# How can I justify what is basically a planetary ring system inside the atmosphere of my world?

I was thinking of making flying islands part of a planet ring, but the problem was that they wouldn't be able to host life unless they were inside the atmosphere of the planet, but at that point it would seem unlikely that they could just float in the sky, so I'm here asking what I should do with the planet ring/flying islands of my world. I think I will have to give up this explanation and come up with something else, otherwise.

If I used the wrong tags, or this question breaks any rule; I am sorry, I just signed up.

• Hello Virdex and welcome to Worldbuilding. I think your tags are fine, but sadly I don't think that what you are looking for is possible as I described below. (unless any other Worldbuilders can come up with anything really novel) – Slarty Sep 30 '17 at 18:24
• Do you really require a planet? Would a setup such as the one in Larry Niven's The Integral Trees work? – Spencer Sep 30 '17 at 18:29
• While not exactly the same question, you might want to also check out Is it possible to orbit inside a gas giant? as your scenario is quite similar. – user Oct 2 '17 at 17:11
• I think the question is fine. We're here to try to help you figure out how to get the elements that you want in a world. Of course, sometimes, the answer is "sorry, but that's not possible". (See also e.g. When is “that's not possible” an appropriate answer? on Worldbuilding Meta.) I also encourage you to use the search feature; we've had some questions involving floating or flying islands, and you may find some of those to be informative. – user Oct 2 '17 at 17:16
• Is it a prerequisite that the planet's native atmosphere MUST be the same kind inside the ring system? Does it have to orbit or stand still? A venus-like atmosphere will make a structure with breathable air buoyant. Think of Geoffrey Landis' cloud city. – Christmas Snow Oct 2 '17 at 17:37

It is not possible to have a planetary ring orbiting within a planetary atmosphere. The very high speed orbital ring would encounter immense frictional heating due to interaction with the atmosphere and would burn up very rapidly. Even a ring orbiting at the very outer reaches of the atmosphere would encounter drag which would cause it to spiral lower and eventually burn up.

$$V=\sqrt{\frac{GM}{r}}$$

Where

V = Orbital velocity in $\frac{m}{s}$
G = Gravitational constant $=6.67\text{e}^{-11}\frac{m^{3}}{s^{2}}kg$
M = Mass of central body $=5.98\text{e}^{24}kg$
r = radius of orbit $\approx6371km$ at the surface

If we apply an orbit at 10km:

$$V=\sqrt{\frac{6.67\text{e}^{-11}\times5.98\text{e}^{24}}{6.381\text{e}^{6}}}\approx\frac{8km}{s}$$

• This answer sounds accurate, but I would love to see a little more detail added about the reasons why: 1. Orbit within atmosphere requires high speed, 2. Orbit at edge of atmosphere encounters drag. Note that I am not disputing either of these, only suggesting that additional detail would make this answer even better! – Dan Henderson Oct 1 '17 at 2:39
• I have added an example assuming an orbit at 10 km, however given the size of the earth 100km either way would not have a very significant effect on the result. The equation for calculating orbital velocity is widely available and is based on Newton’s law of gravity. Does this meet your requirement? I would be happy to provide further details if you are interested. – Slarty Oct 1 '17 at 13:08
• Using an image to display text is not recommended, because text within images can't be read by search engines or screen readers. I made an edit suggestion converting some of the image's contents to text, but I couldn't do all of it because according to the editor "your edit changes too much". I guess you'll have to do the rest of that on your own. – Dan Henderson Oct 2 '17 at 13:50
• Thanks Dan I think thats fine - glad you added the "if we apply an orbit of 10km" otherwise it would have been a daisy cutter orbit lol not that it makes much difference. – Slarty Oct 2 '17 at 16:33

I'm going to go with the Avatar answer, which is at it's most basic, Maglev. The islands of your flying mountain chain contain large concentrations of a room temperature superconductor of sufficient efficiency that it can pull up a large chunks of scenery and support them on the strength of the planetary magnetic field. Such mountains should hold pretty much stationary relative to the ground, they will drift with changes in the planetary magnetosphere and may fall if there is a local weakening in field strength. I'm not sure about the exact atmospheric effects of such a configuration, whether there would be frictional heating etc... High level winds will be effected by such a range, as will Hadley Cell formation and stability.

• This would be my approach as well for trying to semi-scientifically justify floating islands. My only word of caution would be that such a magnetic field would certainly generate a significant affect on an magnetic substance. (like the airship in avatar would have far more erratic operation than just their censors) – anon Oct 2 '17 at 14:51
• @anon You may have a point there, I'm not too well up on the practical math around superconductor effects. What I have read leads me to believe that if you found a really perfect superconductor (our best efforts to date being 60-80% as good as the theoretical limit) then it wouldn't take much field strength at all to maintain a substantial Maglev effect. At any given field strength the size of object that will stay airborne is a function of density. – Ash Oct 2 '17 at 15:18

As Slarty said in their answer, you're not going to get a ring inside of an atmosphere.

However!

You asked for alternatives if your original idea proved infeasible. Without knowing the exact nature of your story, I can't be overly specific, but here are a couple of thoughts:

• The giant chunks of rock float at a considerable height in the atmosphere, but not higher than your sapients can tolerate. The rocks can be held aloft by anti-gravity technology of some kind, or perhaps levitation magic. This could have been done by a past civilization or a current one.

• The rocks could be the remains of asteroids hauled into orbit for exploitation. After creating a living surface, a dome would have been constructed over the top. Artificial gravity keeps the people on the surface, but the remains of the mines below may or may not be affected.

Personally, option 1 sounds really suicidal, unless the original inhabitants were escaping some surface-level catastrophe.

• What do you mean it sounds suicidal? Do you mean it requires suspension of disbelief? – Virdex_ Oct 1 '17 at 5:49
• @Virdex_ I mean that levitating giant rocks as a ground-based civilization means that they can also be dropped on you, either by other civizations with their rocks or your own rocks having some sort of mishap. – Gio Oct 2 '17 at 0:08
• You know what, @Gio, I'm gonna use magic. – Virdex_ Oct 2 '17 at 4:48

Slarty is right. One Alternative would be to make your islands buoyant. I see to ways to go about this, both would be somewhat hand wavy.

• Look into how pumice is formed. It may be possible under certain conditions to produce a pumice that is lighter then air, and would float. This would likely require a very dense atmosphere and hydrogen to be the exsolving gas which would require very high initial pressure.
• hydrogen gas can be produced through biological processes, plant life competing for access to sunlight might evolve hydrogen bladders in order to reach above other plants. Your islands could be composed of a floating tangle of such plants.
• I like that hydrogen bladder / competing for sunlight concept. Kelp do that with air bladders. Plants could put up their floaters by day and reel them in at night. Broken off floaters could produce new plants where they land. But it does not happen - I conclude hydrogen must be very hard to confine in a biological system. – Willk Oct 1 '17 at 0:34
• Pumice is 211kg/M^3 (lowest figure I could find). By comparison, the densest gas, Radon, is 10kg/M^3 at earth sea level. So it would take an air pressure of at least 21 times that of earth's to make rocks float. Interestingly, the Galileo Jupiter probe died at around 24 bar, so that's doable. But then you need a way for pumice to form on a gas giant... – Dewi Morgan Oct 2 '17 at 19:12
• @Dewi Morgan Pumice forms when molten rock undergoes a massive pressure drop while rapidly cooling. Gasses that were in solution at the high pressure suddenly come out of solution but do not have a chance to bubble off since the rock is also rapidly solidifying. With higher initial pressure, more gas could be dissolved in the molten rock then is possible on earth, leading to larger bubbles and lower overall density. – Lex Oct 4 '17 at 18:06
• @Lex: My "that's doable" may have been a little tongue in cheek: I was aiming more to show that floating solids are very improbable, verging on impossible: you can get the conditions to make it float only in a gas giant, where neither of the two elements of earth-style pumice (magma and 1-bar gasses) exist. So it'd have to come from a moon or something (vacuum pumice would be a shade lighter, too!) – Dewi Morgan Oct 6 '17 at 14:59
• Not a floating solid, a floating closed cell foam. Which is perfectly real. Obviously it needs to be more solid then that to form an island, which is why I came at it from the pumice angle. You would be much better off with bubbles filled with hydrogen at atmospheric pressure than vacuum or low pressure gas because the structural stresses on the material would be much lower and gas leaking over a pressure gradient would not be a problem. – Lex Oct 17 '17 at 18:14

Normal physics wouldn't do the job. A planetary ring inside an atmosphere would be moving at approximately 8 km/s. This is the orbital velocity for an Earth mass planet. Atmospheric drag would soon decelerate the pieces of the planetary ring. It wouldn't last very long at all.

As a thought experiment: make a planetary ring magically appear in an atmosphere. At a guesstimate it wouldn't last one whole orbit.

Therefore, if a planetary ring does exist in atmosphere some form of magic or 'magic' must be at work. This could be a barrier to prevent atmospheric drag slowing down the ringlets and the flying islands or a constant force to maintain a constant orbital velocity.

For example, one can imagine a teleportation surface in front of the ringlets instantaneously transporting the atmosphere it impacts to somewhere behind them where it can rematerialize harmlessly. This would eliminate atmospheric drag. But some kind of force-field might necessary to maintain a breathable atmosphere for the flying islands.

In summary, if there is some form of protection to negate atmospheric drag, a form of 'magic' propulsion, if needed, to maintain velocity and a containment field to make the flying islands habitable. Then a planetary ring could orbit inside an atmosphere.

A simpler solution might to have a traditional, scientifically sound planetary ring orbiting outside the atmosphere and have the flying islands periodically rise up from the planet below to join the ring. The flying islands would spend a period of time as part of the ring and later on they would descend back down to the planet.

As has the advantage that the flying islands can be inhabited by lifeforms that evolved on the planet. There can be a form of antigravity that allows them to either move through the atmosphere or rise to join the planetary ring for a time. Again there needs to be mechanism to contain an atmosphere around the flying islands. However, this could be simply part of the antigravity that levitates the islands in the first place.

This idea is that the islands are habitable and can remain habitable while in the planetary ring. Also, they return to the planet to restock air, water, and whatever else they need to survive. They won't be permanently part of the planetary ring. This is only a suggestion.

This alternative solution does turn the original question on its head. But it does that the simplest solution would be to consider to make the flying islands that are part of a planetary ring habitable.

## This would actually be easier if the planetary ring was outside the atmosphere (or inside the mesosphere):

1. You could have your civilization advanced enough to live on the islands while in orbit, or if your main species isn't human (or just not purebred human) they could live in outer space [e.g. waterbears were proven able to live in space].
2. However, if you want your civilization to be less advance, have a previous civilization make the islands hospital. and to explain their absence have them fail to evacuate from what they were trying to escape from ( like a halo ring situation).

If your ring were a hollow vacuum tube, then you could have a single giant maglev train running around the interior at somewhat greater than orbital speed. That would hold up the ring against gravity, and wouldn't intrinsically require a lot of energy to keep running. Since the train would be traveling at thousands of meters per second, you'd need a very high vacuum, and the ring structure would need to be very strong for its weight, though not as much as a space elevator.

Such a ring would have lots of external forces acting on it which, left unchecked, would soon lead to catastrophic oscillations, especially in the vertical direction. This would be controlled by automatically varying the speed of the train along its track to cancel out the oscillation. That would take some energy, but it'd still be pretty efficient because when you slow down a maglev train, its energy is converted into electrical power which you then use to speed up the train somewhere else.