Why Do Some Planets Have Rings? And Why Doesn't Earth?

by Douglas James

The four ‘giant planets’ of our solar system have a couple of things in common, and one of those things is that all of them have planetary rings. The most famous of these is Saturn, but Jupiter, Uranus and Neptune also have rings. But why? Why is it that some planets have rings and others don’t? I’m going to simplify it down for you all...

Well perhaps the most interesting thing about this, is that rings around planets form because of different reasons, and at different times. It used to be thought that Saturn’s rings had formed relatively recently, or at least to the size they are today. However, new research based on evidence picked up by the Cassini spacecraft suggests that Saturn’s rings are almost as old as the Solar System itself, but lots of the ice that forms them, only looks new because ice chunks are constantly colliding with one another - breaking apart and then forming themselves anew. Saturn’s rings were actually formed when many Pluto sized planets passed too far into Saturn’s gravity well and were pulled apart, and those fragments fell into orbit around Saturn, creating the beautiful rings that we can see even from Earth.

Now Jupiter is a different story. The New Horizons spacecraft’s data suggested that it’s rings were formed over a long period of time, mostly from meteorite strikes into Jupiter's moons. What exactly makes up the rings of Uranus and Neptune is still a mystery, as is how they were formed. It will be interesting to find out how similar their ring creations are to both each other, and the other ringed planets.

So why doesn’t Earth have rings? Well it turns out that we may have done a very, very long time ago. Around four and a half billion years ago, a planetary body the size of Mars collided with Earth and send debris from Earth all over our gravity well, and some of this debris famously formed together to create our moon. It is very possible that a temporary planetary ring formed out of these fragments as well, but if it did, then it wasn’t particularly stable, as all the material that was thrown out was gradually sucked back in. This was a particularly… unstable time for our baby planet. I would though recommend looking at pictures of Earth with rings on google, some of them look plain awesome.

The fact is, many rings seem to come from asteroids and such floating around in space, and not very much of that every makes it to our end of the solar system, as it is normally stopped before it can. Jupiter for example acts like a massive shield for Earth, dealing with many potentially life-threatening asteroids that could come hurtling towards us. As does our moon. It also seems as though there is a link between larger planets and rings, suggesting that a more powerful gravity well is needed to sustain them. For now though, yet another integral and beautiful part of our solar system remains a mystery.