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Plenty of other planets in the Solar System have rings. So why not Earth?

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Sources:
http://www.universetoday.com/84129/why-does-saturn-have-rings/
http://saturn.jpl.nasa.gov/science/index.cfm?SciencePageID=55
http://blog.hmns.org/2010/04/the-roche-limit-why-does-saturn-have-rings/
http://io9.com/5931098/using-the-roche-limit-to-put-a-ring-around-earth
http://blogs.scientificamerican.com/observations/2009/11/25/what-would-rings-around-earth-look-like/
http://www.nature.com/nature/journal/v285/n5763/abs/285309a0.html
You wouldn't really want to live on Saturn. It's cold and windy over there. Not to mention the fact that you'd just fall through most of the planet. But there is a certain majesty about the place, what with all those rings. And, they're not as obvious about it, but Jupiter, Uranus, and Neptune all have rings too. So why not Earth?

Turns out that mostly, the answer is luck. And our luck has been a lot different than Saturn's. Its rings consist of billions of particles. Scientists aren't exactly sure how they got there, but many of them probably came from asteroids, comets, and other space rocks that got captured by the planet's gravity. Eventually, they got torn apart into smaller fragments.

But Earth's got a moon, and it hasn't broken up into rings. There's a good reason for that, and it has to do with something called the Roche Limit. An object like a moon will orbit a planet if it's captured by the planet's gravity, but once it's captured it doesn't just sit back and go along for the ride. Instead, gravitation is constantly trying to tear it apart with tidal forces. These forces come from the fact that gravity isn't uniform. The closer you are to the center of the planet, the stronger it gets.

So, the side of the moon that's facing the planet feels a stronger pull than the far side. Normally it's hard to tell that this is happening because moons are made of some pretty tough rock, and the most that happens is that the rock shifts around a little as it orbits. But if a moon gets close enough to its planet, these tidal forces can overcome the very forces that hold the moon together. And when that happens, the moon can slowly disintegrate. Its remnants get spread around and, eventually, you'll end up with rings.

In 1848, a French astronomer named Édouard Roche calculated exactly how close any two objects would have to be for the smaller one to disintegrate in this way. That distance is now called the Roche limit, and it depends on the radius of the planet as well as the ratio of the densities of both objects. So, even for the same planet, you end up with different distances depending on the satellite in question.

And, quite simply, the main reason earth doesn't have rings is that we don't have any comets, asteroids, or moons orbiting within their Roche limits. The Roche limit for the moon, for example, is around 18,000 km but on average, the moon is about 384,000 km from Earth. Not at all close enough for tidal forces to rip it apart.

Even though asteroids and comets do sometimes pass close to Earth, they rarely get close enough to be within their Roche limit. For the average comet, which isn't particularly dense, the Roche limit would probably be around 35,000 km from Earth. For an asteroid, which is usually denser than a comet, the limit would be even smaller. Such a close shave is going to be rare, and even when a comet or asteroid does pass within the Roche limit, it's almost certainly going too fast to be captured by Earth's gravity.

So no rings for us. But buck up! Earth still has a lot going for it! It's warmer than Saturn, not nearly as windy, and you can stand on it.

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