If a center of mass begins to aggregate in an area of space, smaller objects may get captured and will move in an elliptical orbit with the forming object at one of the foci of an ellipse. Because the other foci of the ellipse is also fairly stable, it is also a likely point for smaller objects to aggregate into another larger object. Objects forming in pairs can reinforce each other. Over time when the small objects around the two foci have all aggregated, the objects at the foci will fall together due to gravity.