So, like my question says, I've been wondering if a rocky planet, that looks much like our planet from the surface (oceans, forests, mountains, ext) could be possible.

I've read through a good number of the other giant-planet threads, and those all talk about one being created naturally (which isn't supposed to be possible). The main problem seems to be too much mass.

So, I was thinking, with what we know now, what if the planet was hollow? Obviously there are problems with it falling apart and such, so that's why I say artificially created, as then it could be stabilized. Maybe. I'm also wondering if that could actually work, without just resorting to "It's technology that we can't even imagine."

The main part of my question, though, is how that would affect animals living and evolving on the surface. Assuming that life started on one place in the planet, how much will it have spread by the time intelligent life gets around to evolving? If man could travel faster than life spreads naturally, would they find places where life hadn't grown yet? (probably on the other side of the planet). How would that affect communication and travel, would we decide to explore space or the rest of our planet first? What kind of huge differences would there be on animals? Could life start in multiple places simultaneously?

I hope I'm doing this right, I just really like the idea of a planet so huge that stuff like that happens.

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    $\begingroup$ what your looking for is a shell world youtube.com/watch?v=TfMr_XkWcEs $\endgroup$ Oct 31, 2016 at 22:29
  • $\begingroup$ Can I be a little malicious and ask "Which surface, internal or external?", cue hollow Earth theories.. $\endgroup$ Nov 1, 2016 at 2:34

3 Answers 3


It couldn't exist within the physics as we know.

The problem is hydrostatic equilibrium - You either need some unobtanium, or make it really small. Well under planetary size.

The smallest body confirmed to be in hydrostatic equilibrium is the icy moon Rhea, at 1,528 km, whereas the largest body known to not be in hydrostatic equilibrium is the icy moon Iapetus, at 1,470 km.

Hollow sphere is really far from equilibrium, so you would need to get significantly under 1,470 km or it is the "technology we can't imagine" you wanted to avoid.

So let's use unobtanium. Timeline of the evolutionary history of life shows that life started about 4,000,000 millennia ago. By the time it got photosynthesis 3,500,000 millennia ago it was already widespread in oceans. When Earth got oxygen in the atmosphere, the oceans were full of life (specifically, the oxygen in the atmosphere was produced by life). It was another 2,000,000 millennia before the first intelligent life arose (us). I think it's unlikely that life wouldn't colonize a Jupiter-size Earth-like habitable planet in such a long time. Ocean currents would spread plant seeds and small fish in only a few years or decades at most. On land, winds will do the same for spores and seeds. It'll take longer, decades or centuries for complete colonization. But we have millions of millennia, and that's more than enough time.

  • $\begingroup$ Wow, that was really helpful. I suppose the planet itself really isn't that possible... still, it's really good to know about the life thing. Maybe I'll find another way to do tons of variety/room, so that's a great reference. Currents and Wind makes me wonder about if the continents were very rare and far between, how would that change the time it takes for plants to grow? I suppose it would take longer. $\endgroup$
    – BookWyrm17
    Oct 31, 2016 at 22:52
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    $\begingroup$ @BookWyrm17 it might force plants etc to evolve separately. But after some time you will get fallen trees drifting in the ocean and some contamination. ecosystems could be radically different. And maybe on some "Australia" there would be no advanced life for some reason. Not "no life at all", that's bit much to ask for, but simple single cellular one, colonies, algae etc? $\endgroup$
    – Mołot
    Oct 31, 2016 at 23:08
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    $\begingroup$ @BookWyrm17 If what you want is 'lots and lots of space', and you're okay with it being artificial, how about an Orbital? A ring in space, essentially like the one in the game Halo but bigger, tilted so that one half is lit by the sun and the other half is in shadow, spinning to produce artificial gravity. You can make them almost as large as you need them to be, and (uncertainty from here) I've seen it reported that you can build them out of ordinary steel. $\endgroup$
    – Werrf
    Nov 1, 2016 at 15:58
  • $\begingroup$ The main thing, @Werrf , was that I was hoping for a way to create a planet that has life evolve on it, and then explore the enormous expanse of land available to them. Would they develop space flight before they explored every continent? How advanced would explorers get before everything actually managed to get as explored as we have it today? Things that would fit that. Though, now that I think about it, life evolving on something LIKE that Orbital, aliens built it and then left it to float, that could be interesting. Hmmm... yeah! I like that idea. $\endgroup$
    – BookWyrm17
    Nov 20, 2016 at 1:02

Earth has a density of 5.5 g/cm3, if your planet has a radius 10 times larger then Earth, its mass will grow as cube of the radius thus having 10^3=1000 Earth masses and gravity decreases as square of distance so you'll end up with 10 g surface gravity which is way too much for anything resembling humans.

If you take a look at periodic elements Hydrogen & Helium are gases, Lithium is way too soft, so you best bet is Beryllium 1.848 g/cm3 which is 3 times lighter then Earth. So if you have lot's of Beryllium, I guess you could make it 3 times larger. If you want to make your humans being able to tolerate 2G, then have it be 6 Earth radii large. And that completey ignores the compression, Earth is 4.4 g/cm3 uncompressed.

But without making your planet of some combination of exotic materials graphene, carbon nanotubes, fullerenes of which I don't know anything about, you simply can not have planet with size of a Saturn.

Check this video and linked paper why

If you insist handwave the physics and build whatever you want, your story your rules.

  • $\begingroup$ Being hollow would drastically reduce the density. Balloons are much less heavy than solid rubber balls of the same radius. $\endgroup$ Jan 28, 2019 at 5:03

There will be several differences (assuming the concept of the giant hollow planet itself are ignored).

For starters, there won't be a molten spinning iron core to generate a magnetic field to block solar radiation. This will probably sterilize the planetary surface. But we can handwave that away.

Second, you have WAY more planetary surface without a corresponding increase in gravitational pull. This means that planetary impacts and volcanic eruptions may not be nearly as devastating to the entire planetary ecology, so you won't have as many biological set-backs. So intelligent life may evolve significantly faster than it did on Earth, and you are much more likely to have simultaneously existing sentient species (because the entire planet is less likely to be reset).

Your day/night cycle is going to be MUCH longer unless the planet spins much faster, but we'll just incorporate that into the handwaving of the planets construction in the first place.

What about tectonic activity and volcanic activity? This is very helpful in altering the face of the planet. If the planet is geologically quiet, eventually mountains will wear down and no new ones will be built. Without isolated pockets and the collision of these pockets together (as new land bridges form and continents separate and collide) you may see much less turn over of dominant species. This may accelerate sentience (allowing species to develop unmolested) or greatly retard it (by reducing selection pressures that favor developing intelligence), your call.

A planet like this was featured in the 1980s novel Threshold by David R. Palmer. Massive planet with a mostly Earth like biome of incredibly diverse species.


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