These processes vary in importance from planet to planet so atmospheres can be markedly different although the physical processes are the same. The two larger planets, Jupiter and Saturn, have small, solid cores surrounded by a layer of liquid metallic hydrogen. Uranus and Neptune do not have liquid metallic hydrogen zones because the pressure is not sufficient, but as on Jupiter and Saturn, differentiation has caused virtually all their heavier elements to sink into the core. The cloud bands seen on Jupiter are thought to be layers of varying molecular composition lying at different heights within the atmospheres. We now know that all four gas giants possess ring systems of varying extent.
P are the radius and density of the planet and r O is the density of an orbiting object.
Conversely, bodies could not form by accreting from component parts within this limit. A disc is formed because interactions between the particles cause the distribution of orbits to flatten and slowly spread inwards and outwards. This figure illustrates how a pair of small moons can keep particles trapped in orbit between them. The inner shepherd satellite overtakes the particles, accelerating them if they fall inward and thus moving them out. Similarly, the outer shepherd slows particles that move too far out, so that they fall back in.