I want to write a novel situated in a “reversed planet”: there are two stars orbiting each other, and there is a little spot exactly in the middle of the two that is suitable for life. A normal planet wouldn’t be possible, because of the intense heat, light, and the gravity on the outside of the crust, but the inside could be suitable for life.

The planet is hollow. There is a very bright molten core in the middle of it. The crust is very thick, and although outside of it there is the void of space, neither air nor water pass through it. Because of the gravity of the stars and the rotation of the planet, everything lives pointing outwards: the opposite of our perception of gravity, but the gravity of the dense molten core make it so that the crust doesn’t just crumble away.

On the inside of the crust, there is an environment full of life, and there are humanoid species that live there.

I know that a planet like this can’t really exist, but I would like to know if there are some real theories or just some crazy “pseudo-scientists” that could "scientifically" back up this extravagant planet.

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    $\begingroup$ +1 for specifically saying you know it's not real and may require pseudo science. But consider being slower in accepting. Once you accept an answer, there is less chance for other answers to be posted. Only accept when you're sure you don't need anything better. I try to always wait a day or two. $\endgroup$
    – Mołot
    Nov 22, 2016 at 14:31
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    $\begingroup$ Does the planet have to be natural? $\endgroup$ Nov 22, 2016 at 16:07
  • 2
    $\begingroup$ @TomJNowell no, it could be natural or artificial, although i would prefer if it was natural $\endgroup$
    – JackIta
    Nov 22, 2016 at 16:15
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    $\begingroup$ How sad would that be to have a night sky (if any) without stars :/ $\endgroup$ Nov 22, 2016 at 16:18
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    $\begingroup$ @JulienLachal they don't have a day-night circle, cause they're always under the light of the molten core $\endgroup$
    – JackIta
    Nov 22, 2016 at 16:22

10 Answers 10


The only thing that would let me believe this planet exists is if it were some sort of artificial planet. Like you could say that the thing is actually an artifact of some long lost space faring civilization (or point out that that's what the planets residents theorize if they've gotten to the point where they know that their planet is really really weird).

This is feasible in today's understanding of physics, it's just an absurd engineering problem to build a structure like this. However, I'm sure that some super civilization with vast vast resources and some completely nutter motivation to build this thing could probably do it. It would need to be made of exceptionally strong materials (you probably don't even need unobtanium) and a brilliant structure design. The structure would just need to survive rotating fast enough that anything on the inside of the "crust" is experiencing 1 g of net centrifugal force outwards.

As a result, I can imagine that all sorts of dirt and crud could build up along the equator of this structure (maybe seeded there by the lost civilization). You may be better off with a cylinder or like a ring (i.e. like in Halo) so that you can have a wider strip of "land" on the equator. then on this dirt you could get an atmosphere and critter and whatnot so long as stuff couldn't "spill out" of the planet at comical speeds.

Btw... you're planet would have SUCH an easy time getting ships into space... not so easy getting back though.

Edit: Oh, and I had an Idea about the "molten core". Possibly the stations' original intent was to harness the immense solar energy from the two nearby rotating suns. The core used to house the storage device for this power, but due to centuries without proper maintenance, the whole structure now only collects a small portion of the energy it used to. Another side effect is most of the remaining energy collected is leaking from it's core in the form of radiated heat (i.e. approximated as a mini-sun, possibly a ball of plasma held in place by magnetic suspension).

  • 1
    $\begingroup$ As a note on your last line: The atmosphere is on the inside, so poking a hole in the think crust that keeps it all separated from a vacuum could be......catastrophic. $\endgroup$
    – Anoplexian
    Nov 22, 2016 at 15:30
  • $\begingroup$ @Anoplexian: Oh ya, the structure would have to be extremely robust in seal design. In my mind, if this civilization started digging "out" you would have to get through a couple hundred feet of dirt and rocks. Then a couple hundred feet (or miles depending on planet size) of the actual construction in order to get to space. I think the very first explorers to get down to the deepest "depths" of this planet are in for one heck of a surprise... $\endgroup$
    – padleyj
    Nov 22, 2016 at 15:43
  • $\begingroup$ Perhaps this was originally a normal planet that orbited the stars, but generations ago it was noticed that their orbit was going to be messed up - or maybe their one sun was going to capture another star? - and, lacking the capability of leaving their planet, they decided to go underground and got incredibly, incredibly lucky? $\endgroup$
    – Trevortni
    Nov 22, 2016 at 18:12
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    $\begingroup$ @Trevortni Just not plausible, for a start the orbital position mentioned is completely unstable, something would need to be keeping it there. $\endgroup$
    – Tim B
    Nov 23, 2016 at 12:58
  • $\begingroup$ I know that, but the request was for pseudoscience. $\endgroup$
    – Trevortni
    Nov 23, 2016 at 15:54

Well, as you accept crazy pseudo-physics we can do a lot here:

Assuming the molten inner core keeps its form due to its sheer mass compressing itself we could argue that it forms a magnetic field. Further assuming we can handwave this magnetic field to be very very very strong we can continue with our crazy pseudo-scienceTM by having the very thick outer crust containing ferromagnetic materials that hold the same charge as the magnetic field provided by our core.

Thus we would have an outer shell with a more or less even distance from the inner shell; and, as far as I see it, the core and shell could even rotate in different directions.

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    $\begingroup$ Magnetism is a way stronger force than gravity. If they are polar opposites the crust would collapse on the core. $\endgroup$
    – Hyfnae
    Nov 22, 2016 at 13:25
  • $\begingroup$ @Hyfnae irks, my bad. Meant holding the same charge... Thanks for pointing that out $\endgroup$
    – dot_Sp0T
    Nov 22, 2016 at 13:28
  • $\begingroup$ Magnets don't quite work that way, but for crazy pseudoscience purposes sure why not? (It's not possible to make a sphere such that the inside of the sphere is all north poles) $\endgroup$ Nov 22, 2016 at 22:33
  • $\begingroup$ @immibis but you don't need all north-poles; You just need to align the inner sphere and the outer shell properly so there's an equilibrium. You could probably stabilize it somewhat by making the equators of the shell thicker so there's more of the gravity from the neighbouring planets holding it steady (still pseudoscience, but good pseudoscience) $\endgroup$
    – dot_Sp0T
    Nov 23, 2016 at 9:04
  • $\begingroup$ Note that if the magnetism is what keeps the crust from collapsing into the core... then what is keeping the inhabitants from falling into the core? $\endgroup$
    – ANeves
    Nov 23, 2016 at 15:23

I read a vignette about life on Europa. The twist at the end is when the NASA probe breaks through from the ground. The perception of up/down experienced by the natives is from boyancy, and they live on a reef under the ice cover. Their idea of down is toward the surface.

  • 1
    $\begingroup$ Great idea to use buoyancy instead of gravity, although our frozen seas there are a lot of organisms walking on the bottom and nearly none walking upside down under the ice. $\endgroup$
    – Pere
    Nov 22, 2016 at 23:51
  • $\begingroup$ I think since falling down would take stuff away from the ecosystem and into a totally different domain at hundreds of km down, floating would be an early prerequisite for all the stuff that colonized the ice crust. $\endgroup$
    – JDługosz
    Nov 23, 2016 at 1:18
  • $\begingroup$ Interesting, but if it were humans, then they would experience the same effect we do from being upside down all the time, the blood in their bodies will still flow towards the centre of gravity (which if they are upside down results in too much blood to the brain). $\endgroup$
    – Baldrickk
    May 25, 2017 at 7:28
  • $\begingroup$ I’m sure the indigenous life evolved with valves facing the other way and control mechanisms to keep blood flowing properly that are suitable for their environment. @Baldrickk A visiting human would appear unside down, and we would perceive our submersible as being below the reef on the roof of the world. $\endgroup$
    – JDługosz
    May 25, 2017 at 18:13

I don't know that there is any natural explanation for this planet, even within the confines of Pseudoscience. Certainly it is possible in the realm of sci-fi technology to create such a planet.

there are two stars orbiting each other, and there is a little spot exactly in the middle of the two that is suitable for life.

This is probably possible in theory that a planet happens to have formed exactly at the Lagranian Point between the two stars. I find it extremely unlikely to happen.

TECHNOLOGY SOLUTION: As this is probably extremely unlikely to have occured naturally, it's more likely that someone (doesn't have to be your people) PUT the planet there

Because of the gravity of the stars and the rotation of the planet, everything lives pointing outwards, the opposite of our perception of gravity, but the gravity of the dense molten core make it so that the crust doesn't just crumble away.

This is a very improbable scenario. If the force of gravity pulls people and objects away from the molten core, then it is also pulling the crust away from the molten core at the "surface" level. If the crust is very thick, the gravity on the outward facing crust will be stronger than the gravity at the surface. Further, unless the molten core is extremely large then you are doubling its radius very frequently and it is getting weaker and weaker.

If gravity from the stars is pulling people away from the molten core, then the crust should crumble and fall away.

TECHNOLOGY SOLUTION: The crust is contained inside a stronger substance such as a metallic shell which allows it to maintain its structure. Your planet (or at least the inside) would need to be something other than a sphere (cylinder seems likely) or all the people would mercilessly fall to the equator from rotation/gravity. But since the planet was constructed, picking the shape of the interior is easy -- the outside could still be spherical which would be easier to balance with gravity

Additional Problems: Heat: Your core is molten rock radiating heat towards the crust, your external surface is heated by constant daylight enough that you say the heat on the surface is enough to prevent light. If there is never darkness then your crust will continually get hotter. If you have heat inside and heat outside, where is all the heat going to keep your people from dying?

TECHNOLOGY SOLUTION: If the external surface is engineered instead of natural (see gravity issue). Then surface could be reflective, sending most of the external light away from the planet. OR it could be an advanced solar array

The Stars themselves: I don't think we know of any natural object with a perfectly circular orbit. For two stars to stay in a perfectly circular orbit such that a planet at the center of the orbit was always in a stable position would be extremely difficult. The masses of both stars would have to be just right, the placement and timing would have to be just right and there would have to be nothing of any size operating in their vicinity to disturb the delicate balance.

TECHNOLOGY SOLUTION: This is a hard one, to move stars into an orbit defies even the craziest ideas we have right now. In the pseudo-science realm, I suppose if you had a race that could manipulate gravity to the point they could create black hole levels of gravitational force then they could manipulate the paths of stars and put them where they want them using basic orbital mechanics. More likely I suppose is that they were so determined to find such a place that they put considerable resources into searching, finding and traveling to the only star system within their galaxy that happened to be right.

Side note: If a builder race could control gravity, the planet could be spherical on the inside and natural outside by making gravity pull towards the center of the crust -- this locally over powers the stars, accounts for all the gravitational issues with the crust/people without an external protective shell, and if finely tuned could be suspending the core in the center of the planet (instead of the core holding the planet together in a delicate balance).

This side note actually has me excited as possibilities go though. Let's say some race of builders long ago (maybe your people and they just lost technology over the eons, maybe a race seeding life, whatever). This race could manipulate gravity. They find two stars in a near circular orbit, manipulate gravity a bit to get them perfect, then construct the planet at the center assuming no one would ever find it.

They probably put solar arrays on the surface to power their gravity technology they build into the crust, then put the core in the center to provide heat and natural light (not enough comes from the stars because it's being harnessed for gravity). The core needs a source of energy itself, but this could perhaps be gravity powered as well, IE using tidal forces or such.

When everything was ready, they put their colonists or whatever in the planet, thinking inside a planet in an impossible location in an inhospitable area of a solar system would never be found and would be able to live safely. An Ark of sorts.


The structural problem can be solved with handwavium as others have pointed. Anyway, I think there is a harder problem with temperature: if the outside of the planet is very hot due to the stars, and the interior is under the light of the molten core, after some time the inner surface will reach thermal equilibrium at some temperature between that of the core and that of the outer surface.

Therefore, you need to put the stars far away enough to get a colder outer surface to allow the planet inner side keep a liveable temperature.

Another problem is that the planet is in the inner Lagrange point of the two stars, and that position is unstable. Then, or the planet has an active mechanism to control position (like big rocket engines) or it is massive enough that the stars orbit it - but if the stars orbit it won't be always in the middle or the stars orbits won't be stable.

Edit (about the comment) As cst1992 pointed, an object more massive than a planet would be an star. However there are some practical solutions - at least, as practical as a hollow planet.

The planet or its molten core can be very dense (like a neutron star or just like heavy handwavium) and made of anything unfusionable, like iron. Furthermore, it needs to be fairy cold, so it needs to be different than stars, that would heat up while acreting just by transforming gravitational energy into heat.

Of course, the more massive is the planet, the more difficult becomes to prevent its shell from falling, and the more difficult becomes to keep "gravity" pulling people outwards, but at this point we have supposed so many weird things that a couple more isn't a big deal.

  • $\begingroup$ Stars can't orbit a planet - if the planet were so massive that stars orbited it, then it'd be a star itself. $\endgroup$
    – cst1992
    Nov 23, 2016 at 7:23

There's a problem with your description of this world, but really it ends up helping anyway.

The stars are undoubtably very far away if the planet is in a location suitable for life, which means that their gravitational pull on your lifeforms will be largely undetectable unless they have an extremely sensitive sense of acceleration/orientation.

$$F_g = -\frac{GMm}{r^2}$$

Since the stars are very far away, this $r^2$ factor is very very high, which means the comparatively tiny distance that is the diameter of the planet will make very little difference between the two opposing gravitational forces from the two stars, meaning that the net acceleration due to gravity of these lifeforms will be very small.

But this is a good thing, actually, since your planet will have to rotate much more quickly than your stars will orbit each other, in order to produce an acceleration that the lifeforms can appreciate. If the acceleration due to gravity were significant, then your lifeforms would find themselves experiencing all kinds of weird phenomena, like weighing more or less depending on the time of day, having to stand at an angle on the ground depending on the time of day, or in extreme cases, even falling over and rolling every evening to the side of the planet closest to the nearer star. Architecture would be near-impossible and the whole thing would just be a mess. It would get even messier if the rotation of the planet and the orbit of the stars are not coplanar.

  • $\begingroup$ Would there be a concept of 'day' on this planet? The beings would not know about the outside stars, and their 'star' would be the planet's core(in the sense of source of light). $\endgroup$
    – cst1992
    Nov 23, 2016 at 7:25
  • $\begingroup$ @cst1992 I'm sure they would have some periodic concept of time, as the biology of basically every living thing depends on it (periodic sleep, food intake, waste production, etc). But I was speaking in the sense of one "day" being a revolution of the planet on its axis. $\endgroup$
    – Devsman
    Nov 23, 2016 at 15:56
  • $\begingroup$ If the stars' gravity were strong at the planet's surface, I'd expect the planet to be tidally locked, so the same geography is pointing at each star at all times $\endgroup$ Nov 23, 2016 at 16:38

padleyj explained it well, it could be only an artificial structure.

  • Every orbit between binary stars (two stars) is unstable and every celestial object will be sooner or later kicked out.

  • You cannot have a natural hollow planet. Either the planet is big and that means high gravity and forming a sphere (no hollowness) or it is small, but then it cools out fast and it is impossible to prevent atmospheric loss because gravity is too small.

So as already mentioned, the planet is artifical. A good idea is a combined research station and a new home for emigrants.

  • It has auto correction, it was put in the correct spot and corrects the orbit by shifting internal mass.

  • The "ground" is really a light, strong and airtight material artificially created. Only that is able to prevent catastrophic atmospheric loss.

  • The "molten core" is in reality a fusion reactor, a little sun.

This project failed for unknown reasons, only some creatures as pets/slaves whatever survived and live now in the artifical structure.


Lots of answers here...the Dyson Sphere was also featured in the Star Trek: Next Generation episode, "Relics."

Your molten core's gravity pulls in. The stars pull out. The stars need to have more gravity at the point where people are living, in order to keep things on the inside surface, but the core needs its own gravity to hold itself together.

Somewhere in-between, is a gravity dead zone, which complicates things. Ideally it would be "above" the inside surface, but not close enough to the core that it would be torn apart.

The next issue is the livable area. Your outer binary star system provides the problem of high gravity at the system's equator and, most likely, zero gravity at the system's poles. Gravity would need to be artificially sustained.

I think you've got a big issue having both the molten core and the stars together. Either one could provide energy to sustain life and even having a small star suspended inside a Dyson Sphere seems more believable than anything soaking through the sphere, without tearing it apart. The sphere needs no external help, as it is wholly contained, energy and gravity and all. It can also function as a massive spacecraft, going where its people need.

Based on all of the above, I have to ask – what's the point of the stars on the outside, beyond being an interesting concept? It is an interesting concept, but seems the least realistic part of this fantastic scenario. You'd need some reason for it to really make sense, if you want your audience to buy the story.

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    $\begingroup$ But a Dyson sphere shell has no self-gravity on the inside. $\endgroup$
    – JDługosz
    Nov 22, 2016 at 22:25
  • $\begingroup$ Technically, everything has self-gravity. Presumably by the point any civilization can build inside-out planets, gravity would be among very minor issues. I'm not sure which part of my answer you replied to, but the big problems (as I saw them) in the OP's described situation didn't seem to be related to the sphere itself, but rather the stars, be they inside, outside, or both. The stars would overwhelm the natural gravity of the sphere, making artificial gravity a likely and very expensive necessity. $\endgroup$
    – VoltisArt
    Jan 6, 2017 at 9:53
  • $\begingroup$ «more gravity at the point where people are living» doesn’t work. The expanse of shell on the opposite side is farther away but you are exposed to more of it. A uniform hollow sphere does not attract objects to its inside surface. $\endgroup$
    – JDługosz
    Jan 6, 2017 at 10:14

(Warning: heavy pseudoscience ahead; allergic people should stop reading here. The author holds no liability for any health hazards or other consequences. Proceed at your own risk.)

We all know that, contrary to electromagetism, gravity has no polarity. All matter attracts matter, there is no negative or positive gravity.

Well, we know wrong. There is something like negative gravity. Unlike electromagnetism, where opposite polarities attract each other, and same polarity implies repulsion, gravity works the other way: polar opposites repel each other, the same polarity implies attraction. Which is the reason we never see gravitationally negatively charged objects: they have long be repelled to the outrfrontiers of our observable universe. Except... in very rare cases where a bit of gravitationally negative matter has been accidentaly encapsulated within a sphere of gravitationalyy positive matter (or, of course, the other way round). In these extremely rare "space geodes", the core repels the surface, which is kept in place by its own traditional gravity, which counter balances the neg-gravity of the core.

... it has the advantage that the whole thing is extremely light, for the two opposite kinds of gravity cancel or almost cancel each other, so it doesn't even need to gravitationally interfere with other objects.

  • $\begingroup$ really like this negative gravity $\endgroup$
    – JackIta
    Nov 25, 2016 at 7:22

A planet with two 'cores' isn't outside the realms of possibility, it takes time to even things out, but it's hard to imagine how the planet would maintain equilibrium if either of those cores (or both) were molten. creating chambers inside rock formations isn't difficult at all and nature has dozens and dozens of specific mechanics to 'make it happen,' the problem an 'internal world' has is in structural stability and nature not filling the gap.

Your location in a high solar energy position might help this, as deep crust punctures of a molten core planet might not trigger 'cooling' sufficient to quell the resulting bleed off of mantle and outer core, this would naturally result in a shift in center of mass as a significant proportion of the internal matter bled to the surface and the core is drawn (by gravity and pressure differential) towards the punctures. This would produce an uneven distribution of mass on one side of the object in the short term as the core cooled(lower internal pressure unless the crust suffers a general collapse), but as the surface is molten due to the proximity of the stars, the rotational motion would be accentuated in nature's desire to form a sphere.

It seems to me that the crust could not suffer a general collapse without an incredibly quick exchange of volume, but rather that it would get thicker (internal depth) as the core cooled and 'sloshed' with the rotational characteristics. Atmospheric pressure would clearly initially be intense and poisonous.

Not sure how biogenesis occurs under these conditions.


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