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Some information about their homeworld:

  • Their homeworld, 0.34247 (M⊕) 0.54 G, 10156 km.

The atmospheric pressure of their homeworld is 62% of the Earth's, and the planet has an average temperature of 87.6 kelvin. The atmosphere consists mainly of

  • 68.1% Nitrogen

  • 28.6% Methane

  • 1.71% Carbon dioxide

This is for a story I'm in the process of writing. An alien race from a planet that resembles Titan wants to colonize Earth and terraform it to make it resemble their planet. They are 3ft in height and weigh 60-80 pounds on their own planet. They are also extremely advanced - at least 1000 years ahead of humanity in technology, such as medicine and space travel.

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    $\begingroup$ Oof. Are they sure they want to colonize Earth? They'd weigh nearly twice as much as they're used to, making day to day life pretty unpleasant. $\endgroup$ – Nathaniel Ford Oct 2 '16 at 17:55
  • $\begingroup$ I imagine it would be like humans colonizing a planet with 1.5 G's, something we would do if we really had no other options. And humans surely can adapt to 1.5 G's, would just be somewhat uncomfortable. $\endgroup$ – Stephanie Oct 2 '16 at 17:58
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    $\begingroup$ You listed 0.54G as their native surface gravity, or half of Earth normal. Thus, I assumed it would be more like a human living under 2.0Gs, which is doable but unpleasant. (I assume their advanced medicine can take care of the internal organ bruising.) $\endgroup$ – Nathaniel Ford Oct 2 '16 at 18:00
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    $\begingroup$ As a consistency nitpick, 87.6 K is less than the triple point of methane (91 K) so methane cannot be liquid on the homeworld, regardless of pressure. Since both nitrogen and CO2 are chemically inert, what is the basis for the biochemistry? They can't use methane as a water analog, since it doesn't exist. Also, any answer wthich tries to deal with reducing the temperature needs to factor in the extremely powerful greenhouse effect caused by all that methane, which makes the job that much harder. $\endgroup$ – WhatRoughBeast Oct 2 '16 at 21:37
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    $\begingroup$ Technically, Earth cannot be terraformed. "Terra" means Earth. You can't make Earth more Earth-like. ;) $\endgroup$ – T.J.L. Oct 3 '16 at 4:21
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Terraforming Earth for this species is quite hard work. They need to:

  • Move the planet quite a way further out from the sun, to approximately Saturn's distance, and re-arrange the other planets, including Jupiter and Saturn, so that new-Earth's orbit is stable. Moving Jupiter is a bit of a big job.

  • You do need to move Earth out there, because methane is a very powerful greenhouse gas, and there's no reliable way to make the Earth cold enough while leaving it in its current orbit. You could use a planetary sunshade, but if something goes wrong, your ecosystem cooks.

  • Moving Earth out there will kill off the plant life, so that oxygen stops getting into the atmosphere. You then need to combine all the oxygen in the atmosphere with something, and release all the methane clathrates to get you a start on the necessary level of methane in the atmosphere. Then you need a load more methane from somewhere.

It's really a great deal easier to use Titan. You don't have to move any planets around. The temperature is comfortable for them; the atmospheric pressure is too high, but if they can get rid of a lot of the nitrogen, they'll have a comfortable atmosphere, if they can get more of Titan's methane into gaseous form. This looks like the sensible way to make somewhere to live. If they don't want neighbours, they can just sterilise Earth far more easily than changing it.

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    $\begingroup$ I like your answer John, but your assumption that the sunshade is too dangerous seems unfounded to me. The risk tolerance of these aliens could be quite high or low, depending on the author's wishes, but based on their desire to do a quickie terraforming job, I would suspect it leans more toward the high end. $\endgroup$ – hexagon Oct 2 '16 at 21:03
  • $\begingroup$ @hexagon My understanding is that if a sunshade fails the methane would literally boil off. Like then entire atmosphere would explode in seconds. $\endgroup$ – coteyr Oct 3 '16 at 10:14
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Another option beyond one or more sunshades located at the L1 lagrange point would possibly be a nuclear winter type of effect. The aliens would need to continually seed the stratosphere with smoke particles. If they choose to build space elevators, they could add a very large burner at the right altitude. Otherwise they could maybe use hydrogen balloons tethered to hoses. Presumably they would use pure oxygen as fuel for their burner because the atmosphere has high levels of methane and thus, presumably almost zero oxygen.

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  • $\begingroup$ 87.6 kelvin is very cold - a couple of hundreds degrees lower than present average. A nuclear winter is just expected to cause a drop of a few tens of degrees. en.wikipedia.org/wiki/Nuclear_winter $\endgroup$ – Pere Oct 3 '16 at 8:46
  • $\begingroup$ Large enough sunshades could completely block incoming sun until the desired temperatures are reached, and could then be partially disbanded to keep it there. Note that all that energy would exercise a lot of pressure on the sunshade, which would behave like a solar sail, and would have to be kept in place somehow - either by adding thrusters, playing with angles, or displacing it relative to L1. $\endgroup$ – tucuxi Oct 3 '16 at 8:57

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