World Building 101, Today's Topic: Rings are cool!

Saturn, the second largest planet in our solar system, has a spectacular ring system. Beautiful icy rings that hug the gas planet's equator. So, as a fantasy writer wanting a cool world, rings might be just the thing!

Actually, all four of our solar system's large slushy planets have rings: besides Saturn, Jupiter, Neptune, and yes, even the pun-inducing Uranus has rings (nine dark ones!). So, could our smaller, terrestrial planets have rings? Sure, but it's less likely. Mars might one day end up with rings, as its inner moon is right on the edge of something called the Roche Limit. More about that later...

Rings are formed when dirt, dust, and ice gets captured by a planet's gravity. If the stuff is outside of the Roche Limit it will slowly gather together like a snowball due to its own gravitational pull and eventually form a moon. If the stuff is inside the limit it will either fall to the planet's surface, or continue to orbit planet. The stuff can't collect into a ball to form a moon because the tidal forces of the planet pulls it apart... The explanation is a bit technical, but basically, it's because closer objects have to spin around a planet faster than farther items... So, the outer part of a moon moves slower than the inner part. If the moon is far enough away the tidal force isn't enough to break it apart. But if its close in then the inner part has to move so much faster than the outer that it literally flies apart.

So, if enough debris collects around a planet and orbits it inside the Roche Limit, a ring will form. Over time, rings will form a nice flat disc high above the planet's equator (See this explanation as to why).

The four inner planets of our solar system (the terrestrial or 'rocky' planets) don't have rings mostly because the four outer giants act as vacuum cleaners, sweeping up a lot of the debris as it falls in on the sun's gravity well. And then there's the sun itself, which pulls rather mightily on all this dust and ice. Also, our own moon is quiet large for our size of planet, so it too sweeps up debris that might otherwise fall into an orbit about the Earth, given the chance.

A fantasy world could have a ring. All it takes is enough dirt and ice to collect about the planet. It would also help to have a solar system with fewer large planets in the outer orbits, thereby reducing the amount of clean-up that occurs.

A ring can be bright or dim, gaseous or rough, banded or smooth. It all depends on what it's made up of, and whether there are any 'shepherds'. A Shepard is a large hunk of dense rock -- a tiny moon really, that is orbiting inside a ring. Since it is dense the tidal forces don't pull it apart, and it acts like a little broom, sweeping up the debris in its orbital path. These Shepard moons give a ring two or more distinctive bands (You can read about Saturn's Shepard moons here). Bright rings are composed of ice particles, dark ones of rocky debris. High iron content will give it a reddish color, while other more rare metals can give it a blue (cobalt) or green (copper) sheen. And the planet's magnetic field can have an effect on the metallic rocks as well, moving then into positions in the ring that give rise to faint dark spokes and braided banding.

Millions of ice particles in the ring can refract and diffuse the sun's light, making the ring scintillate with color. Beautiful to behold indeed!

But since a ring will form about a planet's equatorial belt, it will look differently viewed from the planet's surface, depending on what latitude (north-south) position you are at. If you are right on the equator, you'd look up to see a thin line across the middle of the sky, neatly dividing it in two east to west. Rings are quite thin, usually only a few kilometers thick. Meaning that looking at it edge-on from a thousand or so miles away would make it look like a fine demarcation, or even disappear entirely, depending upon just how dense the ring is.

The further one gets away from the planet's equator, the more of the ring will become visible. In the northern hemisphere the ring will look like a giant band going across the southern part of the sky, east to west. In the southern hemisphere one will see the ring in the northern sky.

If the planet has a tilt like Earth's then the band will rise and fall in the sky throughout the year, and the sun will cross behind the ring twice a year. Depending upon just how large the band is, the sun might spend several months behind it. If it is a mostly an ice crystal ring, then the whole band would be diffused with the sun's rays. A brilliant band across the sky for several hours before dawn to after sunset.

As for moons, your fantasy planet could still have them, they would just have to be outside the Roche Limit. The Earth's Roche Limit is about 40,000 km, and our moon is about 400,000 km, easily beyond the limit.

The Earth does have a ring of sorts. Our man-made satellites and various space trash, orbits the Earth in a band of junk... if enough builds up then we'd have our own visible ring! Read more about space trash here.

That finishes my lecture on World Building: Planetary Rings.