Why doesn't the Earth have rings?
Ever wondered why planets like Saturn have beautiful rings, while the Earth only has a moon? The answer lies in the concept of the Roche Limit. This is an imaginary circle around every planet which determines if the planet will have rings. Within this circle, the forces from the planet’s gravity (tidal forces) are strong enough to break a moon into pieces and form a ring. The Earth had rings 4.5 billion years ago when another planet crashed into it and ejected a ring of debris into orbit. However, this ring was outside the Earth’s Roche Limit, which meant that the Earth’s gravity was not strong enough to preserve the ring. Hence, the pieces in the rings started combining and formed a ball which we now know as our Moon. A more scientific explanation An examination of the inverse square law of gravitation will show that the rate at which the gravitational field strength changes increases as you get closer to the planet (the gradient is steeper, as seen on the graph to the right...