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If planets have stable Lagrange points 1/6 of the way around their orbits, why don’t protoplanetary disks typically condense into six mutually-reenforcing planets spaced evenly around their suns?

14 hours ago by AbouBenAdhem to c/askscience

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thenextguy 10 points 13 hours ago

Because the planets have to form first? The Lagrange points don't exist in their own.

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frongt 7 points 13 hours ago

Yeah. Lagrange points are a characteristic of two-body systems. While you'd get areas of increasing stability if a cloud condensed into two bodies, the rotational motion of the stuff in the cloud would outweigh that, and more stuff would be pulled in by the more massive bodies anyway.

If you got really lucky you might end up with a little bit of dust that happened to be going slow enough, but it's still not going to stay there. Solar wind is going to push stuff around, and to stay at a Lagrange point takes active adjusting. It's not a low point in gravity, it's more like balancing on the peak between the two bodies.

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AbouBenAdhem 5 points 13 hours ago

to stay at a Lagrange point takes active adjusting. It’s not a low point in gravity, it’s more like balancing on the peak between the two bodies.

I thought that was true of L1, L2, and L3, but not L4 or L5—hence the stable clusters of trojans at Jupiter’s L4 and L5 points.

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AbouBenAdhem 2 points 13 hours ago

The planets form when the material in an orbit condenses into one body. But as soon as a denser patch starts to condense, it will create stable regions at its L4 and L5 points. That is, the same gravitational force that pulls material together to form a planet should simultaneously be pulling material more than 1/6 of an orbit away to its Lagrange points instead of directly to itself.

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HubertManne 8 points 13 hours ago

I think what you are missing here is just how large a distance 1/6th of its orbit is. Also lagrange points dont pull anything. its a where gravity and orbital speed sync up. so shotting something at a lagrange won't cause it to orbit. you have to get the right orbit going with it at that point and then it will stay in that relative point somewhat. even at lagrange points some adjustments will have to be made. its an island of stability not an attractive force itself. any matter that hits that point and is not the exact right velocity won't stay there and even if it did it would lose it eventually do to outside forces unless it had some mechanism to stabilize.

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