit closer and faster and back on itself?) We might allow one elimination point, but the moon clearly has more than one (1,2, 4A 4B, maybe 4C, and 5)

Populations
Primary Populations might be defined as a group of objects that have these characteristics:
1) Co-orbital (range to be defined, perhaps <1AU between maximum and minimum member perihelion and aphelion)
2) Co-planar (in relation to each other, not necessarily to any external object, but range to be defined, perhaps <10 (20?) degrees variance)
3) Stellar-orbital (not captured as satellites of a primary planet in direct or lagrangian orbits, resonance is ok)
4) No major object in the population has more than 40% of the total population mass. In such a case, the objects may be classified as co-orbital secondaries of the 40%+ primary. However, those secondaries might constitute Secondary or sub-populations (e.g. Trojans, planetary ring system, NEAs)
5) Have similar origins
6) Have similar compositions, or a common range of compositions based on origins.

Feel free to suggest more criteria.

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How do you mean that? For the right choice of "wheels" the earth/moon path looks like a spirograph too