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.
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.