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For a SF story I'm wondering how thick a planet's rings can be? When viewed from the inside, can the ring material be so dense that visibility would only be a couple of meters? What would it take to form a ring this thick, moon collisions, etc?

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  • $\begingroup$ I know that objects in space are REALLY far apart but saturns rings are weird, maybe that can help? sciencing.com/close-rocks-saturns-rings-13152.html $\endgroup$
    – Cupit
    Commented Feb 1, 2019 at 17:28
  • $\begingroup$ Do you need a hard science answer, or is some degree of handwavium acceptable? Just as an example, Star Trek handwaves a lot to achieve dramatically appropriate effects (see comments below referencing The Wrath of Khan). $\endgroup$
    – asgallant
    Commented Feb 1, 2019 at 22:18
  • $\begingroup$ Does your planet specifically need to be a planet? Would a different celestial body suffice? If the body is a black hole and the rings are an accretion disk, then the answer could be "yes", but you probably don't want to find out, empirically ;o). $\endgroup$
    – asgallant
    Commented Feb 1, 2019 at 22:25
  • $\begingroup$ WhatRoughBeast, thanks I thought that it could start out thick but wasn't sure how long that phase lasted. I need to research it. asgallant, yup, hard science answer would be best but I know I'll have to add some hand waving, lol. I am specifically wondering about ringed planets, an accretion disk or protoplanetary disk would change the story too much. $\endgroup$
    – astro
    Commented Feb 2, 2019 at 20:40
  • $\begingroup$ use comments for comments. Don't post answers to comment. $\endgroup$
    – L.Dutch
    Commented Feb 2, 2019 at 21:22

2 Answers 2

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No, they really can't. Planetary rings are like nebulae, only really visible when looked at from a long way away. While they have a lot of material in them it's spread out over even more distance. That scene in the Wrath of Khan where the ships are playing hide and seek in the billowing clouds of a nebula is impossible in realistic space and it's the same way with planetary rings.

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  • $\begingroup$ In the Wrath of Khan it is indeed plausible, as this could easily be some sort of gas in space. Just because the nebula clouds we see from a telescope aren't necessarily visible as a cloud up close does not mean that there cannot be a nebula which actually contains opaque gas, which is a possibility. $\endgroup$ Commented Feb 1, 2019 at 18:08
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    $\begingroup$ @worldbuilder Um, "some sort of gas in space" explains nothing here. Gas opaque at the ranges shown in Wrath of Khan would have to be pretty dense, and it's completely unclear to me how such gas would stick together when surrounded by vacuum. $\endgroup$ Commented Feb 1, 2019 at 18:49
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    $\begingroup$ You realize that the laws of chemistry and physics are universal? At the velocities they talk about in Star Trek, even sublight velocities, if you hit a cloud that is dense enough to be opague you might as well be hitting a brick wall. $\endgroup$
    – pojo-guy
    Commented Feb 1, 2019 at 19:34
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    $\begingroup$ Not just a brick wall, more like "instantly turn yourself into plasma." As for the vacuum comment, gas still has mass, and gravitational and charge attraction will eventually sap enough energy from the atoms and molecules to bring them back together. $\endgroup$
    – Stephan
    Commented Feb 1, 2019 at 19:57
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    $\begingroup$ @worldbuilder a gas cloud that is dense enough to be opaque is either massive enough to collapse under its own gravity (and form a gas giant or a star), or its internal pressure would cause it to disperse, lowering its density past the point where it is no longer opaque. You'd need some exogenous force to prevent its collapse or dispersal. $\endgroup$
    – asgallant
    Commented Feb 1, 2019 at 22:12
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David Johnston's answer to the contrary, such rings can exist - just not for long.

The orbits of the various bits and pieces will intersect each other, and collisions will occur which will gradually force the ring to become thinner and thinner. Eventually, you'll wind up with a very thin ring with each section in its own orbit which does not contact any other - like Saturn's. Either that, or the ring will coalesce into a single large(ish) satellite.

And even Saturn's rings do interact somewhat, due to the presence of moons which perturb the orbits of other, smaller pieces in different orbits.

A thick ring (probably a torus) will be produced by a major collision between two bodies. Such a ring probably existed (briefly) around the earth after the collision which is believed to have created the moon.

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    $\begingroup$ Saturn even has Ring Shepherd Moons that are prime examples of the "eventually coalesce into a single satellite" part. $\endgroup$
    – Stephan
    Commented Feb 1, 2019 at 20:00

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