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Lost moons of Neptune, Still out there?
Rob Pinnegar
post Apr 27 2006, 02:45 PM
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Here's something I've been wondering about for a while:

The general feeling regarding the current Neptunian system seems to be that Triton is a captured Kuiper Belt object. When Triton was captured, any previously existing Neptunian satellite system would have been severely disrupted, which is presumably why we don't see many other moons there now. Only Proteus seems to have survived the catastrophe.

If we assume that Proteus is the "Miranda" of the original Neptunian system, then (using the Uranian system as a model) Neptune is probably missing about four major moons right now. It is of course possible that some of them could have collided with Neptune, or with Triton. However, given Triton's large mass, it's also likely that some would have been ejected into heliocentric orbits. (In fact, this could have helped slow Triton down a bit during the capture process. Transfer of energy and all that.)

Any ejected moons would most likely have made close passes by Neptune within the first few million years after being tossed, which presumably would have caused their orbits to evolve pretty rapidly. I don't claim to understand the subject well enough to predict what would have happened then, but let's suppose for the sake of argument that some mechanism could have gotten them into the Kuiper Belt -- perhaps multiple approaches with KBOs, or something along those lines.

It seems very unlikely, but I wonder if any of those old Neptunian moons are still kicking around in the outer Solar System? At Uranus, Titania and Oberon are both over 1500 kilometres across, and Ariel and Umbriel are over 1100 as well. If Neptune's original satellites were of similar size, any survivors in the Kuiper Belt could rank among the largest objects there. Wouldn't it be a hoot if Ixion, Varuna or Quaoar turned out to be an ex-Neptunian moon?

Intuitively, I think this probably isn't very likely. Neptune's original companions probably ended up as super-sized Centaurs, and got chucked out of the Solar System by Jupiter and Saturn. And really, this hypothesis isn't particularly useful because it won't be falsifiable in any of our lifetimes. But it's neat to think about, anyways.
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JRehling
post Apr 27 2006, 08:47 PM
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Possibly isotopic analysis could reveal a match between a KBO and the existing Neptunian satellites. Don't see an in situ analysis happening soon, though!

Besides ejection, there are two other possible fates for the lost moons, though: Dropping into Neptune or smashing into Triton itself. I think the former is more likely, and would truly leave no fingerprints behind.
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