thenextguy 10 points 14 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 14 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 14 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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motionofethre 5 points 12 hours ago

Correct from what I can see. L4/5 are stable

https://en.wikipedia.org/wiki/Lagrange_point

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AbouBenAdhem 2 points 14 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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AbouBenAdhem 2 points 13 hours ago

My point is, material from the more distant regions of the protoplanetary disk that eventually coalesce into the planet have to move through the Lagrange points to get to the protoplanet. But once they’re near those points, the gravity of the protoplanet is unable to continue drawing them. So how does the protoplanet’s gravity attract material from farther away than its Lagrange points?

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HubertManne 6 points 13 hours ago

again its an island of relative stability. they would have to get to the point at just the right direction and velocity to be stable and even then radiaiton from the sun or other effects can push it out. its stable for a man made sattelite because we make sure the momentum and dirction are just right and make minor adjustments over time to stay there. an object in motion stays in motion and an object at rest stays at rest unless acted on by an external force.

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FaceDeer 9 points 12 hours ago

The Lagrange points are only stable as long as the object in them is much, much smaller than the planet is. Once it becomes large enough to be exerting gravitational influence of its own the Lagrange points become unstable and it drifts out.

I don't think it's widely accepted as plausible any more, but I recall reading a theory that Theia (the planetoid that impacted Earth very early in the solar system's formation to cause the formation of the Moon) formed in one of the L4/L5 points in exactly the manner you're proposing but then collided with Earth once it got too big to remain stable there. IIRC more recent simulations suggest Theia had to hit at a higher velocity than this scenario was capable of producing, so that's why it's been deprecated, but the formation-then-instability bit is still valid.

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OryxAndCake 3 points 13 hours ago

Manually positioned planets sharing an orbit is hella cool as a scifi concept, maybe as an early dyson sphere precursor.

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palordrolap 2 points 7 hours ago

Matter would have to accrete equally at six equally spaced points, which as others have pointed out has a problem with getting started in the first place.

The other problem is that a protoplanetary disk is not uniform, so even if by some miracle, six things of equal mass started out at six equally spaced points along one orbit, one of them would almost certainly accrete more mass than the others.

That would throw the Lagrange points out of alignment, if not also slow the orbit of that body, and I imagine that would quickly lead to runaway loss of equilibrium.

You are free to imagine them going on to happy existences in separate orbits, perhaps with a sweet system of resonances around the star, but something tells me that's unlikely.

On the other hand, there's a neat fiction story in there somewhere if you want to apply it to our own inner planets. Mercury, Venus, proto-Earth, Theia, Mars and the asteroid belt happen to number exactly six.

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