Can terrestrial planets have rings?
I found a site claiming that only Earth has no rings at all, all other planets do. Is there any truth in this?
It turns out that all of the planets, Earth included, did have rings at one time. The thing is, these rings were unstable and the material was either lost to space or collected into the satellites of these planets. The difference between the terrestrial and giant planets is the giant planets have the gravity to capture and hold onto a large satellite system, and these satellite systems are the source of the ring material.
For a ring to be stable, it must be held tightly by the planet's gravity, and the planet must also exert tidal forces on the particles in the ring. Tidal forces result from the fact that the pull of gravity is inversely related to distance, so the farther away an object is, the less force it experiences from the object it is orbiting. Therefore, a planet pulls a little bit more on the inner side of its moons than on the outer side. Close enough to a planet, this might cause a moon to break apart, and also keeps the bits of material that form a ring from collecting together into a moon.
However, it's possible that Mars might develop a ring in the future. Its moon, Phobos, is close enough to the planet that it feels the effects of the planet's tidal forces, and eventually it may break apart and form a ring.
Earth, Mercury, Venus and Pluto will probably never again have observable rings, although if you dumped tons and tons and tons of sand near the planet, it would probably form a ring - pretty cool eh?
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