I'm working on the setting for a fictional story I'm writing and I wanted to inquire as to how possible massive-Universe-sized planets are radius: 5 × 1030M, without me having to re-make the laws of Physics.

I usually dislike it when I encounter works, in which it seems the author doesn't even have a grasp on Physics, and one would have to completely abandon Physics to enjoy such fiction. So I want my story to be coherent with the Laws of Physics as much as possible.

My story, is sci-fi, and fantasy. It uses "magic", but such "magic" is bounded by internal Laws and logic. The "magic" is somewhat similar to "Mahouka Koukou no Rettousei".

Anyway, in the particular realm in which these planets occur, they are bounded by the Laws of Physics as they appear in our Universe.

String Theory has been established as correct.

The Planet-Universe, generate their own source of heat and light.

I want to know what the Physics and Chemistry of such a planet would be like. SO that I can adjust my story suitably while keeping in-line with Physics.

I was thinking of the extreme gravity, condensing the universe, until it was a solid sphere. The ejection jets are absent, due to the gravity, as they too have been solidified.The gravity has caused the planet verse, to attain equilibrium.

How does it sound, I'll appreciate suggestions.

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    $\begingroup$ A planet that large would be a black hole. So somehow general relativity is going to have to go. $\endgroup$
    – David Z
    Mar 24, 2016 at 17:39
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    $\begingroup$ To put David Z's comment in context, any blackhole starts as a tiny cloud of gas ("tiny" in relation to the universe or your planet, so your planet would have to have a density many, many orders of magnitude less than those of an hidrogen cloud in the interestellar void). That would not mix well with the science-based tag. $\endgroup$
    – SJuan76
    Mar 24, 2016 at 17:53
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    $\begingroup$ What do you mean with "somewhat similar to Mahouka Koukou no Rettousei"? I have never heard of that franchise and I am quite sure lots of others haven't either. Don't assume everyone has the same reference pool you have. $\endgroup$
    – Philipp
    Mar 24, 2016 at 18:08
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    $\begingroup$ Your planets have a radius of about 1000x that of the observable universe. If you want the planet to not be a black hole, then you need a density of less than $6.43×10^{-36} \frac{kg}{m^3}$. Deep space is about 14 orders of magnitude denser. $\endgroup$
    – Lacklub
    Mar 24, 2016 at 18:17
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    $\begingroup$ Are you certain you're using the term "universe" correctly here? What are you trying to achieve? Because a planet that's the size of the universe, even if possible (see answers), would take up the entire universe (by definition), so there wouldn't be anywhere for the planet to be. Are you, perhaps, looking for a planet that's sufficiently large to have a number of different environments, so one could travel between them without going to space? Larry Niven thought of this, and it's named Ringworld. $\endgroup$
    – J.D. Ray
    Mar 24, 2016 at 21:36

2 Answers 2


First, a lot of reasons why this won't work in our universe. Then, an interesting solution. Finally, why the exercise is pointless.

5.0x1030 meters is about 10,000 times larger than the observable universe. That doesn't mean it's larger than the universe, but it does mean that an event on one point of the planet has only had enough time to reach 1/10,000th of the rest of the planet. There has not been enough time in the whole universe for light (and thus anything) to go from one side of the planet to the other. This means there hasn't been enough time for gravity to form it into a sphere, the gravity from one side has not had time to affect the other. One has to wonder how this planet formed in the first place.

Either your universe is very, very, very, very much older than ours, or causality (ie. the speed of light) propagates much, much, much, much faster. Both of these have serious consequences. All sorts of important equations depend on the speed of light, so I'd leave that alone. Make your universe tremendously old. At least 1015 years.

Then there's the question of where this planet is getting its light and heat from? A scaled up star would require an even older universe to be given time to form. Then that requires that your planet rotates. With a circumference of 3.14x1031m, even the smallest rotation will have the surface dwellers moving at the speed of light. There's no way to have anything like a normal day/night cycle, even light will take 1023 seconds to get around the surface. One side bakes, the other side freezes.

An alternative heat source would be better. Our universe has no edge, but perhaps your universe does and it produces light and heat as it gobbles up whatever is "outside". Your planet is in the center of the universe and the surrounding edge is like being inside a spherical heat lamp. Your planet would be in perpetual daytime.

Be careful not to cook the planet by adding to the overall energy of your universe.

Then there's gravity. @Philipp already covered that well, can it be solved? One solution is to mess with the gravitational constant. This will severely alter the makeup of the universe on a large scale, but I'd say that's already happened.

How much smaller should G be? We'll solve g = Gm/r2 for G and plug in an Earth g of 9.8 m/s2.

g      = Gm/r^2
gr^2   = Gm
gr^2/m = G

Plug in your numbers... r = 5x10^30 m, m = 2.89×10^96 kg from @Philipp, g = 9.8 m/s^2. We get a G of 8.4x10^-35 N⋅m^2/kg^2 which is 24 orders of magnitude lower than in our universe. Gravity is already very weak, in your universe it's all but undetectable.

Such weak gravity would make your planet take even longer to form. It's questionable gravity could ever form a planet given how much stronger all the other forces would be.

An alternative way to solve the gravity problem is to make the planet hollow. This reduces its mass which reduces its surface gravity. The surface gravity of a hollow planet is a bit harder to calculate, so let's move on to the next problem: how did this shell form and how has it not collapsed?

One possible solution is to put a small black hole in the center of the planet. The "edge" of the universe surrounding the planet is the inverted surface of a white hole. In effect, the planet is both outside and inside a black hole. Material falling into the black hole heats as it falls. This heated material comes out the white hole and creates the warm glow in the sky which heats the planet. As the planet is slowly eaten from the inside, a fine rain of material is falling on the surface from the white hole surrounding it.

This is its geological cycle of renewal, matter on the surface is eventually buried and sinks down to be eaten by the black hole. That same matter falls from the white hole which then rains down on the surface to start the cycle again. Like our own geological cycle, this would be unobservably slow to humans.

The surface of the planet is a standing wave in this flow of particles: a cosmic traffic jam.

This allows your planet without altering gravity or the age of the universe. This is an entirely different universe looping forever.

Even this leaves the basic question of why have a planet so large the inhabitants don't even know they're on a sphere? At your scale the planet would appear to be flat. The horizon would not appear to curve, ships would not sink over the horizon because they could never reach it. There would be no round, orbiting bodies to hint at the shape of the planet because a planet that large with Earth's surface gravity can't hold on to things in orbit.

Your characters would think they live on a flat, infinite plane and there would be little they could do to prove otherwise. Your sphere is so big they could travel in any direction and never have evidence otherwise. Even at the speed of light for 100 years they'd have gone 1018 m or 1/1013 the circumference, not enough to detect a curve.

From a storytelling perspective, what does living on a giant sphere indistinguishable from a flat, infinite plane gain you?

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    $\begingroup$ +1 for pointing out this is functionally indistinguishable from an infinite plane. $\endgroup$
    – Joe Bloggs
    Mar 24, 2016 at 21:04
  • $\begingroup$ The novel "The World is Round" by Tony Rothman (not the one by Gertrude Stein!) takes place on an otherwise earthlike world that is the size of a gas giant. (Maybe bigger.) They even mention the point that ships in the distance disappear before reaching the horizon. Also, the seasons are unusual due to the size and rotation. $\endgroup$ Aug 11, 2016 at 0:14
  • $\begingroup$ That sounds a lot like the universe of Rain World. Who knows, maybe the Void Sea is actually a black hole $\endgroup$ Jul 14, 2020 at 15:23

It's impossible for a planet of that size to exist in a universe with our physical laws.

A planet with a size of 5.0x10^30m would have a volume of 5.24×10^92m³. Assuming the same density as Earth (5514 kg/m³), this planet would have a mass of 2.89×10^96 kg.

The surface gravity would be 7.7×10^24 m/s².

The escape velocity on the surface would be 3×10^19 times the speed of light, and when you have an escape velocity higher than the speed of light, you have a black hole (in this case the event horizon would have a radius of 4.29×10^69 meters, if you really want to know).

I'm sorry, but your mega-planet isn't going to work in this universe, unless you drastically reduce the gravity constant of the universe or construct the planet from some unobtanium material with a ridiculously low density.

  • $\begingroup$ Is it possible within the limits of common science-fiction? $\endgroup$ Mar 24, 2016 at 18:48
  • $\begingroup$ @TobiAlafin Science-fiction has no limits. And there is no such thing as "common" science fiction. That's what makes science fiction so interesting in the first place :) $\endgroup$
    – Philipp
    Mar 24, 2016 at 18:49
  • $\begingroup$ Did the OP mention that the planet is actually a balloon with a rigid shell and absolute vacuum inside? :^) $\endgroup$ Mar 24, 2016 at 19:46
  • $\begingroup$ The tag did mention "common" in its description. Sorry, please can I get an explanation that will allow such planet universes. The density is very high, so I went with the Black Holes solidifying. become a solid sphere, kind of like our moon. The gravity is in an equilibrium state. How does that sound. $\endgroup$ Mar 25, 2016 at 5:07
  • $\begingroup$ @TobiAlafin According to the current state of astrophysics, black holes don't "solidify". $\endgroup$
    – Philipp
    Mar 25, 2016 at 10:38

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