I’m looking to create a situation where there are 3 habitable planets (or moons) in the same system, each with different inhospitable but habitable (by genetically modified humans or humans using supportive technology) environments.

I’ve been roughly basing it on the TRAPPIST-1 system, as it has 4 planets in the habitable zone, but I don’t know if there is a simpler way to have 3 habitable but different environments in relatively close proximity (maybe using moons)?

I thought there could maybe be artificial magnetosphere generators to protect the planets from solar flares (maybe something like this? https://www.sciencedirect.com/science/article/pii/S0094576521005099 ), although maybe the worst of the flares would not affect the planets ( https://phys.org/news/2021-08-superflares-exoplanets-previously-thought.html )

Planet A (closest to the sun, based loosely on TRAPPIST 1d)

Terrestrial planet with orbit of 4 days

Tidally Locked

Eyeball Planet

Closest to Star, so hot and barren on star side, water on night side (flowing sea under ice cap).

0.6 earth mass (in reality Trappist 1d is 0.3 earth mass, so not sure how this change will things)

Life is possible around the terminator (I thought of maybe cloud cover to increase the habitable zone, but apparently that may make it too dark for photosynthesis https://www.centauri-dreams.org/2013/07/05/red-dwarfs-clouds-in-the-habitable-zone/ ).

Requires terraforming to create atmosphere.

Planet B (second closest to the sun, based very loosely on TRAPPIST 1e)

Terrestrial planet with orbit of 6 days

0.8 earth mass

Kind of like a dim earth

I would prefer that the last two planets are not tidally locked, and apparently the presence of an atmosphere avoid tidal locking ( https://physicsworld.com/a/exoplanets-could-avoid-tidal-locking-if-they-have-atmospheres/#:~:text=For%20example%2C%20a%20planet%20orbiting,and%20liquid%20water%20can%20exist. ), so ideally this planet is not tidally locked as it already has an atmosphere (though I don’t know if a thin earth like atmosphere would be sufficient in this case?) Also, at the end of this article it mentions that “the close-in worlds of small stars face other challenges. For example, a planet near its star speeds through space quickly, as Mercury does, so asteroids and comets can smash into the world so fast they may eject its atmosphere”, so perhaps some additional shielding would be required?

Planet C (third closest to the sun, based very loosely on TRAPPIST 1f/g)

Orbit of 8 days

Largest at 1.2 earth mass

Has atmosphere so not tidally locked.

Maybe like an ice world, but not too extreme, so still habitable (though maybe not possible to grow plants on the surface)

I am a complete beginner to both worldbuilding and astronomy, so any input would be greatly appreciated!

  • $\begingroup$ Mars and Venus, the last one up in the clouds only, are arguably habitable by humans using supportive technology, with today's technology, while perhaps not today's economics. $\endgroup$ Jun 3 at 18:34
  • $\begingroup$ Of course it would. Why would there be any problem there? $\endgroup$ Jun 5 at 20:15
  • $\begingroup$ Ever played MOO or any other 5X? $\endgroup$
    – ZOMVID-21
    Jun 7 at 18:40

8 Answers 8


It's totally possible: already in our solar system the habitable zone, that is the region of space where the energy flow granted by the star is sufficient to keep water liquid, span between the orbits of Venus and Mars, with Earth in the middle.

That both Venus and Mars are not habitable at the moment depends on their atmosphere: Venus has a too high greenhouse effect while Mars has not enough of it. You therefore need just to be careful with balancing the stellar radiance and the atmosphere composition to ensure that you are in the sweet spot for habitability.

  • $\begingroup$ My understanding is that early in Mars' history, it would have been habitable to the point of having water seas and (but for oxygen) people might have walked around it without pressure suits. It was habitable, and became less so. We just happen to be at an unlucky point in that history. $\endgroup$
    – John O
    Jun 3 at 13:23
  • 3
    $\begingroup$ We don't know for sure, but Mars could have been habitable if bigger. $\endgroup$
    – FluidCode
    Jun 3 at 15:54
  • $\begingroup$ Thanks for the answer - I was so caught up with exoplanets, I forgot our own solar system! "Discovering" another solar system similar to our own would get also get rid some of the problems associated with orbiting a red dwarf star - I'll look into it :) $\endgroup$
    – user96461
    Jun 3 at 21:59
  • $\begingroup$ Don't forget Earth's moon too, as another planet-sized body that would be habitable were it not for the pesky lack of atmosphere. $\endgroup$
    – N. Virgo
    Jun 5 at 5:39

Double planet though rare is a distinct possibility! even though the 2 planets move though the same habitable zone they need not, indeed probably do not, have the same climate or biosphere. This will negate any need for cramming 3 separate orbits into a small habitable zone. Not to mention an interesting stage on which to play out some interesting questions with how such closely spaced biospheres possibly could interact.


/different inhospitable but habitable (by genetically modified humans or humans using supportive technology) environments. ... if there is a simpler way to have 3 habitable but different environments in relatively close proximity/

Same planet.

The original colonists dug in and made subterranean dwellings, putting to use the extensive cave systems on that planet. These are the least modified of your humans and their civilization is the oldest. They think it is the largest too, and the best.

Later colonists are modified to live in the above ground atmosphere. They find the underworld claustrophobic but find the people down there to be very good looking. You underworlders find the toplands hellish but think the people there are impressive and capable. There is exchange between these people as well as friendly rivalry.

A third wave of colonists lives on and under the oceans. They are the most modifed of all and the first two groups sometimes don't think of them as still being human; there is suggestion that they are not modified like the first two, but crossbred with something else. These people are hard for the first two to communicate with. Sometimes these can also be found in flying habitats.

There may be a fourth group. If so, they are very decentralized. Most interactions with this group are by way of rumor and stories to tell children to keep them from going into dangerous places. This fourth group might actually be the first group, relics of a very early wave of colonizaton in a day when human modification techniques were decidedly different.

  • 1
    $\begingroup$ Thanks for the answer! I hadn't thought of that way to make different environments in close proximity - but I may not make all types of human on the same planet though, as I would like spaceships/space stations to be large part of the world as well :) $\endgroup$
    – user96461
    Jun 3 at 22:08

I’ve been roughly basing it on the TRAPPIST-1 system, as it has 4 planets in the habitable zone, but I don’t know if there is a simpler way to have 3 habitable but different environments in relatively close proximity (maybe using moons)?

The simplest way to have several habitable planets is to have them all about the same size and atmosphere and all about the same distance from the sun.

Since we only know about one habitable planet your planets should be similar in temperature and atmosphere to Earth.

You can go 20 degrees in either direction of course since that sort of variation already exists on Earth. This will give different environments. You can also fiddle with the amount of oxygen in the atmosphere since this also changes in high altitudes and has been much higher in Earth's history.

To stop the planets interfering wit each other too much, just push the colder one further away from the sun and give it a stronger greenhouse effect. Likewise pull the hot planet closer to the sun and give it a weaker greenhouse effect.


If you want three rather hostile but still naturally habitable planets in your system, you might want to find out what the limits of planetary habitability for humans are, in order to create planets within but close to thsoe limits.

In that case you might want to study Habitable Planets for Man, Stephen H. Dole, 1964.


Sean Raymond's blog PlanetPlanet.net has a section Ultimate Solar System where he tries designing planetary systems with as many habitable planets as possible.


At the present time nobody knows how common planetary systems with one habitable planet are, or planetary systems with two habitable planets, or sysems with three habitable planets, and so on with higher numbers of habitable planets.

But suppose that there was a giant planet orbiting in the habitable zone of its star - some have been found in habitable zones - and that giant planet had moons. It would be possible that some of the moons would be large enough, say at least as large as Mars, to be habitable.

I believe in various posts Sean Raymond considers it possible for a giant planet in the habitable zone of its star to have up to five giant and habitable moons.

How many planetary orbits can fit within the habitable zone of a star? That depends onf the mass of the star, the masses of the planets, and the spacing of the planetary orbits. It also depends on how wide the habitable zone of the star is.

So to find out how wide the habitable zone of a star is, we have to find out how luminous the star is compared to the Sun. Then it should be a simple calculation to calculate the inner and outer edges of the star's habitable zone.

But there is not a great deal of agreement about the inner and outer limits of the Sun's habitable zone. The chart here:


Lists about a dozen different estimations or calculations of the inner or outer edges, or both, of the Sun's circumstellar habitable zone. And some of them differ greatly from others in how wide the zone is and thus in how many planetary orbits could be expected to be in the zone.

From what I see Raymond probably uses the estimate of Kasting et al in 1993, which is commonly used, and probably uses their optimistic habitable zone instead of their conservative habitable zone.

In this post:


Raymond says that more low mass planets can fit into the habitable zone of the star he chose to use than high mass planets can.

SUMMARY: The right orbits is the configuration that can squeeze the most planets into the habitable zone. Small planets can be squished tighter than large ones. We can fit 14 of our smallest planets in the habitable zone of our chosen star, or 7 Earth-sized planets, but only 3-4 of our largest (10 Earth mass) planets.

But not to fear: tomorrow’s ninja moves will add two big twists to this story. And blow your mind.

So when starting out to design an ultimate solar system with as many planets as possible in the Habitable zone, Raymond claims that 7 Earth sized planets could be fit into the habitable zone.

A writer might wonder whether a gas giant planet in the habitable zone could really have as many as 5 Earth mass moons habitable moons orbiting it in stable orbits. It is possible that a giant planet could not possibly have more than 2 habitable moons, for example, which would mean that if you wanted three habitable worlds in your system you would have to fit in the orbit of an Earth size planet into the star's habitable zone as well as the orbit of the giant planet.

So if you intend to write about 3 semi habitable moons of a giant planet in the habitable zone of your star, you might want to research many scientific papers about the possibility of havitable exomoons. For example, I have read that calculations show that a habitable exomoon would have to orbit between 5 and 20 planetary radii from the center of the planet, and probably only a few giant exomoons could orbit the same planet within that range.

And if you want to write about three semi habitable planets orbiting the same star in different orbits, you might worry whether the Sun's habitable zone is as wide as some calculations suggest or very narrow A writer wanting a story where no star has more than 1 human habitable world would want to choose a very narrow calculated habitable zone for the Sun to base the habitable zone for his star on.

So some careful writers might want to study the reasons for different calculations of the habitable zone of the Sun, to see which one is most convincing to them, and hope that it will be wide enough to allow three separate planetary orbits.

If you find that you believe all solar systems have habitable zones so narrow they can't possibly have more than one planetary orbit in the habitable zone, you might want to try what Rayomond suggests in this post:


Or here:


[https://planetplanet.net/the-ultimate-solar-system/ 2: https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#Solar_System_estimates


One interesting way to fit more planets into a habitable zone is to have two planets in almost identical orbits that swap orbits with each other. Saturn's moons Epimetheus and Janus do this. They move at different speeds and when they start to get to close to each other, their mutual gravitational attraction will alter their speeds enough to knock them (more or less) into each other's orbit. The process repeats in reverse when one laps the other and catches back up with it. The ratios for mass, orbital radius, etc. have to be just right for it to work, but it's definitely possible.


What you have set up here is definitely plausible! My only concern would be the challenges posed by living on a tidally locked planet, but it sounds like your civilization has the technology to make that possible.

If you don't want to have three different planets, but would still like three different environments, moons are a good idea. Perhaps humans could inhabit three different moons of a large gas giant similar to Jupiter or Saturn.


I think gas giants in the habitable could potentially have quite a few moons with habitable conditions. Titan has a thicker atmosphere than earth, it is just far too cold for us because Saturn is so far out.

They would also be realtively close to each other which would be a huge help in planet hopping. I don't think our exoplanet finding capabilities extends to the moons of gas giants, but I haven't looked into it.


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