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This question is not about terraforming! (or atleast not technological terraforming)

I am writing a fantasy story set in the (real) Trappist-1 system. I am trying to faithfully keep most of the planetary details already known as best as I can, though they can be slightly altered. Magic is present in this world, but I would rather not use it to fix every problem. Currently, I have made it to where three worlds (d,e, and g) are supposed to be inhabited by some form of life (d is only plants and fungi), but I am running into problems with g. The problem is that g receives a similar amount of starlight that mars does. Due to this similarity, I have been trying to use Noahchian mars for inspiration on this planet, but I am not sure how to deal with the earth-like atmospheric pressure I am wanting and the 0 degree axial tilt (there is a daynight cycle similar enough to earths, no matter how realistic this is). Humans are meant to inhabit this fantasy world, so I can't just change the biology of the fantasy creatures till it works. How can I make this planet habitable?

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  • $\begingroup$ Can you explain what you mean with "making habitable but not terraforming"? $\endgroup$
    – L.Dutch
    Nov 18, 2021 at 6:47
  • $\begingroup$ Make a planet with habitable features through natural processes. $\endgroup$
    – skout
    Nov 18, 2021 at 15:12

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The first issue I is that planet c is most likely uninhabitable, due to being a very close analogue to Venus. It orbits relatively close to the sun, receiving even more solar irradiance than Venus does. It's also been observed to have a very thick atmosphere likely composed primarily of water vapour, which would give the surface blazing hot temperatures, as it is actually a much more effective greenhouse gas than carbon dioxide. The planet of those three most likely to be able to support plant life is e, so there's no problems there. Comparison of some physical characteristics of the Trappist-1 system

https://en.wikipedia.org/wiki/TRAPPIST-1

As can be seen above, e, f, and g are the closest to Earth analogues in the system, considering the differing habitable zone. While f will be quite cold, it seems g might be a truly frigid place if it is given a breathable atmosphere, due to only receiving a quarter the sunlight earth does, and I assume this is the habitability problem you are facing. This is tricky because both f and g are cold enough that water would freeze onto their surfaces, without any greenhouse effect that is.

The first solution I can think of would be to take a page out of the other planet's books and give it a much thicker atmosphere, perhaps with a lot of water, which would help it keep a temperature on the equator high enough for liquid water to exist. This atmosphere would be quite deadly to humans at full pressure, so habitation could perhaps only be possible on multifarious plateaus and mountain ranges, pushed up by immense geological processes of your selection. This would also give the planet some quite unique traits, these highlands would be like sky islands in a vast invisible sea of air, that only specially prepared expeditions could cross. To sink into the thicker atmosphere means a slow death as the already thick air gets ever denser. The air pressure would still be quite high on these habitable areas, making aerodynamic flight far easier to accomplish too.

If the three planets are a necessity for the setting, I would propose swapping c with f as the third planet of choice, the main issue with c's habitability is that it is composed in a large part by water, and the atmosphere is entirely steam. If you give it a breathable atmosphere though, you could also exchange a lot of the water content of the planet for normal rock, and you would be left with a habitable, albeit cool world. Even without its wet blanket, c is likely far too hot for liquid water.

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  • $\begingroup$ I am sorry, I accidentally wrote a wrong piece of information. The nearest habitable planet was supposed to be d, not c. It's only inhabitants are meant to be plants, fungi, and microbes though, so I am not worried about it. $\endgroup$
    – skout
    Nov 18, 2021 at 15:12
  • $\begingroup$ I am not sure I need to raise the temperature as high as you are suggesting. Noachian Mars, for example, when simulated at an atmospheric pressure above afew tens of millibars (so maybe the early hundreds) had an average temperature just above the freezing point of water. If it has a similar atmospheric pressure to earth, I would expect something above this value already, which is acceptable since the lack of axial tilt will make the temperature pretty constant. $\endgroup$
    – skout
    Nov 18, 2021 at 15:19
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Sulfur hexafluoride is a highly potent greenhouse gas that is non toxic to humans. It is also significantly more dense then normal air. While care must be taken to design the atmosphere so that people don't asphyxiate, with some of this replacing some of the nitrogen, and an increased percentage of oxygen you can create an atmosphere that is low to the ground and still rather dense and warm.

PS Sulfur hexafluoride is also a mild anesthetic, like nitrous oxide(the gas the dentists use). So visitors may feel... nice...

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  • $\begingroup$ Would this need to be engineered to be part of the atmosphere, or the atmosphere already contain it? I've found that this CAN be naturally created, but vast majority of it is man made. I cannot find the mechanism that would create this naturally in large quantities. $\endgroup$
    – Sonvar
    Nov 18, 2021 at 21:44
  • $\begingroup$ I kind of handwaved that. I figure that some kind of large hot springs with high contents of sulfur, bromine and flourite(one of the mineral forms of flourine) might produce it with the addition of a flourine cycle of the sun breaking apart the gas and it recombining with fresh sulfur. Also, maybe the gas was produced by an event in the geologic past during a period of high concentration of sulfuric acid, allowing large amounts to form. Failing at all that, handwave a plant. $\endgroup$ Nov 18, 2021 at 22:23
  • $\begingroup$ Hmmm.... Fluorine is a hypothetical alternate for respiration. Hydrogen fluoride and Hydrogen Sulfide are hypothetical alternatives for a solvent. If the OP would like to explore a very exotic form of life, there could be a biological mechanism to create this in large enough quantities to effect the climate. However, they probably need a very complex feed back cycle to maintain these in equilibrium, as they have a wide range in boiling and melting points. Maybe if the surface is <0c it could have hydrosulfiric oceans and fluorine atmosphere $\endgroup$
    – Sonvar
    Nov 19, 2021 at 14:56
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I suggest that you look at Habitable planets for Man, Stepehn H. Dole, 1964.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

See what it says about the upper and lower limits of total atmospheric pressure and the minimum and maximum amounts of various atmospheric gases.

You will want the hottest planets to have atmospheres with as little green house gases as possible to kept the temperatures as low as possible. And you will want the coldest planets to have atmospheres with as much green house gases as possible to keep the temperatures as high as possible.

Another possible source of heat for the planets of the Trappist system is the tidal heating resulting from their tidal interactions. The small size of their obits means that they pass very close - compared with palnets in our solar system - to each ohter whenever an inner planet catches up with and passes an other planet.

Thus their tidal interactions should be stronger than those of the planets in our solar system and should produce much more tidal heating.

https://en.wikipedia.org/wiki/TRAPPIST-1#Resonance

Of course I don't know how strong that tidal heating is likely to be in the case of TRAPPIST-1 g.

Nobody knows much about the atmospheres of the TRAPPIST-1 planets, and it is quite possible that none of them has a breathable atmosphere.

But since this is a fantasy story, possibly a character can boast of being descended from a legendary wizard or god, who allegedly created the world. And other characters can say they don't believe any persons be they god or wizard, could be powerful enough to make a world, but maybe someone could be powerful enough to modify a world and make it better to live in.

And maybe your redears might think that it is possible that the legendary person created teh breathable atmosphere of that world. And if they released a lot of oxygen into the atmosphere, they might have also released enough sulfer hexaflouride into the air to make a strong enough greenhouse effect, as thewildnobody suggested..

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