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One of the solution to make a cold planet habitable for us is to increase the greenhouse effect. It works with planets like Mars that are inside the habitable zone but would it be enough for a planet outside of that zone?

It does maximize the heat absorption of the planet but there is a limit to this. We can't increase the gas concentrations too much.

Are there other ways we could heat a cold planet to make it habitable?

For example: In Star Wars, the city-planet-capital Coruscant is heated by mirrors that are deflecting the light of the star toward the planet.

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    $\begingroup$ I wonder why would Coruscant need to be heated. I would think all the waste heat would do the job. $\endgroup$ – HDE 226868 Oct 12 '14 at 13:53
  • $\begingroup$ they do mention the waste, they must have plenty considering the 3 trillions ? people living on the planet. $\endgroup$ – Vincent Oct 12 '14 at 21:56
  • $\begingroup$ Well, they're already discussing this topic with mars... $\endgroup$ – Ethan Bierlein Oct 15 '14 at 12:54
  • $\begingroup$ Mars is still in the habitable zone. It is not so cold. $\endgroup$ – Vincent Oct 15 '14 at 15:58
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Greenhouse gases - the easiest choice

Warming a cold planet should be probably much easier than cooling a hot one. Increasing a planetary albedo by springkling the surface by black, highly absorbing powder or using orbital mirrors might be part of the process, but it seems that the most important will be emission of greenhouse gasses into atmosphere. Atmosphere of Earth weigths approximately $5 \times 10^{18}$ kg, from which there is $3 \times 10^{15}$ kg of CO2.

There are greenhouse gasses, that are much more effective than the carbon dioxide. For example SF6, which is approximately 20,000x more effective. Following analysis is very simplified, but should give us some idea. Total greenhouse effect makes Earth warmer by 30 K. Total contribution of CO2 is around 10-20%, which is 3-6 K. Now, to make the same change by using SF6, we would need approximately $1.5 \times 10^{11}$ kg, or $1.5 \times 10^{8}$ tons. I can imagine that it would not be an easy task, but definitely possible using large industrial scale colony and proper resources.

Now, if we would like to increase the temperature by 30 K or more, the calculations become tricky. Some people say, that the total radiative forcing is logarithmic and for CO2, fit says it should be

$\text{Forcing} = 233.6+9.766*\log({\mathrm{CO}_2 \ \text{in ppm}})$

This, converted to temperature, would be

$\text{Temperature change} = 2.70+0.113*\log({\mathrm{CO}_2 \ \text{in ppm}})$

With dependence like this, it would be almost impossible to warm the planet more. But (for warming of planets luckily), the truth is more complicated:

Firstly, the IPCC scientists don't say this follows a ln function at all. They say it follows whatever their computer models says it follows. This is only a first order solution.

It is hard to predict how much of the greenhouse gas you would need, but the warming should be feasible, at least up to 30 K. Particularly if there is some other frozen gas on the surface, the initial increase in temperature will induce positive feedback, which will make the process much easier.

Very, very cold planets

One might also be interested in terraforming extremely cold planets similar to Pluto. Then the greenhouse gasses will be necessary, but not sufficient. We could try to deploy mirrors on the orbit, but their size would have to be comparable with the surface of the planet or even much bigger, as the radiation is faint. It would be challenge even for a very advanced civilization. Is there an alternative source of energy? The best broadly available source is the fusion 4H->He. How much we would need, though?

Each $m^{2}$ of the Earth's surface radiates away around 240 W in form of heat. This amount can be decreased by the greenhouse gasses, but not too much. To maintain its temperature, Earth would need $1.22 \times 10^{17}$ watts, which is an incredible amount. Even with fantastic efficiency of fusion, you would have to burn 186 kg of hydrogen per second. From the point of view of the fuel alone, it is doable, but the neccessary infrastructure would have to be a true masterpiece of technology. (Many fussion power plants, probably located under water to conduct the heat away.)

The energy could also easily come from tidal forces, if the planet orbits a gas giant or another planet. But the extreme volcanic eruptions would probably be quite aggressive way of providing the energy and containing them might be more problematic than generating the heat artificially. (Not speaking about terribly complicating task of moving the planet into a proper orbit.)

But the heat is not everything. You would probably have to make a big amount of genetic modifications to reduce light requirements of plants, or create a lot of artificial light somehow.

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Using mirrors, it won't be easy.

Suppose you want to make a planet habitable that's the size of Earth but √2 times too far away from its star to be in the habitable zone. What you'll need to do is increase the amount of light it gets by a factor of 2. (Since the light a planet receives is inverse to the second power of its distance from said star.

There are two options, have mirrors orbiting the sun closer than this planet and you'll need a total surface of Ap(rm/rp)2 where rm and rp are the distance to the star from the mirrors and planet respectively and Ap is the surface of the planet interpreted as a flat circle (diameter of the planet times pi). I don't think it would be easy (or cheap) to have those mirrors always redirecting that light at the planet though.

A more viable solution seems to have a bunch of mirrors orbiting the planet, possibly in this configuration. In this case the distance of the mirrors to the star would be equal of the distance between the planet and the star, so the above formula evaluates to: Am = Ap, the surface of the mirrors should equal the apparent surface of the planet . For an earth sized planet, this number is about 113 million km2. If we take circular mirrors with a diameter of 11.2 km (which gives them a surface of 100 km2, we'll need 1.13 million of them.

If you allow those mirrors to be essentially side by side in orbit, you need them to be about 1.13 million km away from your planet, which seems doable.

In order to spread the light reflected by these mirrors nicely across the globe, the mirrors would need to be slightly concave. And will obviously need to be at an angle of about 45°

This will of course heat the poles more than the equators (since the mirrors will always cross over the poles) and will also illuminate the night side of the planet, but I'm sure you could solve some of those issues by adjusting the shape of the mirrors.

If you're wondering how bright these mirrors will be, the will in total be as bright as the sun and have a total angular size equal to that of an earth sized object at a distance of 1.13 million km away. This gives it an angular size of roughly 2.7 times that of the sun, which would make it very bright, but not quite as bright as the sun, I imagine it would look like a very thin bright line in the sky.

All in all this seems achievable, especially if the planet isn't far out of the habitable zone, but I can imagine there might be better solutions.

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  • $\begingroup$ Wow, this is an awesome idea. I mean, ofcourse its really dificult, but it could even be used in real life for warming up jovian moons, mars, or ceres! And it is a bit less violent than throwing nukes or asteroids on them to generate a greenhouse effect. $\endgroup$ – leojg Apr 8 '15 at 19:37
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Orbiting mirrors around the planet is one thing, but if you really want to provide extra energy to the cold planet you might consider establishing a solar laser and beaming energy from the sun.

There are two ways to do this.

You could simply build a fleet of solar power satellites in orbit around Mercury, using the incredible solar energy concentration to generate electricity and drive batteries of huge lasers to beam energy to the distant cold planet. A series of lenses or diffraction gratings would also need to be in various orbits around the solar system to steer the beam since Mercury's orbit around the sun will take the solar power satellites and laser battery in and out of view of the target planet/moon.

A more direct way of doing this would be to use the solar photosphere as the lasing medium. A series of mirrors orbiting inside the photosphere would reflect a driving laser around the sun. The energetic particles in the photosphere would be excited inside the "racetrack", and as they fell back to their ground state, more and more photons would be added to the drive beam. Using a very sophisticated control system, orbiting mirrors would become "half silvered" at the right time to allow the beam to exit the racetrack at the proper moment to illuminate the target planet/moon. Once again, extra mirrors or diffraction gratings would be needed across the Solar System to steer the beam accurately to the target.

With this sort of mega engineering, even bodies as far as the Kruiper belt could be illuminated and warmed.

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