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The idea in its simplest form is that a stellar system of planets is within a transparent gas cloud that includes oxygen. The gas cloud is so incredibly dense (around sea level earth pressure) that humans can breathe and survive in open space.

I would like to use real world physics as much as possible, however some serious wand waving or over-simplifying is necessary and i would love some help.

Obvious issues that come to mind:

  1. Friction. Planets orbit incredibly fast and will burn up. Maybe solved if the gas cloud itself spins at the same rate as the orbiting planets (adjusting for distance from star as well). Space craft could be explained by some king of magnetic shield that magically displaces the gas without huge amounts of energy being expelled.
  2. Oxygen = boom. Maybe the star's gravity will prevent a chain reaction of ignition. Or maybe it won't be an issue, after all a candle draws in oxygen, not the other way around.
  3. Gravity sucking in the gas. I have no solution for this. Although I guess there's only so much gas that can fill the gravity well of a planet.
  4. Air pressure. The combined mass of the gas cloud could explain air pressure in a vague way, however this pressure might increase a lot around the planets? After all, deep sea has more pressure, therefore it stands to reason that an astronomical expanse of gas would result in an enormous amount of pressure. Although that pressure centres on the star, it might still be crushing on each planet.

My goal is to have reasonable explanations that are pseudo-scientific and familiar to the average reader.

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    $\begingroup$ Possibly consider the setup in en.wikipedia.org/wiki/The_Integral_Trees , by Larry Niven? A gas torus rotating around a neutron star. I'm sure there are problems with it, but dang, LN knows how to make something believable. $\endgroup$
    – Qami
    Feb 7, 2023 at 3:52
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    $\begingroup$ As for (1) - the air molecules are, on average, in orbit around the central star, and this would give them the same velocity as the planets. So the planets are not moving through the air - they move along with it. This also accounts correctly for the variation in velocity for planets at different distances from the star. $\endgroup$ Feb 7, 2023 at 15:50
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    $\begingroup$ @DuncanDrake There is no need to postulate a "disk" of gases with a constant angular velocity, because a gas is not a rigid body. If the gas is in orbit, then each region could have its own orbital velocity, as the comment you're responding to suggests. (Also, the angular velocity of a planet around the sun depends on its orbital radius, not on its mass.) $\endgroup$
    – David
    Feb 7, 2023 at 17:05
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    $\begingroup$ @DuncanDrake The angular velocity of an orbit does not depend on mass, only orbital radius. $\endgroup$ Feb 7, 2023 at 21:32
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    $\begingroup$ A problem you don't discuss: noise. If there were a medium to conduct sound, the sun would be a uniform 110 decibels. That's as loud as a rock concert, and can cause permanent hearing damage in 30 minutes. Except that it'd be going on forever. $\endgroup$
    – jdunlop
    Feb 7, 2023 at 22:55

6 Answers 6

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This works best as an accretion disk, although there's still going to be a bunch of hand-waving if you want the science to be hard:

  • Viscosity of the gas is going to be significant on interplanetary scales and will eventually collapse the gas into planets or lead to it either being ejected from the system and/or collapsing onto the star. That being said, "eventually" can be a lot longer than the time your story takes To maximize the believable duration, you want the planets and gas moving together in circular orbits moving at the orbital velocity appropriate to their distances from the central star.
  • Any orbits that cross each other will generate viscous effects which disrupt the disk, again causing collapse into the star / planets. To avoid this, put the atmosphere in a single thin disk. This is why "accretion disks" are common but "accretion spheres" are not a thing. Note that "thin disk" is only thin on interplanetary scales. Having it be 10,000 miles thick is fine as long as the star is at least a few light-minutes away.
  • The heat from the star will affect the disk, and this will tend to blow the gas away from the star itself. The inner radius of the disk will be about as hot as the star and thus the gas molecules are less likely to stay in their nice Keplerian orbits. This mitigates issues like the oxygen catching fire or similar - but the inner edge will probably still be ionized, tenuous, and unsuitable for life.
  • The further reaches of the disk are going to be colder, with the "air" condensing and then freezing out into diffuse "snow". Note that nearby portions of the disk will tend to have similar temperatures. Heat loss will be mostly off the surfaces of the disk, and the heat source will be the sun (plus viscosity...). This will drive some magnificent turbulence, storms and out-of-plane currents.
  • Filling a solar system with gas at atmospheric densities is likely to make the gas opaque or diffusely-lit. The star will be a bright patch in the sky, but you're probably going to lose visibility for individual planets and stars. Interplanetary navigation is going to need a lot of dead-reckoning or trips to the unbreathably-tenuous surface of the disk.
  • Since the gas is moving at stellar orbital velocities and we've cleared out the central regions, you don't have to worry too much about the sun's gravity sucking up gas. Planets still will, but planetary formation is on the scale of a million years or so, so you only need to replenish the disk on those scales (or not... how long does this state last?)
  • That's a lot of oxygen, and oxygen isn't the most abundant gas in the universe. (Hydrogen is, by a lot.) Whether or not you decide to explain it in the story, you might want to come up with some reason (supertech aliens? magic?) to explain how this all came about and whether or not the creators care about maintaining the state. Personally, I find the origin of such a stellar system to be much less plausible than one continuing to exist and be quasi-stable on the timescale of a century or so for "modern technology" to arise.
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Get rid of gravity.

I think the biggest issue is gravity. Like you mentioned in your post, the dense gas will tend to collapse under gravity. There's enough mass in a solar system full of STP air to create thousands of suns. It would happen pretty quickly.

Make gravity some other kind of force of interaction, perhaps something like magnetism, which air does not take part in. Certain elements/molecules are just not pseudo-magnetically "charged" and won't feel an attractive force. That way, planets may still hold themselves together and follow Keplerian orbits, while and people and objects fall with a force inversely proportional to the square of distance.

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You can try making it not a stationary state, but a long lasting transient.

Somewhere there is a source of gas and somewhere else there is a well sucking in all that gas at the same rate which it is emitted. In between the two, due to the flow, there is something akin to an atmosphere, or better said, a rather constant amount of gas.

The stream of gas can also happen in the same direction of the planets motion, so that the induced drag is somewhat reduced.

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Frame challenge: How do I breathe in space? That's the neat part, you don't

There are fewer issues entailed by redesigning the biology of the creatures in your world than by redesigning all astrophysics.

Say that they have a metabolism that works without oxygen or other gases (see this question), and they carry within their bodies a few chemicals that fill in the metabolic gaps.

That allows you to not worry about the problems you raised in your question like stars exploding.

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The early universe was a lot denser than it is now and at one point would have been warm, wet and opaque during it's transition to hot, light and filled, to cold, empty and dark (I forget the time scales), you could set it then, although it would be lacking the heavier elements that make solar systems more interesting. It also didn't last very long. Have a read up on it and see if you are inspired by the setting at all.

So the other option is have the universe not expanding, where gravity and dark energy cancel out. We don't know enough about the expansion of the universe, to know whether or not a universe that didn't expand so rapidly would work with the current laws of physics, so it is not implausible to have the whole universe and not just solar system full of gases. That's your best shot at avoiding to have to tweak all the other laws.

The issue is not the gases clumping but the fact that stars are very good at blowing away the gas clouds they are born in. you'd have to compare solar pressure with atmospheric pressure to see what would roughly happen.

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If you were willing to borrow a bit fantasy you could replace air with something like the classical interpretation of aether as posited by Greek philosophy

In particular Plato and Aristotle both held that this element, Aether was what the gods breathed instead of air. As well as being the material that suspended celestial in objects in place.

You could make something analogous, in particular I believe it might work better as it serves as basically one handwave instead of the hoops you'd need to jump to justify having mundane gas.

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