Posted by Michael Dayah (68.47.233.68) on January 12, 2003 at 01:40:26:
In Reply to: Planets orbiting around each other and sharing atmospheres posted by Alex Glassmann on January 11, 2003 at 12:09:59:
: I was pondering an idea that recently came to me. Would it be at all conceiveable for several small "planets" (smaller or equal in mass to our moon) to orbit around eachother with no large parent planet. For example and clarification, two or more planets with an unprecedented similarity in mass, orbiting around each other. Assuming the preceding is possible, is it at all pheasable that if this grouping or clumping of planets (if there were quite a large number--ten or more)could in affect share atmospheres?
Let's start with two planets. Binary planetary systems can exist. I'm not sure where the line is between planet and satellite and binary planets, but Pluto and Charon have been considered both. It's also speculated that they may share atmospheres—molecules of methane drawn from Pluto to Charon. It sounds like you're talking about two equal-sized planets orbiting very near each other. Again, possible. The problem with orbits is that they decay because space isn't a perfect vacuum. All orbiting bodies lose their energy to particles they hit in space. With planets very close together, there's not much room to decay. When you introduce atmosphere, which creates so much friction our launched satellites regularly fall to earth and require thrusters, the decay becomes a much greater problem, especially at the close distances you describe.
It sounds like you're more interested in multiple planet systems. It's very difficult to get more than two bodies to orbit each other. Two bodies generally represent stable equilibrium. If the orbit decays or some other perturbation is introduced, they'll probably continue to orbit each other. Multiple bodies generally represent unstable equilibrium. You can find positions and trajectories that'll allow multiple planets to all orbit each other indefinitely, but if something is slightly nudged, a body may be ejected or fall into another. There's some complex mathematics behind this that I don't fully understand. To really get a feel for this stable and unstable equilibrium yourself, I strongly suggest you download this small gravity simulation program. It's from the early '90s and for Windows 3.1, but it works on XP. After a little fooling around, you should be able to master making binary stars orbit each other. If you're able to get 3 stars to orbit each other for more than half a dozen cycles, save the simulation and send it to me, because I can't. The chances are very low that something will randomly occur if we have trouble making it work in a simple program with a goal in mind, not to mention the decay problem that this program doesn't take into account.