One Comment

# Why Doesn’t Earth Have Rings Around It Like Saturn?

By | On + -

Jupiter, Saturn, Uranus and Neptune all have rings, so why doesn’t Earth have rings too? Turns out that, mostly, the answer is luck.

Our luck has been a lot different than Saturn. It rings consists 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 in 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 the planet if it’s captured by the planet’s gravity, but once its 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 up from some pretty tough rock and the most that happens is that the rock shifts around a little as it orbits, but if the 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 residents gets spread around and eventually you’ll end up with rings.

In 1848, a French astronomer name 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 the Roche limit. The Roche limit for the moon, for example, is around 18,000 kilometer, but on average, the moon is about 384,000 kilometers 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 the Roche limit. For the average, comets, which isn’t particularly dense, the Roche limit would probably be around 35,000 kilometers 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.

.

##### Tags

1. Thanks for this. I’ve been thinking about it for the 5 years since this article :).
Earth appears to have had little other than the moon in orbit with it until we started littering our near space in 1957.
Why was that?
Although I understand the need for just the right velocity, presumably over 4.5bn years there would still have been significant stuff from “outer space” that could/should have been captured in orbit with us.
Why was it so clean (or was it?)
Is it that, over a long enough period, it is self-cleaning?
How?