For a long, long, long time I have had a concept of a world that exists on the inside of a hallowed-out sphere.

The basic construction of the world is like this: It is a large world, at the center of which is a very small but self-sustained star. Surrounding this star is an array of gasses that comprise an inner-atmosphere protecting the world from extreme radiation. Because the atmosphere needs to be so thick, rainfall is incredibly heavy, and because the 'sun' is so close to the surface, dangerous electromagnetic storms are common.

There are areas of much thicker atmosphere, which result in incredibly harsh cold climates, and due to the strong gravitational pull of the 'sun', there are bodies of water that lift right out of the ground and flow through the air, creating fast channels to other parts of the world.

Mining occurs, and overmining is causing a loss of atmosphere in this insulated inner-world, so this is already a problem that I'd be well aware of for such a world. And of course, there is always daylight, except during periods of high cloud coverage.

What other possible problems would I need to address to create a realistic sci-fi world that is essentially an insulated sphere like this one?

  • $\begingroup$ Which dimensions are we talking about here? Is it a Dyson Sphere, or a hollow Earth? In the latter case, please specify what this very small star in the center should be. I am no expert in astrophysics, plasma physics, or solar engineering (lol), but the questions that come in mind are like: How big/heavy is it? How much distance shall be there? What is the energy output? Isn't it actually just a monstrous fusion reactor? $\endgroup$
    – MauganRa
    Nov 22, 2017 at 12:08

6 Answers 6


Let's flesh out your planet some more so I can point out some potential problems. (Which admittedly are exacerbated by my "fleshing"). I hope I didn't fail in my physics but even if I did the first set of problems applies even to a Dyson Sphere or Ringworld.

Alright first the wishes/problems-list:

  • sun inside hollowed sphere.
  • variable thickness atmosphere protecting the world from extreme radiation.
  • floating rivers that return back to the surface
  • mining, causing a loss of atmosphere
  • rainfall and clouds

Resulting in:

Without resorting to gravity manipulation (in which case at that scale you could probably tackle any problem facing your world with technology) your hardest one is the rivers. Just a spinning shell wouldn't work in this case as there's no force to drag a floating river back towards the shell. A very fast spinning sun could maybe work by the pressure exerted, this could help stabilize the sphere as well. In order to "launch" a river you'd either need an area with much weaker gravity or a shell that has more inertia than your water. Both require your shell to be higher density than water at the very least and preferably much denser. In order for mining to cause a loss of atmosphere (gradually I'm assuming) it needs to go somewhere. Since digging a deep pit would tend to fill it, that could be the entire cause if the atmosphere was relatively thin. To launch a river your shell would be tidally locked which would force fresh rain up a mountain. The tidal locking would prevent extreme mixing of the atmosphere and could allow some Earth-like layers to develop.

Then the problems:

  • Earth receives a very small amount of the sun's energy and as your capturing all of it you need a way to capture and store it or a way to radiate it outside the shell quickly otherwise your going to fry. As it gets hotter your clouds don't rain as much and the atmosphere starts mixing more.
  • Electrical storms would be frequent regardless from the quick build-up of ionization.
  • Solar flares would be killers at that distance.
  • Even though the pressure would be providing the counter-force for the sun's gravity you would have its full contribution on the outside and as such would probably never be able to leave even if you had access to the outer surface.
  • Puncturing the shell would result in rapid loss of the atmosphere.
  • Existing on the inside of the airtight sphere you would never see the outside and even if you had created the world and moved in from the outside the existence of something beyond the sphere would soon become mythical. Scientific progress in related areas would slow.
  • It would be extremely bright. Pollution may actually be encouraged.
  • Conversely pollution would act in a thinner layer and have greater health effects by being closer to the surface.
  • The horizon and sky would be a flat color (star's) making visual navigation impossible on the ocean.
  • Mining would slowly unbalance the shell leading to collapse.
  • You are very vulnerable to apocalyptic attacks both external and internal. Punctures and extreme pushes. Both are more likely as object fall faster towards the sun. A solitary person can become a world-destroyer.

Problems exacerbated by the "fleshing":

  • A spinning sun would increase mixing in the upper layers and surface heat would increase mixing in the lower layers. You're civilizations ultimate fear would be additional factors or increased heat making the two zones merge. The effects would include runaway heating.

Problems created by the "fleshing":

  • A spinning sun would constantly be ejecting its atmosphere and would probably die much quicker than normal.

To be realistic you would need to address each of these problems in your depiction.

  • 1
    $\begingroup$ The heating could be reduced by having massive metallic spikes (mountains) going from the surface of the sphere and close to the sun, effectively functioning as a radiator. Coupled with an outside atmosphere, you could get rid of excessive heat and avoid loosing the inner atmosphere from punctures. $\endgroup$
    – Clearer
    Oct 18, 2014 at 7:56

You're basically describing a Dyson Sphere.

Spin it fast, and you have gravity from centripetal force at or near the equator. It would have to cancel out the gravity from the star in the middle.

  • 1
    $\begingroup$ It would have to do more than cancel it out, it would need to overpower it -- if you merely cancel it out, you're weightless. Of course, no matter how fast it's spinning, you get less and less of it as you move toward either pole, to the point that, barring climbing gear, you never would -- you'd eventually reach a point where the centripetal force and the star's gravity cancel each other out, making you weightless, and your next step would send you flying away from the sphere and begin your fall into the star. This point would likely be this world's equivalent of our Arctic/Antarctic Circles. $\endgroup$
    – Kromey
    Oct 7, 2014 at 20:35
  • $\begingroup$ Yes, could have been more clear. Thanks for elaborating. $\endgroup$
    – user772
    Oct 8, 2014 at 0:02
  • $\begingroup$ @Kromey, is that area of weightlessness sustainable? $\endgroup$
    – Len
    Jan 18, 2018 at 17:00
  • $\begingroup$ @Len If you mean will it remain "weightless", then yes -- there's simply nothing there to provide weight. If you mean sustainable in terms of atmosphere etc., I don't know offhand. $\endgroup$
    – Kromey
    Jan 19, 2018 at 18:00

First issue (pointed out in other answers) is gravity. I can see a single ring around a sun being feasible (an asteroid belt that became a planet?) but there are quite a few issues in a full sphere.

curious...How thick are you thinking the sphere would be?

Gravity issues:

  • The gravity has to be strong enough to hold the sphere together and spinning fast enough to give gravity, but not fast enough to send the spinning mass off into space.

  • Gravity will be strong in the equator where the spin is greatest, but almost non existent as you got to the top or bottom of the sphere

Formation issues:

Our current understanding of how planets for unfortunately makes this setup very very unlikely if not impossible. There is no mass 'above' or 'below' the sun during solar system formation, it's kind of a '2d cloud' that planets form in around a sun. Admittedly this is drawing from an exceedingly narrow scope of knowledge that we currently have on planetary formations.

Heat release:

I really can't see how a system like this releases heat and energy. A sun (even a small one) puts out a lot of energy...in a standard case, a planet like earth radiates it back out to space. However, radiation in this sphere case would simply go back towards the sun. There would have to be a manner in which heat and energy travels through the shell of the sphere and releases out into space...and I have no clue how to do this in a manner that wouldn't melt the sphere.


Our sun is an incredibly strong magnet when it comes to it...only very recent research has been able to link the sun's magnetic cycles (sunspots in particular) to earth weather (yes, at the distance out sun is from earth, we see very consistent effects of the suns magnetic field from this distance). I'd be concerned that anything metallic that could be magnetized will be. This magnetism could be used to explain how the sphere holds together, but I would think any residents of the sphere wouldn't be compelled to use much for metal.

Reversely...the atmosphere has nothing to do with protecting earth from the majority of nasty space weather coming out of the sun. It's the Earths magnetic field that deflects this (around or through) the Earth. A planet needs a few elements to generate it's own magnetic field, nothing that a sphere could contain.

What happens if a Delta class sunspot throws a CME towards the sphere? At this distance, the atmosphere would actually fall apart at an atomic level and be forced to reform as the CME impact cascades through the atmosphere and into the land of the sphere itself.

I hate to say it...but with only an atmosphere to protect vs solar weather, there would be little chance of anything surviving (including water) on the inner surface of the sphere.


Unfortunately, had such a spherical world come into existence, I don't see it as 'stable' for long lengths of time. A single comet that would harmlessly plunge into the sun is now in a position to tear this sphere a completely new hole with all the matter from it either being flung into space or sucked into the center sun. Worse yet, after the hole is formed, there is nothing to hold in the atmosphere from leeching out as the entire system spins...and worse yet, this impact might very well collapse the whole system.

  • $\begingroup$ Commenting on myself...just to add, I don't think ocean formation would be really feasible except as a giant ring around the equator of the inside of the sphere...Heh, it might even cause a bulk of water at the equator thats higher than the land around it. $\endgroup$
    – Twelfth
    Oct 7, 2014 at 23:44

I think the biggest problem would be that everything would fall to the center into the sun. The center of mass for the entire globe is just that in the center, it wouldn't be any different than having a hole that goes to the center of the earth, granted there is likely a bit less mass (well quite a bit less), so the pull would be weaker and the fall slower but everything not nailed down to the sides would slowly fall into the sun, at least without some kind of wings to 'fly' back to the edge of the sphere.

Of course this is really a mini-Dyson sphere.


There would be no gravity from the inside of the sphere, the pull from all the various parts of it cancels out so you end up in zero gravity. As a result everything either needs to be in orbit or it will fall into the sun.

You should look at Ringworlds if you want to see an alternative or you will need some sort of gravity generator in the sphere that generates an artificial gravity field that falls off faster than a normal one.

  • $\begingroup$ What is the planet spins fast enough to simulate gravity? You'd get strong gravity along the equator and less and less as you travel to the poles. Wrong? $\endgroup$
    – Sheraff
    Oct 7, 2014 at 20:19
  • 1
    $\begingroup$ That is correct, but the effective gravity would always be towards the direction of the equator... $\endgroup$
    – Tim B
    Oct 7, 2014 at 21:34
  • $\begingroup$ Wait, towards the equator? or towards the outside, stronger at the equator? $\endgroup$
    – Sheraff
    Oct 7, 2014 at 21:37
  • $\begingroup$ Always towards the same direction. If it's a circle : O with vertical axis of rotation | then "gravity" simulated by the spin would always be <-> no matter where you were on the sphere. The strength would change but the direction would always be the same (so not directly "down" except on the equator). Moving away from the equator would start to feel more and more like climbing a hill. $\endgroup$
    – Tim B
    Oct 7, 2014 at 22:22

The thing would wobble. There's nothing keeping the sun at the center of the sphere and it would eventually migrate to some other point. Or the sphere would migrate around it.

  • 2
    $\begingroup$ Welcome to worldbuilding! Would you be able to describe this in more detail? How would the sun migrate if it's at the centre of the sphere? (What do you mean by 'migrate'?) - would the sphere move with the sun? If not, why not? What effects would this have on all the things mentioned in the question? would the wobble/migration be noticeable? $\endgroup$ Nov 21, 2017 at 22:57

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