Quote:
Originally Posted by Molokh
Aren't Lagrange points special precisely because they're stable?
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L1, L2, and L3 aren't stable, but are where gravitational forces either cancel each other out or cancel out centrifugal force. L4 and L5 are both stable; these are 60 degrees forward and behind the orbiting body in the same orbit. Not only are these stable, but several bodies in our solar system are at the L4 or L5 point of other orbiting bodies. Saturn has several such moons (called trojans), and I believe Ceres or Orcus has its own trojan, though I could be mistaken. I get the dwarf planets, plutoids, and KBOs all mixed up.
Theoretically, if two moons orbited a point outside themselves, their center of mass could probably orbit a planet. I don't know of any instances thereof or how it'd come to be, but considering the nature of so many of hte planets being found of late, I hesitate to discount any occurance, no matter how bizarre. I mean, they've found planets orbiting pulsars and white dwarfs that formed after the star died, and they've found proto-planetary disks around brown dwarfs.
Anothe interesting, though improbable system might be that of a binary star system with one main, hot star being orbited by a brown dwarf or suitably small, dim star (like a white dwarf) with a planet chasing it at L5.
Another option I used once for a fantasy setting was that of a planet with one large moon similar to our own and one tiny black hole orbiting the planet further out. As long as the gravitational fields canceled out it would remain stable, though I sort of ignored all the radation the black hole would be spewing out.
With ragard to planetary rings, don't discount such visions as Arthur C Clarke's in
The Fountains of Paradise, either. It could be possible to artificially manufacture a planetary ring and even connect it to the planet with several space elevators. Such a world would look much like a wagon wheel and also be plausible (provided materials science is sufficiently advanced).