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It is impossible to have a continuous mostly liquid water ring around a star?

Probably such a configuration would be unstable under normal planetary forming condition. I am looking for a good physical reason why it could never be possible.
It is related to these questions :

• Can I have a very dense asteroid belt ring around a star?


• Is it possible for a planet to have a liquid ring?


• https://physics.stackexchange.com/questions/41254/why-is-larry-nivens-ringworld-unstable

solar wind, internal friction forces, water triple
point, axial-transverse displacements, roche limit.

• assemble various materials


• phase states


• internal movements


• stabilizing cycles

Every orbiting ring-like structure is unstable due to gravity. Yes, even Saturn's rings and the Asteroid Belt in our Solar System are unstable: they constantly lose and rearrange "particles". The Asteroid Belt is more an intersection of asteroid orbits than some ring structure. There is always a tendency to form some clusters even inside Roche limit. That means that this liquid ring, at best, would become a ring of water drops, blobs and one or two liquid planetoids (like in Saturn's system – they are needed for ring quasi-stability).

This can be easily understood if you consider the inner and outer parts of this ring, as well as the ring at periapsis and apoapsis. They would always have considerable velocity difference, resulting in massive whirls forming. Those whirls would inevitably separate and form planetoids, which would then evolve to something like Saturn's rings & moons, or to a single planet (depending on starting mass and orbit)

Сontinuous liquid ring is impossible.

Friction of the fluid against itself as a wave-propagating medium results in local concentrations of density, and ultimately in the continuous material being broken up. The only stable condition for fluid particles is that they must isolate themselves from the wave field through cohesion into local, disconnected bodies or else lose energy to internal motion and fall from their orbit.

It was James Clerk Maxwell who first discovered that rings around a planetary body (including a star) could not possibly consist of a contiguous fluid:

"Supposing the ring to be fluid and continuous, we found that it will
be necessarily broken up into small portions. We conclude, therefore,
that the rings must consist of disconnected particles; these may be
either solid or liquid, but they must be independent. The entire
system of rings must therefore consist either of a series of many
concentric rings, each moving with its own velocity, and having its
own systems of waves, or else of a confused multitude of revolving
particles, not arranged in rings, and continually coming into
collision with each other."

https://archive.org/details/onstabilityofmot00maxw/page/66

Incidentally, it was in this work that Maxwell derived the Criterion for the Stability of a Dynamical System, which is the key to all modern control theory and practice, including robots, automobiles, airplanes, biochemical control, and so on.

Mankind learned how to stabilize mechanical engines and to create robots using sensor feedback loops for control as a direct consequence of Maxwell's investigation into Saturn's rings.

Liquid is hard to get in space: you can get either solid or gas. You therefore might get ice particles of various size, or water vapor.

Ice would slowly sublimate to gas, and gas would be blown away by the stellar wind. The rate at which this happens would depend on the distance from the star. On the far side of the goldilocks zone ices can live as long as the star.

But they won't form a ring all around the orbit, unless they are extremely sparse. If they are dense enough, gravity will soon coalesce them into a single body.

Here are a few points

• Friction: If your water needs to be high up enough so that it isn't affected by the friction of particles in the atmosphere which will slow it down and eventually cause it to rain down.




• Solar Winds: Your water needs to be in the atmosphere to offer it protection from solar winds. If it is too high, the solar winds will hit your water and eventually strip it from the planet or cause it to enter the atmosphere. This conflicts with the Friction part.




• Space: Space is essentially a vacuum and when water is placed in a vacuum it will first boil and then freeze, creating a powder of frozen ice crystals. Basically your water needs to be close enough to the sun, so that it can remain in a liquid state, however at this height, it will be close enough to the sun to be impacted by solar winds. This will cause a conflict with the Solar Winds Part.




• Asteroids: There are a lot of asteroids that hit the earths atmosphere and burn up. Unfortunately, due to the amount of light pollution, we cannot see them unless we are in very remote places. If your layer of water is outside of the atmosphere, the asteroids will hit your water rings and cause it to rain back down on earth. After several million years, there won't be much left (especially since water will try to stick to itself due to surface tension).




• Surface Tension: Water doesn't like to act alone. It likes to stick to itself and link up. Anything that impacts a pure water ring will have rippling effects along the entire ring as the shock is dispersed through the water. Anything outside of a perfect laminar flowing ring of water will cause small discrepancies and a buildup in volume at one location which will eventually cause it to be pulled down. For examples of water tension in space have a look at the videos on the international space station. It will literally stick to your skin.

Yes it is impossible:

Liquid water cannot exist at pressures below 0.006Bar so in order to have liquid water in free orbit around a star you would first need a gas torus. Now the highest density we've directly observed in such a torus is around the planet Jupiter it has an amazing 2000 particles per cubic centimetre (that's not in any way measurable as a pressure). Getting a torus dense enough around a normal star is probably not realistic, there is a possible solution for forming such a torus but I'm not completely sure that it's accuracy.

Conditions which allow you to have liquid water are almost impossible but even if you could have them a water ring wouldn't be stable. Due to differences in the orbital velocity and momentum of various parts of the ring very few water molecules are actually moving in identical orbital tracks. Parts of the ring are moving in close to the same way as each other but not the same way as their neighbours, this will cause the ring to disintegrate into smaller and smaller droplets as smaller and smaller differences in velocity add up. You could have a thick ring of air that is above 100% relative humidity such that water vapour was constantly coalescing into clouds, droplets and even larger spheroidal pools, lakes and oceans that are then pulled apart by gravitational and orbital forces but not a single permanently contiguous ring of liquid.

At this time the other answers either miss or gloss over the core of the reason why such a structure could not be possible [by natural means]:

Fission and materials filtering.

With the right star and enough H20 in the correct initial orbits, there isn't a lot to actually stop such a structure from existing for a decent chunk of time on a geological timescale.

Sure, the structure is going to fail eventually, and fairly quickly on astronomical timescales, but in the grand scheme of things our own solar system is going to fail relatively fast compared to the overarching universe... [And we seem to be doing somewhat okay for the time being...]

The point however is that despite how implausible it is to have that much material in that kind of orbit, the far more implausible point is getting that material in the first place...

So the core of our real issue here is all the 'extra' elements that get created along the way when you start off with Hydrogen and run things along well enough to come up with notable amounts of oxygen: There is 'other stuff' in there, which will heavily contaminate our 'water source' even if we otherwise have perfect conditions for creating the 'temporary' water ring.

You would need to have had enough 'free' hydrogen and oxygen in the system for all the water required, while not having a volume of stuff heavier than oxygen to group up and form cores that interfere with your water ring, and having somehow filtered the bulk of everything between Hydrogen and Oxygen out of that region... All without having displayed the target water from its required initial formation motions.

• I can napkin math a star with a water ring that exists for a time, but I'm not seeing any kind of a starting point to napkin math anything close to a filtering mechanic short of "god/aliens did it".

Such a thing cannot exist, because the ring is not in the lowest energy state, and so it is in an unstable state. Note that the star has nothing whatsoever to do with it. A ring of material is unstable with or without the star, for the exact same reason. So, without loss of generality, simply imagine the situation where there is no star. This ring of material will naturally collapse in upon itself due to gravity. The result is that the ring slowly collapses in to form a sphere, which is the lowest energy state.

In the presence of a star, the gravity field that the ring finds itself in isn't flat as in the situation where there is no star, but the water still all wants to collapse down. Note that it won't collapse to a single sphere, though, because of the influence of the star. It should all collapse down to 3 spheres, with two smaller ones at the 5th and 4th Lagrange points