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Suppose, in the future, an alien species (I'll refer to them as the Destroyers) develops an extreme dislike for humanity. The Destroyers track human ships back to Earth and take actions that lead to the extermination of all life on Earth, as well as rendering the Earth unfit for human habitation. Over the following years, Earth is forgotten by humanity until, thousands of years later, it is rediscovered.

The catch is, they weren't the first to arrive.

Another alien species (I'll refer to them as the Rebuilders) long since discovered Earth. The Rebuilders decided to restore the Earth to a habitable state and seed new life, mostly to prove they can, but also to bolster their god complex. To accomplish this, the Rebuilders harvested material from the other celestial bodies in the Solar System and set to work terraforming Earth to their specifications.

How reasonable is this scenario? Are there sufficient materials in the Solar System to restore a barren, rocky Earth devoid of an atmosphere to a lush, green-and-blue pearl of a planet complete with complex life? How much of those resources would need to be consumed to rejuvenate the Earth?

Note that I am not asking about a reasonable timeframe for evolution to produce complex lifeforms; this can be taken care of by the Rebuilders' hyper-advanced technology and/or handwavium (which is pretty much the same thing).

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  • $\begingroup$ Tangential Question: Can we still call it terraforming if an alien species makes changes to a planet to make it unlike Earth? $\endgroup$
    – Frostfyre
    Commented Aug 17, 2016 at 21:05
  • $\begingroup$ You say "rendering the Earth unfit for human habitation" but don't say how, then later indicate "restore a barren, rocky Earth devoid of an atmosphere" I mean it would mostly render earth uninhabitable just raising the average global temperature 10 degrees or so, but might not eradicate all life. Seeding the planet with radioisotopes would probably kill everything, and force you to do some drastic stuff to make it useful again. Not sure what might cause it to lose atmo, and stay lost, plus blow off all the topsoil. Sounds like the water may be gone too? $\endgroup$
    – Seeds
    Commented Aug 17, 2016 at 21:13
  • $\begingroup$ It is still probably best called terraforming as it is a well known term. Xenoforming maybe? $\endgroup$
    – Bellerophon
    Commented Aug 17, 2016 at 21:13
  • $\begingroup$ @Seeds I excluded what happened because it isn't relevant to this discussion. But, yes, there is no water left after the event. $\endgroup$
    – Frostfyre
    Commented Aug 17, 2016 at 21:40
  • $\begingroup$ Just because something is unfit for human habitation doesn't mean it can't support other life. See Chernobyl for a real life example. Bacteria can live happily in volcanic springs and high radiation environments. Cockroaches are said to be able to survive a nuclear winter. Given enough time, enough species should survive in a viable world $\endgroup$
    – nzaman
    Commented Aug 18, 2016 at 4:45

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The short answer is there is more than enough materials in the solar system to do that and more. Europa has 3X the water as all the oceans of Earth, there are megatonnes of Nitrogen on Titan as well as something like 100X the hydrocarbons compared to Earth. Lots of raw materials for complex chemistry.

There is also a great deal of energy available to perform the actions you want to do, ranging from the luminous output of the sun to more local resources like the trillions of watts of electrical energy in the flux tube between Jupiter and Io.

There are even millions of asteroids and comets which can be used to apply gravitational torque to move the Earth into a more suitable orbit, since it is very unlikely the aliens have the same preferences for heat and light as we do.

That is probably the more interesting issue in this scenario. Since the aliens are, well, alien, their needs and wants will be quite different from our own. Maybe their home planet is a cold, dry desert like Mars, or maybe the percentage of surface water is closer to 90% to support their aquatic lifestyle.

Since the time and energy costs of terraforming a planet are immense anyway (and the cost only rises as you try to speed things up), the motivations and technological savvy of the aliens needs to be addressed. Obviously they have fairly high level of technology to begin with, in order to travel between the stars. Shipping raw materials in bulk across the solar system and moving the planet around isn't going to be much of a chore, and trading time for energy is going to speak to the motives of the aliens.

The ability to manipulate matter at the molecular level will also be important. Creating biomes and putting together an ecosphere on a planetary scale in a reasonable time will require a lot of work. Manipulating the nitrogen and hydrocarbons to create organic molecules, laying down "soil" and planting waves of colonizing creatures could happen very rapidly if they had the resources and energy budget. If they are like us, we could terraform Mars in a matter of millennia by pumping hardened micro organisms on the surface and waiting patiently for them to build the ecosystem from the ground up. There will be space between the two extremes where the aliens will be working.

So the short answer is "yes", everything is possible, so long as they devote sufficient time and energy to the project.

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A brief answer to your tangential question: Can we still call it terraforming if an alien species makes changes to a planet to make it unlike Earth?

The problem with xenoforming as a word is that it literally means 'forming something that is strange'.

Joe Haldeman called terrforming "geomorphy" (used, possibly, in Mindbridge [1976]). The 'geo' doesn't need to be specific to 'earth' it could be applied generically to planets of any type.

Of course, if you wanted to relate it to sapient beings modifying planets to be more like their home planet, then calling it home building does sound a trifle banal. The 'eco' in 'economics' and 'ecology' comes from 'oikos' the ancient Greek word for 'home'. This suggests the appropriate name for terraforming to be like their alien home planet could be either 'ecomorphy' (home-shaping) or ecotropy (home-directing). 'Ecoforming' is possible, but the word looks odd on the page and may not sound well too. 'Echo-forming' sounds like echo-making. And ecoforming could be pronounced ek-cough-or-ming. Not so good.

This leaves ecomorphy or ecotropy, but most people can understand terraforming even if the transformation doesn't result in an earthlike planet. I suspect people will prefer terraforming.

My personal favourite replacement word for terraforming can be found in Harry Harrison's The Stainless Steel Rat (1961). He called it 'terrafying'.

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