The large, gaseous outer planets all have ring systems, whereas the small, rocky inner planets do not. At present, scientists aren’t entirely sure how these ring systems came about. They may have formed from leftover material from the formation of the planet, or be the remains of a moon that was destroyed by an impact or simply broken apart by the gravitational force of the parent planet.
Scientists are also unsure why only the gas giants have rings. They think it may be related to the same process that resulted in gas giants forming only in the outer Solar System while rocky planets formed only in the inner Solar System. It is thought that the energy given off by the infant Sun expelled most of the light gases and other volatile molecules from the inner regions of the Solar System, leaving heavier elements able to form inner rocky planets. This process also seems to have made it easier for the outer planets to form moons.
So, the combination of large gravitational forces, the existence of volatile materials such as ices and the shepherding of material by numerous moons probably mean the outer planets were far more likely to form and keep planetary rings.
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