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Now- this is probably a stupid question but hear me out. If the planet I want to imagine were much larger than earth, would it have to have a massive difference in gravity? It sounds extremely implausible to make it hollow (how would that happen naturally, etc.), but if the planet were packed with a large amount of dense, swirling air or gas, kept there by a sort of hard outer layer, would it change the gravity? I know it couldn’t be possible in the real universe, but with the inventions of a few new, extra hard minerals and the like to make it more plausible, I’d like to know the effects of a hollow planet. (Afterthought: If some dense gasses gathered themselves naturally, could they slowly pull rocks and minerals from space and form a cover? Take into account this would be in a fictional world)

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  • $\begingroup$ If you invented new minerals then that would be magic, and magic doesn't require any explanation, right? $\endgroup$
    – Alastor
    Commented Oct 9, 2022 at 14:54

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Trivially, Yes

If you imagine some sort of supermaterial like the stuff of which Larry Niven's Ringworld is made, formed into a hollow sphere under however much crust you want, then you could spread all the Earth's mass (and then some, because of the increased radius) over this shell.

(I specify the supermaterial because there is no known material that would let you put a rocky shell on the scale of a planet over a void, and no gas that would allow the same effect. If you have a planetary mass of gas, the core is solid, and if there were enough gas to gather solids, the solids would fall to the core, not form a shell. So we'd have to assume that this planet was manufactured, maybe by the Magratheans.)

So if we assume the mass of the supermaterial to be negligible, and the faux-crust to be, say, 10km thick and of similar composition to Earth's crust ($2.6 g/cm^3$), then we could make a sphere of Earth's mass ($5.972 × 10^24 kg$) with a radius of 135 202km.

At that size, of course, surface gravity would be substantially lower than Earth's, so you could definitely go bigger; I suspect integration would get you the exact size at which you could get 1g.

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Theoretically yes, since the strength of the gravitation field of any object is determined by its mass not its shape. The shape of the object will effect the shape of the field however. For example a cylinder with Earths mass would not have a uniform 1 G gravitation field but that's not the issue in this case since your planet is still roughly spherical.

If you want your world to have a earth like outer surface environment and temperature however there would be issues with structural integrity, magnetic field strength & thermal equilibrium since there would be no (natural) way to maintain an Earth like mantle or core which is needed to maintain the Earths magnetic field, surface temperature & plate tectonics etc.

Basically to have the same gravity all the mass contained inside the Earths inner crust would have to be either (pasted) on to the inner surface or you have some kind of 'air gap.' between the inner and outer crusts of the Earth,

There would also be a upper limit on how big the circumference of your world could be before the gravitational field at any one point on the surface started to decrease notably due to the distance separating it from the rest of the earths mass. (I have no grasp of the maths involved in working out the numbers involved above BTW.)

Frame check: It might be simpler to hypothesize some kind of 'Clarketech' level artificial construct (like a Dyson Sphere but on a smaller scale). Basically an artificial station that has been built to emulate Earth like conditions on its outer surface with complex machinery inside dealing with issues like magnetic fields and heat distribution etc.

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  • $\begingroup$ There’s no upper limit on circumference as you suggest. Mathematically, the gravitational field of a hollow but spherical planet is indistinguishable from that of a solid planet at all points above the surface. $\endgroup$
    – Mike Scott
    Commented Sep 10, 2023 at 12:07
  • $\begingroup$ Would that apply to a hollow sphere whose shell had the thickness/density that equaled the Earth's diameter though? Assuming a relatively 'small' hollow core would not polar opposite points on the sphere be affected by additional gravitational forces generated by the mass of the opposing side? $\endgroup$
    – Mon
    Commented Sep 10, 2023 at 23:53
  • $\begingroup$ No, it’s a relatively straightforward application of integral calculus to show that it makes no difference. All that matters is the mass and the radius, assuming that the density everywhere is the same at the same depth. $\endgroup$
    – Mike Scott
    Commented Sep 11, 2023 at 7:37
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The technical definition of a planet requires it to be large enough to compact itself into a relatively spherical form by its own gravity. If it's hollow, it thus obviously violates that definition, at least in terms of naturally occurring bodies. As for the artificial creation of such a hollow "planet" (here, meaning something that at least externally looks like a planet and would be generally called one by the less-scientific), you run into some issues.

Your idea of filling the centre with gases instead of minerals sounds nice in theory, but in reality it doesn't work on a planetary scale. The reason is simple: gravity. As overall mass goes up and gravitational pull increases, pressure on the centre materials (the gases, in this case) will increase due to the weight of everything above them. By the time you get near Earth's gravitational pull, that force will be easily sufficient to compress the gases to the point where they're no longer gases but liquid or solid, decreasing their volume. That reduction will create a relative vacuum (or at least lower pressure zones) under the crust, which under that strain will buckle and fall in (this is the principle behind underwater torpedoes in naval combat, incidentally, as the momentary vacuum caused by their explosion driving away the water can cause battleships to literally snap under their own weight no matter how much armour they have), being drawn towards the core. You get a vicious loop at this point, as the collapsing crust will then apply still more pressure to the core, compacting it further, and so on and so on until your planet is no longer hollow.

If you're willing to handwave some details, you could stipulate an arbitrarily tough crust that will somehow not buckle under planetary stresses for your fictional world; I don't think there is a material sturdy enough for the task in reality, but I'm not a materials scientist. This, however, runs into other complications. A crust that unbreakable won't permit normal planetary processes because the material from the mantle won't be able to reach the surface. You're going to miss out on any sort of plate tectonics, for instance, since this crust layer will be impossible to move: no earthquakes or volcanoes. I'm not an expert on geology, but I don't doubt there would be other likely-catastrophic consequences over longer periods.

Frankly, your goal is impossible without a completely artificial construction on a scale beyond sanity. Your best bet, as far as I can see, is to imagine building an outer shell around an ordinary planet (of whatever desired radius); Earth's gravitational field at the level of the ISS is around 90% of its strength at ground level, so you could start with a slightly larger planet than Earth and work from there. Fill the gap between your shell and the actual planet with whatever gases you desire: I doubt it matters much what you use, as long as you have suitable mass/volume. I have no idea what you'd make the shell out of to withstand the stresses on it, as noted before, so you'd have to handwave that. Give it a good push to set up its initial rotation and install mechanisms underneath to maintain that rotation in the face of friction from the gases below (note that literally attaching the outer shell to the planet would likely rip apart the planet or the shell due to inevitable differences in applied forces). If you want a realistic planetary surface for your outer shell, you're going to have to convincingly fake a planet's mantle between your shell and whatever its outer surface is, so that shell is likely to be dozens of kilometres thick due to the need for such secondary mechanisms. EDIT: I knew there was a term for these sorts of constructs. Look into Dyson Spheres if you want to do further research, as you'd pretty much be building one of those around a planet rather than a star.

Conclusion: this is a classical case of pleasant theory foundering on the harsh rocks of science and realism. You could technically get an Earth-like gravity on a much larger object if it were hollow, simply by picturing Saturn or Jupiter with an Earth-like crust layer above the gases. In practice, though, there is no outer shell that could possibly withstand the stresses involved of such a crust layer without breaking apart and being absorbed into the planet.

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  • $\begingroup$ I'm not convinced by "By the time you get near Earth's gravitational pull, that force will be easily sufficient to compress the gases to the point where they're no longer gases but liquid or solid, decreasing their volume". I recall being taught in high school physics that the gravitational force acting on an object inside a uniform spherical shell is always zero. Of course, there's no such thing as a perfect sphere, and then there's other objects in the universe also exerting a pull, so perhaps you're still right! $\endgroup$
    – Auspex
    Commented Sep 9, 2022 at 11:06
  • $\begingroup$ @Auspex The gravitational force is pulling everything towards the centre. Technically, yes, gravity at the centre is zero, but when you draw everything towards the centre, whatever is on the upper layers will compress everything closer to the centre. Pressure (and temperature) rises, and rapidly, as you go closer to the Earth's core; at any given point within the Earth, everything between there and the surface is pressing down on that point. This is why the Earth has a solid core, even though at several thousand degrees Celsius it would be pure gas if under normal pressure. $\endgroup$
    – Palarran
    Commented Sep 9, 2022 at 11:12
  • $\begingroup$ No, anywhere within the sphere, if you sum all the force vectors it comes to zero. btw, the physics teacher who taught this was named "Moon" :) $\endgroup$
    – Auspex
    Commented Sep 9, 2022 at 11:18
  • $\begingroup$ The earth has a solid core because there isn't a shell around the coalescing matter. $\endgroup$
    – Auspex
    Commented Sep 9, 2022 at 11:19
  • $\begingroup$ @Auspex In the sphere, you only get net-zero force vectors if you include the normal force, unless I'm mistaken. That's the force that keeps gravity from pulling people straight through the earth: it's the resistance of the ground, and if the downward force is great enough, it will break through the surface. In a hollow-Earth scenario, gases simply cannot offer sufficient resistance to balance the force of gravity, and the outer shell will then buckle under the strain; picture a house being dropped on a balloon, perhaps, for an idea of the relative forces and strengths involved. $\endgroup$
    – Palarran
    Commented Sep 10, 2022 at 0:15

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