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I want to make a hot planet habitable for mankind. It could be a planet in the habitable zone like Venus or outside of the zone like Mercury. These are just examples.

Someone said here: Terraforming a very cold planet? that terraforming a cold planet is easier that a hot one. Still, is it possible to make a planet colder?

Example 1: A planet in the habitable zone is too hot for human habitation due to atmospheric reasons. Can it be cooled down?

Example 2: A planet is too hot due to solar proximity. Can it be cooled down? Can it be moved away from the star?

What are the different techniques or technologies that could be used to lower the surface temperature of a planet?

*including possible not too far future technologies.

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    $\begingroup$ Can you specify why the planet is hot? Lack of atmosphere, too much atmosphere, volcanic, solar proximity? This will help focus the question a bit. $\endgroup$ – James Oct 15 '14 at 18:15
  • $\begingroup$ Yea, all of these factors except for the volcanic activity. I think this could be for another question maybe. Is it too broad ? $\endgroup$ – Vincent Oct 15 '14 at 18:16
  • $\begingroup$ I suggested and edit of what I had in mind if you want to take a look. $\endgroup$ – James Oct 15 '14 at 18:25
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    $\begingroup$ Some of these proposals for fighting global warming on earth might (in a larger scale) also be applicable to hot planets. $\endgroup$ – celtschk Oct 15 '14 at 20:30
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Fundamentally you have two choices (assuming we don't have some sort of super future tech like planetary force fields, portals, moving the planet into a larger orbit, cooling the sun, etc).

  • Increase outgoing heat
  • Reduce incoming heat

The first one could be tried a little using giant heat sinks or similar but is not really practical so that leaves the second option of reducing incoming heat.

Now you have three choices:

  • Make the planet's surface more reflective

For example ice caps reflect away a lot of incoming heat. Similar could be done with artificial substances.

  • Make the planet's atmosphere retain less heat/more reflective

Dust in the upper atmosphere similar to an Impact/Volcanic/Nuclear winter could reflect away heat. Stripping out greenhouse gasses would cause it to retain less heat.

  • Block the heat before it reaches the planet

A giant mirror, sail or similar placed in space between the sun and the planet could reduce the incoming sunlight.

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  • $\begingroup$ I don't think you'd want a single giant mirror. Use a swarm of thousands/millions of small ones. They'd only need to be a few atoms thick, so wouldn't require that much total mass (maybe a smallish asteroid), and could to some extent be maneuvered as solar sails. $\endgroup$ – jamesqf Jan 14 '15 at 23:35
  • $\begingroup$ @jamesqf Not a bad idea, you'd tend to lose the small mirrors over time and need to keep replacing them though. They'd also be harder to keep in position. $\endgroup$ – Tim B Jan 15 '15 at 0:12
  • $\begingroup$ Do they need to be mirrors? Aren't they simply casting a shadow on the planet? If the "shaders" themselves warm up, what does it matter? That heat won't be transferred to the planet. $\endgroup$ – wetcircuit Apr 11 '17 at 2:14
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    $\begingroup$ @wetcircuit true enough. Mirrors will radiate less and might let you harness the energy somehow. Just shades is simpler though. $\endgroup$ – Tim B Apr 11 '17 at 7:23

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