Why does Earth not have rings?
Dear Urban Astronomer
After reading your article about the rings that they found around that exoplanet, I wondered why Earth does not have rings? You mentioned that a lot of the planets in our own solar system have them, so why not us?
The planets in our Solar System which have rings are Jupiter, Saturn, Uranus and Neptune. These four planets have a few other things in common as well: They all have lots of moons, and they’re all extremely massive gas giants. They’re big enough to capture passing asteroids, which is how they gained most of their moons. They’re also big enough that if a captured asteroid swings in close enough, it could be ripped to fragments by powerful tidal forces. The debris from such an event would spread out into a flat circular region, which explains how each of the ring systems could have been formed, and why the smaller rocky planets (including Earth) don’t have ring systems of their own.
As we’ve just discussed, rings are composed of uncountable small bits of material orbiting in a plane around a planet. They’re easy enough to form but they’re also dynamically unstable, so that they dissipate over the course of only a few thousands of years. After a while, all that would be left would be the sort of faint and diffuse ghosts of rings that we see around three of the gas giants. But what makes Saturn so special, that its rings are so bright and obvious? It turns out that Saturn has tiny “shepherd” moons, buried in and around the rings. These tiny moonlets are so small they they were only seen for the first time when unmanned spacecraft first visited the planet, but they still have enough mass to drag any wandering debris right back into place within the rings where it belongs. Interestingly, although the moonlets are invisible from Earth, we can see their orbits as thin grooves separating the various rings within the ring system.
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