Can a habitable planet have two separate crusts, both of which can sustain life? I am wondering if through the planet's creation, over time through some form of mantle cooling/erosion combination, without any intentional artificial construction, a planet can have two crusts, with air in between.

The larger the distance between the two, the better. Ideally, the distance between the two crusts would be greater than 12 km, allowing for flight. I know the atmospheric conditions would be vastly different between the two crusts, and I'm wanting the bottom crust to have similar conditions atmospheric conditions to those on earth, even having weather patterns. This would lead to an insanely thin atmosphere around the top crust.

The only way I can see working is some form of massive cave system forming in one of the thickest crusts ever developed or some columns that remain from erosion that has removed an area to create a troposphere. For simplicity sake, I'd say we just assume the top crust has enough holes to allow in light to sustain life on the bottom crust.

I do not know how to get the area between the two crusts sufficiently lit, but that would help too. The implications on society, evolution, and life in general, would be extremely fun to explore, but I want to know if the world is even possible without some sort of magic intervention.

  • $\begingroup$ Welcome to Worldbuilding, Son of Thrain! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$
    – Dubukay
    Jul 26, 2018 at 2:50
  • $\begingroup$ Related, if not even duplicate: worldbuilding.stackexchange.com/q/116146/30492 $\endgroup$
    – L.Dutch
    Jul 26, 2018 at 3:58
  • $\begingroup$ After reflection, and to further distinguish this question from the very good question posed by @crZyhamSter, I think it would be more interesting to theorize the creation of the crusts as completely natural. $\endgroup$ Jul 26, 2018 at 4:07
  • 1
    $\begingroup$ I added a tag to this question. You probably want to check out the other hollow-Earth tagged questions. $\endgroup$
    – SRM
    Jul 26, 2018 at 4:22
  • 1
    $\begingroup$ You want a habitable planet. Do you want the two levels habitable for the same species? Something human-like, even? $\endgroup$
    – o.m.
    Jul 26, 2018 at 15:49

6 Answers 6


For a given value of 'atmosphere' we already know of one such planet. Well, Moon.


The smallest moon of Jupiter, Europa is interesting because it is hypothesised that the surface is made entirely of a thick layer of ice. How does this match your scenario? It's further hypothesised that under that ice, thanks to the heating produced by the Tides of Jupiter (awesome title for a book if ever I heard one) there is a liquid subsurface ocean.

It's not too much of a stretch to imagine a world with a rocky, pretty hot core entirely covered in water and cold enough at the surface that there is a constant layer of ice. Now: We know that life can evolve in oceanic abysses and we also know that life can survive in arctic conditions, so it's entirely possible to get two interlinked biospheres going (though interestingly you'd probably evolve photosynthetic life really late).

You'll also have to watch out for global warming.



At least not for a world. Part of the definition of a planet is a body so large that gravity pulls it into a roughly round shape.

You can't get two surfaces without pillars and roof beams to hold the outer shell up and those are not going to be natural.

The suggestion of light rock floating on atmosphere are unworkable. You might get rock floating on water, but water is much denser than any atmosphere of a "habitable" world for life as we know it.

  • 1
    $\begingroup$ Yes, that is certainly the consensus that was reached when I asked my own question about unusual planet formation. Answerers contended that above a certain mass, the body will be sphere-like and anything else would not be natural. This has bearing on whether two crusts can exist or not as well. The pillar and roof analogy is very fitting. $\endgroup$ Jul 26, 2018 at 10:06
  • 4
    $\begingroup$ Another answer speaks of Europa, which has a thick layer of ice above a liquid ocean. In such a planet, is there a reason why you couldn't have seamounts or other volcanic formations that build to the surface, creating a system of pillars, releasing ash and other materiel into the atmosphere, which could accumulate to form a crust? $\endgroup$
    – Dent7777
    Jul 26, 2018 at 13:12
  • 5
    $\begingroup$ @Dent7777 Europa might be the closest we can get to at least sustaining life in the bottom crust. However, the issue with Europa is that it's a liquid world close to the bottom crust. The OP seems to want an atmosphere similar to Earth's in the lower crust. $\endgroup$ Jul 26, 2018 at 14:46
  • 1
    $\begingroup$ @ArashHowaida I'm getting away from hard science here, but the liquid could theoretically be drained or removed in some way after the creation of the seamounts and crust, leaving a pillars-and-roof setup. $\endgroup$
    – Dent7777
    Jul 26, 2018 at 15:16
  • 2
    $\begingroup$ @Dent7777 Neither rock nor ice is strong enough to allow that. It would bend and break as soon as the water level started dropping. We know that from right here on Earth, that is how sinkholes form. $\endgroup$ Jul 26, 2018 at 19:06

One possibility is that the upper "crust" is alive. Imagine that the entire planet became dominated by a single type of tree, hundreds of feet tall (or even thousands for a lower-gravity planet). At the top it spreads out into an extremely dense layer of branches and leaves a few tens of feet thick that capture essentially al the sunlight, leaving everything below that layer in perpetual near-darkness.

In such an environment the tops of the trees would be a host to a wide variety of plants, animals, and fungi that never touch the ground. Air plants would try to get sunlight by sitting above these enormous trees. Climbing and flying animals would feed on leaves, sap, fruit, and each other.

The ground would be like the deep sea on Earth. Animals and fungi would feed on things falling from above. A few very tough, very slow-growing plants or lichens might be able to live off the tiny amount of sunlight.

But the area between the ground and the tops of the trees would be essentially devoid of life. There would be nothing to eat there, so only a few large flying or climbing hunters that hunt both on the surface and on the ground would ever travel between the layers

Since the branches are so thick, cities could be built on top assuming they are careful and spread out

You could also add some interesting mechanics this way. Even trees eventually die, which means these massive trees will eventually fall. This would be like a skyscraper collapsing, causing massive destruction to everything above and below, but also allowing sunlight down to the ground for a little while, which causes a massive flurry of plant and corresponding animal growth in that area.

You could extend it even further by having the trees be massive horizontally as well. Such tall trees would be able to fall over easily, so as they spread out horizontally the lower trunks from their branches to the ground banyan-style. These start of like vines, but once they reach the ground they thicken and harden into new trunks. So a single tree could have many trunks and cover an enormous area. If you want you could even make them country-sized. Imagine what would happen when on of those dies. For a primitive culture it would be civilization-ending.

  • $\begingroup$ David Drake set one of the Hammer's Slammers stories on a continent like this. $\endgroup$
    – Jasper
    Jul 27, 2018 at 1:28

A ceiling made of foam balloons

Suppose there is a life-form that produces thick, lightweight, closed-cell foams. Perhaps polyurethane with a density of 2 - 3 pounds per cubic foot (about 0.03 - 0.05 g / cc).

Suppose that a version of this life-form evolves that forms giant bubbles, whose surfaces are made of the foam, and whose interior is a flotation gas (such as hydrogen or even methane). The bubble-former could even be a different organism from the foam-former.

Suppose that more of these giant bubbles rise than fall, for a long period.

Suppose that the floating bubbles can get glued together.

You could wind up with an earth-like atmosphere, which has a foam ceiling. The topside of the foam ceiling might even be walkable.

The ceiling height would probably only be a few thousand feet high, instead of the 40,000 feet requested in the original post. But it would still be high enough to enable flying underneath the ceiling.

The ceiling would probably interfere with rainstorms and other weather phenomena.


Possibly if you vastly different densities. You could have rock, then a heavy gas or liquid, with a lighter rock floating on it or protruding through it into another atmosphere.

We have that on Earth if you think of the oceans as an atmosphere, both atmospheres on Earth support life.

  • $\begingroup$ Very true! I hadn't thought about having a liquid in between! Although, I'm wondering if it'd work with a gas. Lighter rocks wouldn't be able to float on a gas, despite their little density. $\endgroup$ Jul 26, 2018 at 4:04
  • $\begingroup$ Not without using another force to combat gravity for the rocks, but you could just have the rocks rise through both mediums as on earth, islands and continents don't float, they come all the way from the sea floor $\endgroup$
    – Kilisi
    Jul 26, 2018 at 4:22
  • $\begingroup$ An "iceball" world like Europa does have two hard surfaces, with an ocean in between. I'm not sure if the ice surface is coherent enough to be called a crust, though. $\endgroup$
    – Jasper
    Jul 26, 2018 at 5:49
  • $\begingroup$ @Jasper Dammit: You ninja'd my answer with a comment. $\endgroup$
    – Joe Bloggs
    Jul 26, 2018 at 5:53
  • $\begingroup$ Miles of ice sounds pretty coherent to me! $\endgroup$
    – Ruadhan
    Jul 26, 2018 at 8:09

I was thinking that a Dyson's sphere almost answers this question. There isn't the atmosphere involved, nor is there an inner crust, but rather a gaseous surface. So what if you had a Dyson's sphere inside a Dyson's sphere. The inner Dyson's sphere could have maybe 50% of its surface as holes to allow sufficient sunlight through to power the second Dyson's sphere.

  • $\begingroup$ Welcome to Worldbuilding, Paul! If you have a moment, please take the tour and visit the help center to learn more about the site. $\endgroup$ Jul 26, 2018 at 17:36

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .