# Could a planetary disk create a shared atmosphere?

So essentially, we have a protoplanetary disk filled with asteroids ranging from boulders to 30km long. Could they sustain a breathable atmosphere for humans, or would there have to be oxygen tanks and such needed. Additionally, could a ‘magic’ atmosphere generator help make the atmosphere breathable?

• If there is a shared atmosphere then it will slow down any relative movement of the rocks, and they will very quickly collapse into one big rock. (This is why a satellite cannot realistically orbit the Earth at less than about 200 km altitude.) May 14, 2022 at 22:05
• Q: What's the size ? A "protoplanetary disk" is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star. You're talking about a habitable DISK of that size or a RING ? May 14, 2022 at 23:44
• Clearly "a ‘magic’ atmosphere generator" could do anything. Meanwhile except huge amounts of luck, what would stop this protoplanetary disk from need the same squillions of years of development that Earth needed to produce either a breathable atmosphere or life, let alone both? May 15, 2022 at 22:50

A protoplanetary disk is, in astronomical terms, a violent and deadly place. It is not somewhere you want to live. Gravity has spun all of this debris from the birth of the star into a disk. Everything that wasn't consumed by the star is now spinning around it.

# Land

Land to live is going to be an issue. You say there are asteroids of varying sizes. These asteroids are in an inherently chaotic system. Unlike 'normal' asteroids we have now, they're actively colliding because that's how we get planets (rocky ones at any rate).

This happens over millions of years, it is true, so in terms of a human lifespan it might be feasible and relatively safe, but the ultimate fate of that region of space is that there will be one thing there: a planet. Planets 'clear' their surrounding space of debris, since anything there gets sucked in and added to the planet. That's one of the reasons there are very few asteroids among the inner planets.

This can lead to incredibly violence. Earth was impacted by a planetoid the size of Mars billions of years ago, the debris from which formed the moon. The impact would have rendered both bodies practically liquid. This would be a somewhat regular occurrence.

# Temperature

Another word for a protoplanetary disk is accretion disk. This is what forms when material spins around a source, whether it be a star or a black hole. The issue is that spinning things make friction, which makes heat. Black hole accretion disks can be incredibly bright due to all the radiation they spit out (remember, heat is radiation too!).

This paper puts the average temperature of the Sun's accretion disk at roughly 230 degrees celsius for the first 5 million years. These things are hot compared to normal space stuff. This paper has some more information.

# Atmosphere

Now, onto atmosphere which is what you wanted. It is true that all elements came from the nebula, so all the elements needed for a breathable atmosphere are there too. Unfortunately, if we look at Earth, it's unlikely any of it would be breathable.

Most of the gasses in the accretion disk is just hydrogen. That's no good. The rest is probably like a gas giant; you'd find water, ammonia, methane.

Earth got it's atmosphere in a few ways. Volcanoes added nitrogen and carbon dioxide, as did asteroids slamming into earth. It did not have any oxygen. Any free oxygen was immediately taken into minerals, such as iron, to make iron oxide.

Oxygen didn't become a major part of Earth's atmosphere until the Great Oxidation Event, which is still the largest mass extinction event in the planet's history. This was done by bacteria who, like plants today, generated oxygen as part of their food cycle. It took a long time to raise oxygen levels and it required a lot of prep beforehand, like making sure that carbon dioxide was present beforehand. If that wasn't the case, life might have stayed anaerobic.

TLDR: if your humans want to breathe, they need to bring air themselves, if they can survive at all.

• Ok, so I’m going to have to add some heavy magic for it to work then. May 15, 2022 at 3:58
• With technology or magic it might be feasible, that's why they call it fiction :) Living inside an asteroid for example might negate a lot of the issues, as long as they have an air supply and a way to dispose of waste heat. As long as they choose a good asteroid that won't collide with anything anytime soon, that is. May 16, 2022 at 0:23

Normally, no. BUT. In The Integral Trees by Larry Niven, he proposes such a world, a gas torus around a neutron star and main sequence binary pair.

• Essentially, a ring of air that happens to orbit around the same area as the protoplanetary disk? May 14, 2022 at 21:59
• @RYANLANDELS - not exactly. Niven's proposed torus depends strongly on the nature of the binary star - particularly the neutron star. The additional gravity is necessary to let the gas remaining inside a habitable radius be at all plausible. That said, it's also complete nonsense from a reality-check perspective, but it is precedent in science fiction. May 14, 2022 at 22:27
• An excellent sum-up of the problems: worldbuilding.stackexchange.com/a/96348/36850 May 14, 2022 at 23:59