What is the most efficient way of reducing the temperature of a planet?

Question

As you might know, most planets that have atmospheres are not the ideal temperature to live on. For example, Venus is a very, very, very hot planet with constant volcanic eruptions and especially the greenhouse effect. I was wondering if there exists any practical way to 'cool off' the planet.

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Obviously, you cannot eject the excess temperature through space (there has to be something for the heat to transfer through). What is the most efficient (and not ridiculously expensive) material to heat up and then eject into space to remove heat?

Answer 1

Heat can actually be transferred through space, that is called radiation. A planet with a constant temperature radiates an amount of energy equal to what it receives, and the energy radiated is proportional to its absolute temperature to the fourth power. (see the Stefan-Boltzmann law)

A possible solution is to cool Venus is to use a gigantic but thin sunshade balancing at the L1 Lagrangian point. A $100nm$ thick aluminium sheet blocking half the solar insulation of Venus is just going to mass $15,000,000$ metric tons. That may sound like a lot, but it is just a cube of aluminium with a side length of $180 m$, or about a third of the annual world production.

Another possibility is to use a similar sheet of aluminium as a mirror to instead heat up the planet. That may sound counter productive, but a higher temperature will allow heavier molecules to escape from the atmosphere into space. This will reduce the extreme greenhouse effect of Venus, over time decreasing the surface temperature (currently 462 °C). Hydrogen and helium are already gone from the Venus atmosphere, and carbon and oxygen are leaking too, although very slowly. A higher temperature speeds up the leakage.

Answer 2

Another way is to reflect as much sunlight as possible at ground level during the day but encourage radiation at night. Cover a large expanse of desert with aluminised plastic sheet reflector material on motorised rollers. By day have them unrolled. Sunlight is reflected back up through the atmosphere. At night roll them up. The ground can then radiate into the clear sky.

Overall you would be cooling the planet but there would be severe climate distortions caused by, say, making the Sahara into a snowy wasteland. You might try to distribute the mirrors but they'll work a lot less well in a place with higher cloud cover. Also they'll remove land useful for agriculture etc.

Sorry just realized question was specific to Venus. It would not work there because it's git 100% cloud cover and greenhouse effect has already run away. I was thinking Earthlike planet, perhaps geo-engineering if we let greenhouse effect become critical.

Answer 3

To remove heat by ejecting material you have to have concentrated it (i.e., by compressing gas) so that you have moved heat from the general environment into a concentrated state. Unfortunately, the work needed to concentrate the heat is always greater than the heat transferred - creating more heat than is being gotten rid of. The only effective way to remove heat from a planet is by radiation from the surface. You might be able to affect the efficiency of radiation and reduce heat influx by modifying large surface areas to be black at night (black is the most effective radiator color) and white at daytime (to reflect heat back into space.) Perhaps genetically modified plants or algae that change colors with the light?