# Could a planet sized bubble of breatheable atmostphere exist?

I'm reading a book (Sun of Suns by Karl Schroeder) that the main location is a planet called Virga, which contains air, water, and floating chunks of rock, and has no or a very small amount of gravity. There is a main 'sun' at the center of the planet, which provides the heat for weather.

Could a 'planet' of this type exist?

• No, not with real physics forces. Gravity would pile up everything on the "sun". – anna v Apr 7 '13 at 18:24
• Science Fiction author Larry Niven has a story, 'Smoke Ring' I think it was, that had a ring of atmosphere orbiting a neutron star or some such thing. But that would be a slightly different question: 'Is Larry Niven's 'Smoke Ring' possible?' I guess that question would be. – Bobbi Bennett Apr 7 '13 at 19:40
• @BobbiBennett Like this question: physics.stackexchange.com/questions/41254 – user10851 Apr 7 '13 at 19:50
• Found this: larryniven.net/physics.shtml. No ball of air, though. – Bobbi Bennett Apr 7 '13 at 21:27
• The OP is forgetting to mention that Schroeders Virga is a huge bubble; the bubble keeping the atmosphere trapped. – Len Feb 1 '18 at 21:38

If the cloud is in thermal equilibrium, then the typical molecular speeds go like $mv^2\sim kT$, and escape velocity is given roughly by $v^2\sim\Phi$, where $\Phi$ is the gravitational potential. The result is that the maximum temperature of such a cloud is $T\sim m\Phi/k$. If you put in a typical numbers, you find that even for a body with gravity as strong as the moon's, it's not possible to have air and water (high $T$ and low $m$). But note that the result depends on the gravitational potential, not the gravitational field, so in theory this could work if the body is very large in linear dimensions. Also, it would be possible, for example, to give the moon a permament atmosphere of heavy molecules such as long-chain fluorocarbons, making it a shirtsleeve environment where all you'd need was an oxygen tank.