Edit: Before I give up on this I wonder how could an advanced civilization create conditions on a small planetoid (Ceres sized?) to keep an Earth level breathable atmosphere? It seems like its impossible in nature, but what about artificially? Some handwavium is allowed of course.

Edit: I previously wrote "within" a small planetoid. Apologies. I meant ON a small planetoid.

Original question: How can small planetoids (Charon, Ceres, Dysnomia sized[?]) have, and keep, an atmosphere with breathable gases for any kind of extended period of time?

I realize that their gravity is low and that might be a reason why it couldn't, but are there any conditions under which they could? For example(s):

  • Could it be releasing gases of its own from within its core?
  • Could the core have a tiny bit of Neutronium in it (don't laugh)?
  • Could a comet have hit it and given it the gases?

I am looking for an answer that would exist in nature, but am open to artificial means as well. Thanks.


8 Answers 8


Put a bag around it.

This isn't as far fetched as it might sound. I've seen numerous references to it in science fiction and other discussions. Sorry that I can't find them at the moment. Off of memory this was used in the Quantum Vibe web comic at some point in it's history.

The main point is to cover the planet in a very thin plastic sphere and then inflate that sphere with air. If you build some support structure (a ring around the planetoid, for example, you can build airlocks and docking ports. That way you can get to and from the planetoid without ripping the bag. Minor tears from micrometeorites should be small enough to not cause a significant loss of air (small holes in a really big bag). It is best if the polymer is self healing but could be patched or replaced in sections. One story had the self healing aspect so high that ships could fly through it and the tears would heal before there was significant loss.

One discussion talked about capping one of the Moon's craters with a mylar cover but that was a long time ago and might not have made it to the web.

  • 3
    $\begingroup$ Don't manufacture it, grow it on the surface of the planetoid, in one piece, with bio/nano-technology. $\endgroup$
    – Josh King
    Jan 26, 2018 at 21:04
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    $\begingroup$ @Len, I'd say manufacture on the ground. Plastic sheet is pretty easy to make if you have the tools. I'd start from the poles and work toward the equator. You would have to build it with lots of pleats to make sure it will have enough surface area. Then you inflate it. It'll be a huge engineering task but I don't think that it would take tech that we don't have. Of course, if you posit better tech, you can create a stronger bad faster and with less cost than we could now. $\endgroup$
    – ShadoCat
    Jan 27, 2018 at 0:33
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    $\begingroup$ the big issue with this is micrometeorites will have torn dozens of holes in it by the end of the first day. $\endgroup$
    – John
    Jan 27, 2018 at 5:51
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    $\begingroup$ @Len: Don't do it all at once. Start with a small domed colony. As it grows, and new colonies are started, you get bigger & bigger domes, until the whole body is one big dome. $\endgroup$
    – jamesqf
    Jan 27, 2018 at 19:27
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    $\begingroup$ 2900-7300 kilograms of material in the 10 gram to 1 kilogram size range strikes the earth each year and if anything the earth has less impacts than other planets because we have such a large moon. Even lighter material under a gram could puncture your bag, at orbital speeds a fleck of paint is like a bullet. $\endgroup$
    – John
    Jan 29, 2018 at 15:01

If your gravity is low, you need to be cold

enter image description here

I think this is the 25th time this graphic has been posted to Worldbuilding.

As you can see, the ability of a planet to hold an atmosphere depends on both the escape velocity and temperature. Lighter gasses (hydrogen and helium) required lower temperatures to escape, heavier gasses (like krypton, sulfur dioxide, or xenon) need higher temperatures.

You can see Eris, Pluto, and Triton plotted down there. Pluto does have a tenuous atmosphere of sorts, made of nitrogen, methane, and carbon monoxide. But these gasses are constantly refreshed from the surface of the planet. With surface temperatures in the 40-60 K range, Pluto's surface is cold enough that all those listed gasses are solids. So occasionally some sunlight vaporizes those gasses, and they form an atmosphere. Those gasses then escape into space, becuase Pluto's gravity isn't strong enough to hold on to them, but because of the cold, some freeze back onto the surface and being the cycle over again. Since pluto has a large reservoir of these materials, Pluto isn't going to run out of atmosphere any time soon. But again, keep in mind that Pluto's atmosphere has a pressure of about 0.00001 bar; not much atmosphere at all.


Ceres and the like are far too small to hold an atmosphere, at any practical temperature. Even Pluto is too small. The best you can hope for is geological phenomena like cryo-volcanoes to blow a lot of materials out of the center of the planet. These will be lost over time, but could form a modest amount of atmosphere before they were driven off. But then again, Enceladus is also small and has these same conditions, but it isn't holding any atmosphere.


Could it be releasing gases of its own from within its core?

Possibly. Ceres does that. She just doesn't do it enough to have a breathable atmosphere. From the link:

In early 2014, using data from the Herschel Space Observatory, it was discovered that there are several localized (not more than 60 km in diameter) mid-latitude sources of water vapor on Ceres, which each give off approximately 1026 molecules (or 3 kg) of water per second.

And unfortunately, even if she upped the amount of gas she generates, she probably would not be able to keep it (see the page 19 of this article).

Now, Ceres is mostly rock and has little water. But think of Europa. That one has enough ice to make for an atmosphere if heated up enough.

Could the core have a tiny bit of Neutronium in it (don't laugh)?

Very unrealistic, and even if you handwave it, very troublesome.

Neutronium should only exists in the core of dead stars. It is very dense, by definition. So a very small portion of it would do for a planet that is over 99.9999% gas in volume. However, remember that even if gravity is close to zero in the center, that is still where everything is pulled towards. Every dead mass in your planet would clobber up there.

Could a comet have hit it and given it the gases?

One single comet, no. But multiple bombardments through millions or billions of years could do the trick.

Remember that with little gravity, you may lose a lot of gas to space on every impact. So your net gain from each individual impact might be little, or even negative.

  • $\begingroup$ So the neutronium would work for a planet that was 99.9999% gases? Lets say I handwave how the neutronium got there, is it possible that any of that mass of gases be breathable, creating a habitable zone for life? $\endgroup$
    – Len
    Jan 19, 2018 at 20:38
  • $\begingroup$ Len - Neutronium won't work. Neutronium is dense because of the intense gravity of a neutron star. In the much slighter gravity of any other type of planet or star (except from a black hole) the pressure will not be enough to maintain the density of the Neutronium. Any tiny world that had enough neutronium to give it sufficient escape velocity to retain a dense atmosphere would have a high enough mass and density that we would already know about it, if it was anywhere in our solar system except for the farthest and least known dwarf planets. $\endgroup$ Jan 20, 2018 at 6:59
  • $\begingroup$ @len - everything thaf could not float permanently and indefinitely would clobber up at the center. $\endgroup$ Jan 20, 2018 at 23:35

Since you asked, "... within a small planetoid...," then may answer is to hollow out the planetoid and live inside it. The gases that you want may be trapped inside an existing rock, much like helium is trapped by layers of rock in American natural gas fields.

Living on the surface of the Earth is possible partly due to:

  • The thick atmosphere protects us from most space debris and radiation.
  • The thick atmosphere regulates day and night time temperatures.
  • The magnetosphere protects us from the solar wind.

To make up for this on Ceres, you need buildings with shielding and the most economical choice is the local rock.

But the bag idea has some merit.

It should be possible to create a hollow sphere with a 600 mile diameter by heating and inflating a balloon. Unlike a traditional balloon where all parts expand at the same time, I envision inflation plants roving the surface of the growing sphere. These plants heat local areas and allow the sphere to expand as a series of 'warts'. As the sphere expands the material thickness decreases, but this is an opportunity for vacuum welding. New material is added by the inflation plants to restore the thickness of the sphere for optimal heating and expansion during the next pass.


After all of that work, you have a giant bubble that will fit Ceres and it will have a slight atmosphere inside, but very little structural support. So the bubble would be layers of bubbles, which also provides redundancy against leaks and acts as insulation.


I have found "shellworld" article to match your description. The shell floats above the atmosphere and its sheer weight adds atmospheric pressure. Just don't understand why it is opaque with artificial lighting. I can picture autonomous robots roving the surface and mending meteorite punctures. Obviosly, the shell must have an area designated to carry a spaceport and a gate.


Ok my first thought was Larry Niven's "Integral Trees"... which if memory serves, describes an entire torus of environment in orbit around a star. The titular trees grow inside the band, in a state of perpetual free fall. Individually they are too small for their own environment, so they live inside a self contained environment.

What about a parent planet...

The planetoid doesn't need to OWN the environment, just be inside it... a gas giant made of oxygen and hydrogen with the planetoid inside it's bounds? A tiny singularity generating enough gravity to hold oxygen, but not enough to devour it, with the planetoid orbiting that, while the singularity orbits the sun?


It could exist inside an unusually dense gas torus. Similar to Niven's Smoke Ring / Integral Trees.

This would be more likely if some gas giants are involved in addition to the small planetoids, especially if you want it to be stable for any period of time. It would require just the right positioning of planets for the gravity and magnetic fields to maintain the gases in place.

The small planetoids gravity would cause a localized increase in density in the torus making a denser atmosphere around the planet. Atmospheric gases lost to space from the planet would go back into the torus and be replenished from the same source.

This is highly unlikely, but could potentially occur naturally or artificially by a very advanced civilization who decided to move some planets around.


To keep an atmosphere all that you need is a gravity well. The most obvious way of compensating for a shallow gravity well is a lower temperature, which decreases the gases' average speed to well below the escape velocity.

But if alien technology is in the equation, you can place a stabilized black hole in the center of the planetoid, so that surface gravity increases.

Having Earth surface gravity will not be quite enough because the gravity well, while now deep enough, is small - gravity falls off more sharply than on Earth.

I think you can safely enough imagine a surface gravity as high as 1.5, and some outgassing from the planet's mantle should do the rest. Being a planetoid, it's doable to have huge deep deposits of water ice and frozen nitrogen.

The timeline goes more or less like this:

  • the planetoid (total mass that of, say, Ceres) forms, with lots of frozen water ice, nitrogen, carbon dioxide, perhaps methane.
  • the planetoid gets covered in cosmic dust, fragments, loose rocks and dirt from the asteroid belt.
  • the Handwavers come by and drill a hole to the center of the planetoid, and place a Cosmic Cruncher in position.
  • the Cruncher activates and a shielded black hole forms, crushing most of the planetoid into a volume one tenth as before, and increasing its temperature through gravitational collapse. An atmosphere forms, and small liquid seas.
  • on average, accounting the central black hole, the "density" of the planetoid is one hundred times Earth's. So its radius could be one hundredth of Earth's - just 67 kilometers - and the surface gravity would be the same (a smaller radius translates to a higher surface gravity).

Of course, having one hundredth Earth's radius means 1/1003 its volume, and a density 100 times higher only corrects mass to 1/1002 - one tenth of a thousandth - of Earth's. This means that atmosphere retention capacity (which depends on mass) is also about 0.01% of Earth; if Earth can keep its atmosphere for ten billion years, our planetoid's will only last one million years; of those, perhaps one-two hundred thousand years will have an acceptable atmosphere. Outgassing will help there, if needed.


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