From: Henry Baker <hbaker1@pipeline.com>

what makes a centrifuge different from the solar system? I.e., at what densities of planetary matter would things start working differently?

From: "Paul R. Pudaite" <paul.r.pudaite@gmail.com>

The solar system doesn't have a wall. (?)

Gravity, to things in orbits, is not like a wall because the force of gravity and the momentum are both proportional to the mass. With a gas of particles of different densities, in orbit, the gas pressure will be equal from all directions and particles of different densities are happy to share similar orbits. (The pressure is related to "temperature" of the particles moving randomly relative to a lockstep orbit, and some of the energy of collision is dissipated by radiated heat... and yet they can't settle to a lockstep because rings at different distances have to go at different speeds... Does that mean everything has to become separated solid rings?) If everything stopped in its orbit, and was only held up by bouncing against other things (and against the Sun??) it would be like Earth's atmosphere or oceans, or a centrifuge, and then dense stuff would settle toward the center. Then, you've got the fluid pressure pushing up in proportion to size and the gravity pulling down in proportion to mass. I think if the force of gravity dropped off more slowly or more quickly, you'd have the effect of a wall pushing inward or outward, respectively, so this raises a more metaphysical question, why is gravity just so? I wonder whether slight changes in gravity's exponent either way would mean chaotic orbits that collide with each other much more often. --Steve