Unimportant Questions About Planets and Rings

[Courk]

1. Is it possible for a rocky planet to have a ring system? I don't see why it couldn't, but I don't pretend to have any knowledge about this.

2. Does a planet have to be a certain size before it can have a ring system?

3a. What would a ring system look like from a planet's surface?

3b. What would Saturn's rings look like from the surface if Saturn had a surface?

[Chuck]

Does abandoned space junk count as a ring?

[Samadhi]

1. We had a ring before it coalesced into a moon (by most theories).
2. It has to be bigger than the mass of the ring.
3a. Pretty freaking cool.
3b. Really freaking cool.
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[Courk]

So it's theorized that we had a ring that smooshed together to form a moon? IIRC, an article on wikipedia said that a theory for rings was that moons got ripped apart when they got too close to the planet. These two theories don't seem to match up.

[Quailman]

There's a competing theory that says the rings of Saturn are the remains of an ancient death star that was destroyed in a conflict a long time ago.

[Chuck]

Maybe the part of earth's ring that was far enough out formed the moon while the part that was too close eventually fell into the atmosphere.

[Lepton*]

I'll try to answer... I put a "rings primer" at the bottom of my post.
1. A rocky planet *could* have rings. The defining characteristic here is the planet's mass. You need a very large mass in for the Roche Limit to be far away from the planet, and you don't want to be near the planet because a ring can't form too close because the material would fall onto the planet too fast (from orbital drag). So the rocky planet would have to be very large. Probably a couple times the size (radius) of Earth.

But in a sense we do have rings. The Van Allen belts are created by some interesting electrodynamics. They are able to keep some material in "rings" around Earth. Not much, though. It wouldn't be visible from space like Saturn's rings are.

2. Roche Limit. (I try to explain it at the end)

3. I'm not entirely sure. If you were standing directly underneath them, on the equator, you probably wouldn't see much. At a higher latitude, you would probably see a bright glowing band that goes across the sky. Remember, however, that the rings are only going to be lit up when they are on the sun-facing side, which is day, so it won't be as spectacular. On the other hand, since the rings do extend a fair bit beyond the planet, you might be able to see the edges of the parts that are illuminated at night.

Here's how I imagine it: At a higher latitude, looking South, the inside of the rings will be near or below the horizon. The top of the rings will be near -- but not too near -- the overhead. So there will be a huge, thick band running across the sky between East and West. At midnight, the rings directly overhead will be blocked from the sun, but the rings at the side of the planet will be illuminated. So the bands will run a bit up into the sky and then stop like two sides of a stone bridge that is far from completion. It would be absolutely gorgeous, I think.

Samadhi: Some ~recent work has shown that it is almost impossible for the moon to pick up enough debris to form around the Earth. Computer simulations of the formation of the solar system seem to be showing that it is extremely unlikely for a planet like Earth to have a moon like we have. Venus and Mars both lack sizable moons. In fact, the best guess right now is that a Mars-sized object hit the Earth in the formation stage. As a result, a huge amount of the Earth's mantle was knocked out into space and later collected itself into the moon.

There is some evidence of this strange theory. The moon, despite being a sizable object, lacks any internal structure, as if it formed all at once and didn't exist in the earliest days of the solar system when things were very hot (which would give the core a high temperature and thus cause differentiation). In addition, the moon seems to be made entirely of the same stuff as the Earth's mantle, and that's pretty hard to explain in any other way.

Hopefully I haven't mis-spoken.

The Roche Limit of a heavenly body is the distance from its core beyond which its gravity doesn't break up clumps of material. That's a poor description, so here's what I mean: if we were to put a large spherical clump of gravel and dirt into low orbit around the earth, the earth's gravity would pull it apart as it orbited. Think of how the gravitational attraction would be larger on one side of the dirtball than the other and that sort of thing.

For the massive planets, what we think happens is that small moons get "too close" to the planet and get broken up into rings. This process can take millions of years (but not billions, so it is sort of a fast process). The rings themselves aren't stable -- passing moons stir them up and so on -- and they would dissipitate on a timescale of millions of years if they weren't replenished. If I was given the opportunity to watch anything in the universe, one of my top picks would have to be the long-term evolution of the rings. I'm sure that it would be absolutely beautiful.

The Cassini space probe has been doing some wondering work around Saturn lately, so the emphasis of current research is on Saturn. There seem to be small moons within and outside the rings (but none inside of them). The small moons near the rings act to carve out the interesting forms that we see in the rings. For example, a gravitational resonance, which is sort of a "sweet spot" between an outer moon and the planet, seems to be the causes of many of the gaps in the rings. Others are caused by the moons moving through the rings.

The rings themselves are extremely diffuse. They are probably less dense than most of our best vacuums that we can create on earth. However, the material of which they are made -- regular dust, tiny rocks, and bit of ice -- are extrodinarily reflective.

[Courk]

[Chuck]

We wouldn't be here to see the rings anyway. Our ancestors would have fallen off cliffs while wandering around looking up.

[Lepton*]

[Chuck]

[Neo]