What's A Ring Plane?

 … and why, if you land on a moon of a ringed planet, you’re not likely to see a ring. Just some shadows on a planet. Not even the great Chesley Bonestell captured it quite right.

Author Paul Gilster, at his excellent blog Centauri Dreams, recounts a passage from Jack McDevitt‘s (recommended!) sf novel Polaris:

Most of the planet’s contemporary inhabitants — there are fewer than three hundred thousand altogether — live along a seacoast that’s usually warm and invigorating. Lots of beach and sun. Great sky views. They haven’t yet achieved tidal lock, so if you time things right you can sit out on the beach and watch Gobulus, with its rings and its system of moons, rise out of the ocean.

I would love to live on a moon of a gas giant. A beach would work, yes. Earth-sized with gravity, yes. But the vision of a romantic planet rising over the ocean with big dominant rings and a bevy of big moons is mostly fantasy. There are scientific reasons why:

Tidal lock

Jack McDevitt alludes to this in the quote above. The moon is tidally locked to Earth; one hemisphere faces the planet. The closer to the planet you are, the more likely tidal lock is in effect. Saturn’s itty bitty moons (about city and county and Connecticut-sized) even align so the long axis points at the planet. Like Prometheus:

A larger, rounder moon will rotate once on its axis (a day) for each orbit around the planet (sort of like a month in reverse). If you’re really close to the planet, you’ll see spectacular sights. But only if you’re on the side of the moon that faces said planet. There’s another reason why, even if you’re really close to a planet, the rings won’t be all that spectacular.

Orderly Orbits

Over millions to billions of years, planets of a sun, or moons of a planet settle into regular orbits. The more planets and moons, the more regular the close-in orbits are. Generally speaking. Innermost planet Mercury has something of a tilt as it moves around the sun. But Saturn’s inner dozen-and-a-half moons orbit in near-circles: everything about as far out as Titan. They also orbit in pretty much the same plane, like marbles on the flat floor of a gym or playground. The rings are in the same plane. Close to a planet or star, nobody’s orbiting up near the basketball hoops or down in the subcellar. Check the image of Cassini of Saturn, rings, and a few moons below.

Try an experiment at home. Find a big ball, like this:

This is your planet.

Then cut a big circle out of a newspaper. Here is your ring:

Put the planet in the middle of the ring and look at it edge-on. What do you see more of: the planet or the ring?

Thanks to the young miss for providing technical support for this. We couldn’t get the ring really flat like it would be for real. Here’s Saturn with the rings edge-on:

Not much ring to see from Cassini in this image, but the winter shadow of the ring on the northern hemisphere is pretty impressive.

Disorderly Orbits

Some of Saturn’s moons tumble like a juggled potato. But they’re so far away from the planet that the view would be less impressive. Saturn might appear as large as the dipper of the Big Dipper. Any bigger, the moon would be closer and you’d have tidal lock.

It is possible for a moon to be captured into an irregular orbit by a planet. Problem is that an interloper orbiting close to a planet to give good views would send other moons scurrying into their own irregular orbits. Not a recipe for long-term safety and stability.

I don’t mean to spoil the images of great science fiction writers and quench the imagination of the rest of us. But people on a moon like McDevitt’s will see a planet bigger than the sun, little moons flicking around like bright stars, and probably only the shadow of a ring on the big planet. It’s most likely their moon will orbit in or very near the ring plane. Sights like this …

… will have to wait for space probes.

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