# Can a planet's atmosphere be in orbit around the planet? [duplicate]

Imagine a planet somewhat like Saturn except that its rings are its atmosphere instead of rocks and ice.

Is this theoretically possible?

(a) If so, what conditions would need to be true and what shape would the atmosphere be? (e.g. a torus)

(b) If not, why not?

Thanks

Notes

I am not asking how such a thing could come about - just if it could work once in place.

At this stage there is no requirement for life to be sustainable in this system or for what the planet itself should be made of.

• Hard science on a theory is going to be challenging. Are you asking for a situation where the wind speed is so fast, that it actually lifts off the ground and creates a vacuum between itself and the ground? Jan 30 '19 at 18:06
• Gases being gases they don't like to stay put unless constrained by a containing force. What force do you suppose will keep the gaseous rings from dissipating into the vacuum of space like any self-respecting gas would love to do? Jan 30 '19 at 18:10
• This sounds like it more belongs on a physics site. But the short answer is no, not without magic. Jan 30 '19 at 18:14
• "If not, why not?" Because a gas will expand to fill available space. A planet's atmosphere is kept in place by the force of gravity which pulls the gas towards the center of the planet; some small amount of gas still escapes in space, but if the gravitational pull is strong enough this amount reduces to a trickle. If a blob of gas somehow finds itself in orbit around a planet it will expand into nothingness because there is no force to oppose the natural tendency of the gas to expand. Sure, the center of gravity of the expanding gas will remain in orbit; but the gas will expand to infinity. Jan 30 '19 at 18:21
• Also relevant XKCD what-if.xkcd.com/6 Jan 30 '19 at 18:21

I wouldn't really call that an atmosphere, so let's rephrase it: Can you have a ring of gas around a planet?

Yes. But not a very dense one. Jupiter, for example, has a plasma torus around it corresponding to the obit of Io, consisting of material from Io's atmosphere. It's not stable long-term, and has to be continually replenished by the loss of material from Io's surface... but that's true of Saturn's solid rings as well. They are a temporary structure which will last for a few hundred million years at most. And in fact, Saturn kinda already has a gas ring of its own, consisting of material ejected from Enceladus.

In all those cases, however, the gas tori are so diffuse that they would still be considered hard lab-grade vacuum here on Earth. They are so diffuse that the very existence of other examples of them in our solar system is debatable--there's some evidence that they exist, but they aren't the kind of dense structures that you can obviously see through a telescope!

So, can you have a dense ring of gas with significant internal pressure, which might reasonably be termed an "atmosphere", orbiting a planet? Well, kind of, although it is debatable whether the central object should really be called a "planet" in such cases, as they will necessarily be extremely massive. And the "gas ring" is likely to be more of a "liquid and gas ring", as it will also need to be massive enough to hold high-pressure gasses in place with its own self-gravity; i.e., you'll be looking at something like a toroidal gas-giant planet.

See, for example, General Relativistics Structure of Star-Toroid Systems. The math does not depend on the central object actually being a luminous star, but it does depend on the central object being massive--and the gas ring also being extremely massive. It cannot be stabilized only by the gravity of the central mass. Such structures have been observed in nature (see, for example, Hoag's Object), but only on galactic scales--not even the supposedly-stable stellar ones--so the possibility of something like that naturally forming on the scale of a planet seems Extremely Low.

• Once more science destroys all the fun, so here are my annotations: Io's plasma torus is connecting to Jupiter's atmosphere and therefore probably not what OP had in mind. Enceladus doesn't have a gas ring, that's a dust ring, huge difference, dust doesn't feel pressure but gas does, which is why this whole thing is impossible. Saturns rings being only 100 myears old: Yeah, there was a paper about that, and ofc blown up media coverage, but that result and the timescale is more than questionable. Jan 31 '19 at 0:47

If there was gas orbiting the planet and that gas reached down to the solid surface, if any, of the planet, then the solid surface of the planet would be scoured by winds travelling at orbital velocities, tens of thousands of miles per hour. Any life forms on the surface would be destroyed by the winds.

The winds would destroy every surface feature and would be slowed down to lower speeds by the collisions. So the lower air molecules would drop out of orbit and crash into the planet's surface at speeds slightly less than orbital speeds. Kaboom! So the only air molecules left in the orbital atmosphere would be orbiting the planet somewhat higher than the surface of the planet.

So it would be impossible for the orbital atmosphere to reach down to the surface. If the planet is to be habitable for humans, or have multi celled lifeforms like plants and animals, or intelligent natives, it has to have an atmosphere.

Thus the planet would need to have two different atmospheres. One reaching up from the surface of the planet and becoming thinner and thinner with altitude, and moving at the same speed as the planet rotates, and another atmosphere far above which travels at orbital speeds, probably in a torus above the planet's equator.

The two atmospheres can't touch, because the orbital speed of the orbital atmosphere should be many times as fast as the rotational speed of the planet. So if an orbiting air molecule hits a slower moving molecule from the lower atmosphere one or both molecules are likely to be knocked out of the zone where the two atmospheres meet, which should thus become cleared of air molecules.

as I remember, Larry Niven, who writes somewhat "harder" than usual science fiction, did write a novel, The Integral Trees, set in a gas torus of breathable air around a star. Thus it is possible that he carefully calculated that there could be a gas torus of breathable air in space that wouldn't swiftly dissipate. You should look for discussions of the scientific plausible of The Integral Trees to see how plausible a torus of dense gas in space is.

I suppose that it is theoretically possible for the lower atmosphere and the orbital atmosphere to meet under extreme conditions. Planets are mostly held together by their gravity. If a planet rotates too fast it will become more and more oblate, getting wider and flatter until it resembles a pancake, and if it rotates too fast it will fly apart when gravity is no longer strong enough to hold it together.

So possibly a planet that rotates fast enough and becomes pancake shaped as a result might rotate at orbital speed at its equator. Thus a hypothetical orbital atmosphere might possibly extend down to the planet's equator and interact with the lower atmosphere around the planet without the air molecules in the two atmospheres having different enough speeds that differ.the zone of interaction between the two atmospheres would be cleared out.

• "And another atmosphere far above which travels at orbital speeds, probably in a torus above the planet's equator": to repeat my question to the original poster: what force is keeping the gas in the torus, when, as any gas does, it would very much like to expand into the void of space? Consider that inside the torus there must be some pressure greater than zero; outside the torus there is vacuum at zero pressure. Why isn't the gas expanding into the vacuum? Jan 30 '19 at 18:33
• Please note that this question is tagged hard-science. Answers that lack equations, citations, and empirical evidence are likely to be deleted. Jan 30 '19 at 18:35
• Thanks for your answer. I didn't specify that the planet should be inhabited. Jan 30 '19 at 18:57
• There are too many instances of "possibly" to meet the hard-science mandate. (Chasly, that tag is ruthless. You should only be using it if you actually need the formal proofs it requires. It is NOT (not, not, not, not, not) intended to be a super science-based tag. Answers that don't meet the hard-science tag can and do get deleted, no matter how useful they are.) Jan 30 '19 at 19:52