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So, my planet has (around) an Earth year long day-night cycle. How could its temperatures not reach extremes that would disallow life? Assume that everything is Earth like except for multiple moons (everything can be modified, if needed).

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Have a thick atmosphere with a high heat capacity.

Venus has an extremely long day length--about a third of an Earth year long--but a nearly-uniform temperature across its entire surface due to the thick atmosphere.

Give your planet a thicker atmosphere with plenty of CO2, and reduced insolation (either from a smaller star or a wider orbit) to make up for it, and you can keep temperatures across the globe uniformly in a range comfortable to Earthlike life (rather than uniformly in a range that will melt lead) over an indefinitely long nighttime period.

Water is also good for moving and retaining heat; note that coastal areas tend to have more stable temperatures than inland ones, getting both less hot and less cold than inland deserts, and northern Europe--particularly Britain--is temperate entirely because of the heat carried north on ocean currents. Equivalent latitudes in the interior of Russia and North America are frigid. So, give your world lots of ocean and little land, or at least landmasses that are broken up into small bits with plenty of ocean intrusion while leaving clear lanes for global circulation, and you will reduce the thermal load that must be born by the atmosphere.

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There are only two possible solutions:

  1. An immensely thick atmosphere. Something on the order of Venus' atmosphere, so the sheer thermal mass of the air conveys heat to the shade side, and cools the hot side.
    Pro: more uniform temperatures on planetary surface.
    Con:
  • Massive atmospheric pressure.
  • winds are much more powerful.
  • possible problems with greenhouse effect. This could make your planet as balmy as Venus(450C-ish)
  • If you are adapted to 100bar atmosphere, then space travel becomes a bit problematic. Your spaceship now needs to be an incredibly strong pressure vessel, thus heavy.
  • Just how do you plan to grow anything under all those clouds? Very little light will reach the surface! So it's artificial light all the way, including for crops.
    How exactly is being on this planet better than making a Space Habitat?
  1. Build your entire colony underground.
    Pro: its a nice constant temperature down there
    Con:
  • Air would be handy. But up on the surface, air temperature and composition fluctuates cyclically, and quite wildly. CO2 snow, anyone? So you need to have very fancy filters, or huge storage ability, or huge air reprocessing ability.
  • Lots of digging. Actually, this is not so bad, just takes a bit of time. You want to dig anyways, to mine minerals and metals.
  • Just how do you plan to grow anything under all that rock? NO light will reach the tunnels! So it's artificial light all the way, including for crops.
    How exactly is being on this planet better than making a Space Habitat?
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