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Originally Posted by jkmccrann
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.
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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.
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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?
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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!
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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.
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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.
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Would those figures approach the 98% mass dominance enjoyed by the Earth? Or at least under 99%?
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nope. Good questions, though.
(note: questions added to comments section of original post)