The particles are not in orbit. They are in a gas, colliding exceedingly frequently one with another, though going in straight lines between collisions. Each center of gravitational attraction is simply a region of denser gas compared to 'outlying' areas (made denser by their mutual gravitational attraction), and the two centers, as a whole, are in orbit about each other. Were it not for the loss of energy due to radiation (the clouds are glowing white hot, in a 'fog' of hot gas surrounding it), the clouds would not contract and get denser. However, because they do radiate away their energy, gravitational collapse ensues, causing the gas to become denser and denser (higher internal pressure) until eventually the point is reached at which the gas molecules begin to turn to liquid. Imagine a gas of two components - say Radon and Nitrogen, one much heavier than another. In a box in deep space, they would be evenly mixed. Place that box in gravity, say a basement of a house. One would now expect a gradient, with the heavier ones spending more time closer to the earth's center. Or, take a centrifuge with two different molecules of different weights in the tube, and apply many g's to the molecules. Again, there will be a gradient. Gaseous centrifugal enrichment of U-235 works by a similar principle. The greater the difference in the masses, the greater the ease of obtaining the gradient. Any additonal insight by others on how to express this well, would be appreciated.