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I've had an idea for several unique planets in my sci-fi fantasy world, perhaps the most unique is the capital of the alliance between several interstellar governments. The planet is made of thousands of floating islands of varying size, orbiting a core which orbits a star.

Although many of these islands are incredibly small, some are large enough to contain entire settlements, the inner capital city is made of seven of them orbiting one holding the central government building.

I'm just wondering if this is possible in any capacity by the laws of physics, if it isn't I can always just explain it away with magic but I'd like to have that as a last resort.

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    $\begingroup$ I believe the duplicate question flagging is misplaced here. This question seems to be asking about a planet-sized object made of independent chunks of rock, from surface to core, rather than a few islands floating in the sky of a whole, ordinary planet. $\endgroup$
    – BMF
    Commented Feb 26, 2023 at 17:44
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    $\begingroup$ When you say "islands" do you actually mean "small moons" or do you mean "chunks of rock floating in an atmosphere"? Put another way - are inter-island trips done using aircraft or spacecraft? $\endgroup$ Commented Feb 27, 2023 at 3:33
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    $\begingroup$ @KerrAvon2055 Even if it's a bunch of asteroids suspended in a bubble of atmosphere for example, I believe it qualifies as a totally different question. The underlying premise is just vastly different. $\endgroup$
    – BMF
    Commented Feb 27, 2023 at 4:19
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    $\begingroup$ @BMF we don't know enough until the OP provides some more detail to know how different the underling premise is. Even if the question is re-opened as "not a duplicate" it is likely to be closed again due to a lack of sufficient detail. $\endgroup$ Commented Feb 28, 2023 at 3:35
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    $\begingroup$ I don't think anyone has asked this question before, as the author seems to want a gravitationally bound collection of unconnected asteroids, with an enveloping atmosphere. The questions people link are an entirely different setup. That said, I can't fathom how the answer can be anything other than "no". But I won't let my lack of an answer presume other people might not think one up! (Nor do I feel confident enough in orbital physics to really spell out a proper "no" answer. Hm, what if they all orbit a gas giant...somethingsomething Lagrange points... I dunno.) $\endgroup$
    – JamieB
    Commented Mar 1, 2023 at 1:32

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closest possible is an asteroid belt

Let's skip the discussion about planet classification and go straight to the physics. You're basically describing an asteroid field with I assume an atmosphere. Herein lies the problem. You need enough gravity to hold the atmosphere. If you have enough gravity to hokd the atmosphere, then the gravity is too strong to make individual islands. It will be like any pebble on Earth. It'll fall down towards the Earth, sinking through atmosphere and water until it reaches any solid matter more dense and heavy than itself. This means all floating islands will float back to the proposed core, making an actual planet instead of a lot of floating islands.

If we assume no atmosphere, or the rocks are outside of the atmosphere, you have different problems. We can see that moons and asteroids exist, so in theory it is possible. However, it is neigh impossible to balance it all without active thrust from the rocks. For each extra rock has a gravity field that influences the others, making instable orbits. They then either escape the orbit, or crash onto the core. You would need constant correction to prevent this from happening.

Just a random asteroid field or asteroid belt seems best. Most asteroid belt or fields are actually very well spaced out, Contrary to what movies have you believe. It is much, much more space and very little rock. It still gives plenty of space for building and travelling. Older asteroid belts or fields are also pretty stable, making crashes and instability much more rare.

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    $\begingroup$ How about an "atmosphere" of something really dense like uranium hexachloride? OK, it's radioactive and decomposes easily, but.... $\endgroup$ Commented Mar 1, 2023 at 15:13
  • $\begingroup$ Yeah this seems like the easiest fix. The floating chunks are just a dense asteroid field, and he has to solve the atmosphere problem some other way. Magic or high tech. $\endgroup$
    – JamieB
    Commented Mar 1, 2023 at 15:14
  • $\begingroup$ @AngryMuppet hey whatever floats your rocks. If the aliens or their machines can survive it and use it, it should be fine right? Oxygen is highly reactive. Uranium hexachloride sounds highly reactive and full with energy. Good for all kinds of uses. $\endgroup$
    – Trioxidane
    Commented Mar 1, 2023 at 15:36
  • $\begingroup$ "An atmosphere of uranium hexachloride" uranium hexachloride does not occur in nature it is synthesised by man and rocks would not float in it. Uranium hexachloride is hydroscopic and reactive so the rocks would not last long. Especially so if the rocks were moving at high speed in some sort of "orbit". Probably best just to draw a veil over the whole proceedings with magic rather than prolonging the agony. $\endgroup$
    – Slarty
    Commented Mar 1, 2023 at 17:25
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    $\begingroup$ hmmm well yes my bad sorry ;o) LOL $\endgroup$
    – Slarty
    Commented Mar 1, 2023 at 21:27
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Technically, it is not a planet, but otherwise it could be plausible. I suppose it would be a bit like shattered Moon. The problem is that the rocks cannot have stable orbits because they all affect each other, and they cannot orbit the core the same way as if they were part of a whole planet, so their positions have to be maintained on a constant basis.

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  • $\begingroup$ Now you have me wondering: what's the timeline of a "shattered moon" coming back together? I kinda picture Hollywood style smashing of rocks together but if it's a slow process (say, 100 years, slow collapse by human eye standards) then it could be feasible... the civilization needs to expend some effort to maintain separation. If it's a fast collapse though, then no bueno. $\endgroup$
    – JamieB
    Commented Mar 1, 2023 at 15:12
  • $\begingroup$ This is the answer that probably best suits OP. There are clever orbital arrangements that make the orbital maintenance easier, but never redundant. $\endgroup$
    – BMF
    Commented Mar 1, 2023 at 15:54
  • $\begingroup$ Seveneves makes it very clear what happens with a shattered moon. It's not stable, even over a period of single-digit years. $\endgroup$
    – jdunlop
    Commented Mar 1, 2023 at 17:49
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Megascale nuclear force

There is no method for building this within the current standard model of physics, but we can step outside of known physics and use cosmological observations to theorize.

If you look at a physical map of the galaxies and galaxy clusters and super-clusters, you eventually get to the point where most of the matter exists in thin walls around huge voids, like soap bubbles. This suggests that there's something in those voids that repels normal matter, but we can't see it, but that's unprovable speculation.

We think of forces as being like gravity or magnetism, where the closer you get the stronger they are, but they aren't all like that. The strong nuclear force, for instance, pulls things together until you get to a certain distance, then starts pushing things away.

Let's theorize an exotic form of matter that interacts with other instances of itself like the nuclear force, but on human scales. Instead of reversing at .8x10^-15 meters, it reverses at something like 10^6 or 10^8 meters. If matter affected by this force gets pushed together hard enough, it will clump. Maybe the core of the planet has a big clump in the middle, held together by normal matter that it's chemically bonded to. Smaller clumps with normal matter attached would be drawn to this core by gravity at long distances, but then stop where the meganuclear repulsion exceeds the gravitational pull.

So, there you go. Floating rocks.

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No. Orbits require massive speed relative to the object it is orbiting. satellites in low earth orbit complete their circuit in about an hour and a half.

Any orbit shallow enough to be in atmosphere would be unstable at the best of times, but if there actually is atmosphere, it's completely impossible for the same reason airplanes don't stay in the air forever. The same air resistance that slows airplanes down when their engines aren't running will make any floating rocks lose their orbital speed, assuming they don't just burst into flame like an asteroid (which they would).

If the atmosphere were moving fast enough relative to the core, and in the same direction, not to turn the floating rocks into plasma, the air would be thrown off the planet by the force, if it somehow weren't thrown off, it would tear the core apart. And if the core were spinning fast enough in the right direction too, the very spin alone would tear the core apart.

Y'all gonna need some space fantasy junk for that

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I’m not sure if this is what you are looking for but maybe it could be possible if the core was super magnetic and there were massive amounts of super magnets in the floating islands. If none of the current known magnets have the capability to produce that effect then you could explain it with an element that hasn’t been discovered in real life that is more magnetic than any other.

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