At some future juncture, we’re going to need more living space, whether it be found on another planet or through the expanse of our planet’s existing surface area. In his latest venture into worldbuilding, Oxford University research fellow Anders Sandberg explores some of the more extreme possibilities.

For the thought experiment, “What is the largest possible inhabitable world,” Sandberg limited his investigation to describing hypothetical worlds — both natural and artificial — with “maximal surface area that can be inhabited by at least terrestrial-style organic life of human size and is allowed by the known laws of physics.”

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That’s a lot to work with, including heavy super-Earths, large low-density worlds (like a carbon planet), and artificial worlds like shellworlds, gas-propped bubbleworlds, and topopolises (dubbed “cosmic spaghetti”).

A depiction of a carbon world surface. “The local geology is dominated by graphite and tar deposits, with diamond crystals and heavy hydrocarbon lakes,” writes Sandberg. “The atmosphere is largely carbon monoxide and volatile hydrocarbons, with a fair amount of soot.”

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The most obvious candidates for natural large worlds are super-Earths, including Sandberg’s double Earth, which are rocky planets with lots of mass. He describes variations of these worlds depending on their chemical composition and density. In many cases, these large planets would take on the form of waterworlds and warm Neptunes, making habitability impossible, or at least very difficult. In one instance, he describes a hypothetical iron-rich planet 274 times heavier and 2.7 times larger than Earth featuring a surface gravity 38 times Earth’s.

“This is not habitable for humans (although immersion in a liquid tank and breathing through oxygenated liquids might allow survival),” he writes. “However, bacteria have been cultured at 403,627 g in centrifuges!”

More reasonably, he describes a 359 times heavier and 5 times large ice world with “just” 14.3 times our surface gravity. “Humans could probably survive if they were lying down, although this is way above any long-term limits found by NASA,” he writes.

To get around the gravity issue, Sandberg takes a page from Hal Clement’s Mission of Gravity in which a planet is rotated fast enough to produce tolerable gravitational conditions along the equator. Again, not ideal.

A sketch of a shellworld via Andart II.

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Sandberg’s analysis of artificial worlds, i.e. megascale structures, is particularly fun. On the prospect of onion-like shellworlds he writes:

Consider roofing over the entire Earth’s surface: it would take a fair amount of material, but we could mine it by digging tunnels under the surface. At the end we would have more than doubled the available surface (roof, old ground, plus some tunnels). We can continue the process, digging up material to build a giant onion of concentric floors and giant pillars holding up the rest. The end result is akin to the megastructure in Iain M. Banks’ Matter.

Sandberg goes so far as to describe a shell world with a surface area of 1.2 x 1022 square meters, or about 23 million times Earth’s area. The trouble with this world, and many like it, is the dearth of gravity.

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Indeed, a central theme that runs through Sandberg’s analysis is how impractical and unwieldy these large planets and structures might actually be. Sandberg concludes his thought experiment with this discussion:

Why aim for a large world in the first place? There are three apparent reasons. The first is simply survival, or perhaps Lebensraum: large worlds have more space for more beings, and this may be a good thing in itself. The second is to have more space for stuff of value, whether that is toys, gardens or wilderness. The third is to desire for diversity: a large world can have more places that are different from each other. There is more space for exploration, for divergent evolution. Even if the world is deliberately made parts can become different and unique.

Planets are neat, self-assembling systems. They also use a lot of mass to provide gravity and are not very good at producing living space. Artificial constructs can become far larger and are far more efficient at living space per kilogram. But in the end they tend to be limited by gravity.

Our search for the largest possible world demonstrates that demanding a singular world may be a foolish constraint: a swarm of O’Neill cylinders, or a Dyson swarm surrounding a star, has enormously much more area than any singular structure and few of the mechanical problems. Even a carefully arranged solar system could have far more habitable worlds within (relatively) easy reach.

There’s much, much more to Sandberg’s analysis. Be sure to read it all.


Contact the author at george@io9.com and @dvorsky. Top image of Gliese 667 Cb by ESO/L. Calçada - ESO