The planet's radius is 15km and has a similar density and gravity to Earth. For this question, most laws of our universe do not apply, meaning this is possible without any serious consequences. Could this planet support a population of about 650,000 people using only materials from the planet? What percent of the planet would they need to use?

Other info (most likely won't be needed):

  • The day/night cycle is 8 hours long total.
  • The sun's energy given per km² is about the same as earth.
  • The planet is 2/7 water, 3/7 used land, and 2/7 unused land.
  • The planet is made of mostly the same things as Earth.

EDIT: Some of you were confused by what I meant by disregarding physics. I meant disregarding the fact that a planet with this size and gravity is impossible, and most other things should by mostly similar to Earth.

EDIT 2: Please assume that the planet is completely composed of crust. By people I mean humans, and the technology level is around Earth's 1960s. If the current population of 650k cannot be supported, it would be great if you tell what number could.

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    $\begingroup$ What is it (disregarding physics) made of? $\endgroup$ Commented May 11 at 19:25
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    $\begingroup$ If we are disregarding physics, what do the words day, night, energy, water and life even mean? Anyway, when disregarding physics everything is possible. $\endgroup$
    – AlexP
    Commented May 11 at 19:32
  • $\begingroup$ Could you give us a bit more to go on - Earth's core is about 3500 km in radius, the crust 35km deep. Could you tell us about the geological history, the atmospheric depth, oceans, minerals. There's a lot missing from the question, a lot of assumptions we'd need to make before answering. Best you fill us in. $\endgroup$ Commented May 11 at 20:07
  • $\begingroup$ You may want to ask about an isolated valley in the mountains with an area of 2,800 sq km... The point is that because the planet is so small, the gravitational acceleration will vary a lot with altitude, and the atmosphere will behave very differently than how it does on Earth. (Behave very differently means that it will be almost all gone in a blink of a geological eye.) $\endgroup$
    – AlexP
    Commented May 11 at 20:08
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    $\begingroup$ @g s Actually it is. There are plenty of small countries(mostly islands) that are similarly small, but either were able to are did have the agriculture to feed everyone. Eg Mauritus, while it technically imported 2 thirds of it's food, only 4% of it's crop production was for eating(with most of the rest being sugar cane), and if instead of growing cane they grew grains and such, they would have been fully self-sufficient. With 680k people. $\endgroup$
    – Bubbles
    Commented May 13 at 1:11

5 Answers 5



The key is preparation and efficiency.

To kick-start the colony, take computers, LASER sintering equipment for building, ceramic crucibles, rations, 3D printers (+ 3D LASER-sintering for metals) and plenty of power supplies in the form of solar + batteries or fusion-based (+ mattresses, bedding, cloths and personal stuff). You'll also need some raw materials to get you started: Phosphorous, copper, sulphur, calcium, molybdenum, sodium, potassium, zinc etc.. The total you'll need of these shouldn't be more than a few metric tons.

Infrastructure for dwellings and streets would be assembled by basic robots gathering the local equivalent of regolith, LASER sintering bots would then move-on to making room for administrative buildings, entertainment, manufacturing areas, laboratories and importantly food-production.


Caloric density: Sweet potatoes can give us 70,000 kcal per hectare per day. Which is enough energy yield for 2300 people. That's conventional farming.

We can do better.

Aeroponics can give us a vastly increased yield. Ultrasonic foggers are used to create a fine mist of nutrients surrounding the roots of your chosen crop-plants. Oxygen is allowed to circulate freely, giving the root-system greater chances to thrive than in soil or hydroponics. It allows strict monitoring of nutrient uptake, oxygen and CO2 and tweaking of the balance according to the plants' needs. This all occurs within a sealed and environmentally-controlled space, all that's needed from outside is sun.

Yield over conventional farming when using vertical aeroponic systems are estimated to be between 220-600 times greater than conventional farming. For caloric value alone, that would mean supplying the energy needs of 1,300,000 people per hectare (with sweet-potatoes alone). Various types of bean, brassicas and berries should be grown to round-off the diet. Animal protein can be supplied from gastropods, yeast or fish-farming, perhaps insects - if thought necessary. Alternatively, meat culturing has come a long way in the last few years, but I'm not able to find the per-land-yield at present.

Just for reference, figures of 1,300,000 (1.3 million) people's energy-needs met per hectare, refer to an area 1/100 of a square kilometer - that's less than a thousandth of a percent of the total area available on your planet.

Multi-layered systems would give a greater yield (with the addition of artificial lighting). Advanced closed-loop recycling systems much like the ESA's CO2 recycling systems would apply to all nutrients from crop-waste through poop, all essentials would be ploughed back into the system. Including of course, water.


Glass is made from silicon dioxide (found everywhere) with a few additives and is recyclable 100%. Metals are recyclable, stainless should be preferred for longevity over iron. Plastics can be manufactured from plant-waste, same with rubbers - silicon-based rubbers (containing silicon, carbon and hydrogen), all easily available. Just requires energy - for which you've the sun. (Possibly you can assume that your space-faring race has perfected fusion-reactors, adding even more easily available sources). Clothes and household materials can be made from plant-fibers/bioplastics and fungi-based fabric.

Play it right, and your colony could become very successful indeed and a shining beacon of what's possible in terms of efficient use of land.

  • $\begingroup$ Thanks! Helpful, but probably not in the way you would expect ;) $\endgroup$
    – value1
    Commented Jun 3 at 15:16

I think other proposed answers are a bit unimaginative. Of course you can have a 15km radius planet with Earth-identical conditions on the surface. It just needs a small black hole at its core and to be surrounded by a belt of extremely dense matter which cancels out the excesses of the black hole gravity. Or something. Terry Pratchett has a planet balancing on the back of four elephants which are standing on a giant turtle which swims through space; so disregarding physics is fine.

But to answer your original question, as another commenter points out, this gives a total surface area of 2827km2. If it were a flat rectangle, that would be 40km by 70km. That's a tiny area of land for anything to take place. Given your proposed population of 650K, that's a population density of 229 people per km2. In the "used land" (1211km2), it's a population density of 536 people per km2, which is about the same as the Netherlands.

A bit of searching on Wikipedia reveals that Luxembourg has an area roughly the same as your planet and a population density about the same, although it's less than 1% water. This list might help your thinking further: https://en.wikipedia.org/wiki/List_of_countries_and_dependencies_by_area

Could the population of 650,000 be supported? Well, anything is possible, as I say above, but with 1960s technology and only using materials from the planet, I'd say no. The 1960s were already globalised and getting 650,000 people to mine and refine all their minerals, produce fertiliser, maintain a telecommunications system, educate future generations, provide health care, etc. would be a lot.

However, it would all depend on the starting conditions. Nuclear power is 1960s technology, so if they were dropped there with a long-lasting nuclear power plant plus a bunch of other utilities that last essentially in perpetuity, they might manage to survive.

What population could be supported at 1960s technology? I'd say in the long term, 0.

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    $\begingroup$ It is probably worth noting that Luxembourg is a net importer of food - this page says that although it produces all the and dairy than it needs and even exports some, it only makes 3-5% of the vegetables it needs and only 1% of the fruit. Annoyingly the page doesn't mention grain, but I imagine they import a lot of that as well. So to support a population the size of Luxembourg probably requires a land area somewhat bigger than Luxembourg, although if there is a lot of good arable land it might be possible. $\endgroup$
    – N. Virgo
    Commented May 12 at 9:54

No(unless you ignore a lot of physics). The problem is that bodies so small have very weak gravities and almost none of the things needed for sustaining life. It would have no atmosphere, which means no liquids(some kind of liquid is needed for organic life), and the gravity would be almost zero. Something this small probably cannot support any life without extensive assistance(domes), and this is not possible with 1960's tech. To make it habitable, you either have to throw out a lot of physics, or use tech that is far past the tech level described.

However, if you ignore all that, and only look at the space available, absolutely. There are many countries with much higher population densities, so you could support that many people in that amount of space. Of course, though, the above problems apply.

Remember though, the rule of cool reigns supreme, so if you are fine with disregarding those issues, go for it!


I like Escaped dental patient's answer for a maximalist estimate.

If instead we're using conventional agriculture, and we assume crops have been specially modified or selected to perform as well as Earth crops despite the changed day cycle: you need about 1.2 hectares per person for agriculture. That's 0.012 square kilometers per human.

Assuming that the 3/7 of used land means land usable for agriculture: 3/7 of the surface area of a 15km sphere is about 1200 square kilometers. Divide to get the maximum sustainable population.

$\dfrac{1200 km^2}{0.012 \frac{km^2}{\text{human}}} = 100,000 \text{human}$


You need some way to provide gravity for them.

Natural control mechanisms of our atmosphere mostly won't work. For example, probably there will be no rains, instead a continuous, more vaporous weather.

Things will be yet more unstable as on the Earth. If anything badly, there will be nowhere to go and nothing will compensate it.

However, the temperature difference between the polar and equatorial regions will be much smaller, maybe even nothing.

They will likely need many terraforming and weather control machines to keep things in track.

Probably they could not survive with the today technology. You have only 800 $m^2$ used land for a human. On the today Earth, we have about 4 times more.

If gravity decreases quadratically with the distance, then it decreases much faster as on the Earth. That would mean that in 15km high we only have 1/4g acceleration. That will make the atmosphere escape in hours, maybe in days.

The thing what provides the gravity, must keep an Earth-like gravitational field even a hundred kilometer high, not only on the surface.

  • $\begingroup$ Sorry if I wasn't clear; I'm looking for if it's possible for a planet of that size to support the given population materialwise. $\endgroup$
    – value1
    Commented May 12 at 4:09

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