In figuring out the rough timeline for my sci-fi, I've run into an interesting dilemma. As it stands right now, There are about 90 years in between one of my civilizations discovering space travel and them building an empire spanning several dozen planets and two or three hundred star systems in their neighborhood of the galaxy. Now, with the level of technology that they start with, I'm not too worried about their ability to reach and establish outposts and industry on these planets. The problem comes when I look at the list of three or four city-planets in the new empire.

So, to the main issue in this question. Assuming that establishing the colony is easy enough and no diseases or natural disasters wreck the population, how long would it take for a colony of sentients with human-like lifespans and reproductive cycles to turn an Earth-like planet into an ecumenopolis?

Notes and rules:

The empire in question starts with a planet with a population of about 20 trillion.

The empire in question starts its expansion with conquest and federation, adding 4 other developed planets and 1 ecumenopolis with 120 trillion inhabitants in the ten years after gaining space-faring status. The inhabitants of these other planets are of different species. They can still be used for colonization purposes, though they would not be the preferred type.

Cloning technology is available but seen as immoral by the empire.

Don't depopulate the starting planets by more than 10% of their populations.

Food is plentiful.

Robotics and large scale industry makes building the cities relatively easy.

Minimum population density for me to consider it an ecumenopolis is 2000 people per square mile.

Let me know if any other details are needed.

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    $\begingroup$ The title says "ecumopolis" (which is bad Greek) and the body uses "ecumenopolis" (which good Greek, albeit with American spelling). What I never undestood is what is the purpose of those all-world cities: what do they do, why doesn't the empire spread out the population to thousands of planets which at least could grow their own food... $\endgroup$
    – AlexP
    Mar 12, 2019 at 0:11
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    $\begingroup$ the 120 trillion then are of another species? If not you've already established that the species has about 110 children per mated pair(assuming mated pairings ofc.) $\endgroup$
    – Giu Piete
    Mar 12, 2019 at 0:21
  • $\begingroup$ Read the classic Foundation series by Isaac Asimov. The great writer wrote about a galactic empire with just one city-planet in an entire galaxy and the novels cover why even one of these is a problem. As @AlexP said, what do these cities do ? Why don't they live in space instead (as in e.g. IM Bank's Culture novels) ? What's the advantage of a city-planet ? $\endgroup$ Mar 12, 2019 at 1:52
  • $\begingroup$ What's depopulating got to do with anything? Are you saying we should take 10% of the original population and make 120 trillion out of them? That's every individual adult producing 60 adult offspring in 10 years, is that what you're asking for? They're definitley not human then. If not what are you asking? $\endgroup$ Mar 12, 2019 at 2:03
  • $\begingroup$ I'm guessing the depopulating means from emigration $\endgroup$
    – coagmano
    Mar 12, 2019 at 5:19

2 Answers 2



Without limitations on population growth like disease, food or space, this is going to be the same sort of curve regardless of your starting assumptions. In this curve I started with 2 individuals, assumed 5 offspring per 25 years for each 2 individuals < 50 years old, each person dies at 100.

1e+13 is 10 trillion. Of course the more people you start with the faster you get to 10 trillion. Starting with 2 it takes 600 years. Starting with 10,000 it takes 425 years. Starting with 100,000,000 it takes 225 years.

  • $\begingroup$ Excellent, thanks. Could you share your methods/formulas for making the graph? It would be useful to change around the variables and run it again. $\endgroup$ Mar 12, 2019 at 3:12
  • $\begingroup$ It is Excel. My profile has my email. Send me an email and I will send you the file. If you are unfamiliar with excel let me know and I will add remarks explaining. $\endgroup$
    – Willk
    Mar 12, 2019 at 14:25

FTL Changes Everything

With FTL space travel, more and more people will move offworld where they don't have to compete for homes, jobs and resources. Orbital habitats and colony worlds offer cheap land where you can raise your family as well as easy to find work and other opportunities.

Any growth in the population needs to be put against migration outwards. Population increases where there is no ability to expand outwards.

  • $\begingroup$ you have to put your comment against the difficulties of moving to a rural world. Look at the current situation of our world, population is moving towards big cities and not the other way arround $\endgroup$
    – Sxubach
    Jan 20, 2020 at 11:01

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