# Binary Moons?

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In a dream I once had there was a planet and had two moons, doing a binary star system style orbit-like thing. Is this possible in real-life?

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So the two moons orbited each other, while they both orbited the larger planet? I don't think that would be at all stable.

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Hum, I guess it would depend on the size of the Moons and the mother planet, how big were they all?

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I'd asssume as long as the moons' orbit was big enough, it could be possible. But... I'm no astrophyisist.

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Hum, I guess it would depend on the size of the Moons and the mother planet, how big were they all?

I don't actually remember the size of the moons and planet from the dream. If such a thing would exist IRL, it would have to be a certain size to work.

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Possible, especially after a recent collision. But emphasis on the "recent", it wouldn't stay stable for long. Eventually they'd either collide and smush together into a larger moon, or split up and start occupying the same orbit (either settling at L4/L5 or, as before, colliding)

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It could work but it would have to have a very big parent planet. It this point it would properbly be a gas giant and a very dense one. It would properbly be so dense that it would almost be a brown dwarf or at least right on the border. The two moons would also have to be very close to each other. And for that they'd have to be small. But I guess you already figured this out. I will try and find some numbers on it.

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If they were far enough out so that the differences in the gravity that pulled on them were small enough, and the moons themselves were small enough, I could see it. Trinary star systems work in the exact opposite fashion where there is a third star orbiting the barycenter of the inner binary.

I think a really good question to ask on this subject though, is how stable they would be. As the moons orbited the barycenter, the moon closer to the host planet would have a greater force of gravity acting on it than the outer one, so (as I said) unless they are really far away from the planet where the difference is small, I doubt it would be stable for more than a few million to a few tens of millions of years.

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Wouldn't 2 moons of nearly identical mass orbiting each other be able to orbit another body? Since the barycenter of the binary system would be stable and therefore they would orbit along the path of their barycenter? I know this question was posed a few weeks ago, but it was asking if a moon could have a moon, not a binary moon.

Edited by Kilmeister
Terminology
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-snip- I doubt it would be stable for more than a few million to a few tens of millions of years.

Jeez, that's barely any time at all. (Gotta love astronomical time scales.

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Wouldn't 2 moons of nearly identical mass orbiting each other be able to orbit another body? Since the barycenter of the binary system would be stable and therefore they would orbit along the path of their barycenter? I know this question was posed a few weeks ago, but it was asking if a moon could have a moon, not a binary moon.

I think you'd have to have absolutely perfect orbits for that to work, and even then tidal effects would have more of an effect on whichever moon was closer to the planet, so it'd begin to destabalise itself.

There are binary trojans though, so presumably it would work if you're far enough from the parent body. Neptune's moon Neso has a SMA greater than Mercury's perihelion. I'm sure a binary moon could easily exist at such a distance.

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