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With terraforming, how to do it and how long it takes depends largely on the planet in question. Worlds such as Venus may be nigh-impossible to terraform, whereas worlds more similar to prebiotic Earth might be very easy to do so. Ideally a human race that is new to terraforming would start with a planet that is “easy”, so what type of terrestrial planet would be easiest to terraform, and how would we do this?

Some background information: by this point civilisation has developed to a post-scarcity condition, possessing nanofactories and advanced robot drones to perform heavy labour. These terraformers have just arrived on-site in their new solar system and have no contact with earth. They have access to the in-situ resources on this planet and any moons it may have but none other than this.

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    $\begingroup$ "What would the easiest planet to terraform look like?" Terra (aka Earth), we wouldn't need to do anything, instant job done. .. next question? $\endgroup$
    – Pelinore
    Jan 4 at 20:25
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    $\begingroup$ Besides robots what technology do your terraformers have? If they have a genesis device then terraforming any planet is equally trivial. If they can barely make it to space then terraforming any planet is equally impossible. In between there is a whole range. $\endgroup$
    – sphennings
    Jan 5 at 6:51
  • $\begingroup$ The prebiotic Earth was "terraformed" (a lot - think oxygen) by e.g. cyanobacteria (biological oxygen nanofactories - ha!). Plus millions of years... $\endgroup$
    – frIT
    Jan 6 at 10:37

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/What would the easiest planet to terraform look like?/

Accidentally terraformed.

Separated from his mission to the target system, Daron made landfall on this lifeless world suspecting that there might be giant jewels laying on the ground. There actually were, and so he spent a few days collecting them, then got back into his ship and hurried to catch up.

The microbial occupants of his leavings found circumstances in their new world much to their liking, and began to remake the surface according to their abilities and needs. Return visitors decades later found the surface teeming with life. Also pretty smelly. The world is referred to as "Daron's Dump" because of its provenance.

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    $\begingroup$ In the excellent (free to read online) saga The Deathworlders, a planet's ecosystem is irreversibly changed by a human's toilet breaks as she travels across it; the resulting stripe of ecological damage becomes known as 'the Skidmark' by later human colonists. $\endgroup$
    – dbmag9
    Jan 5 at 12:24
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    $\begingroup$ This would be the easiest, but decades is not nearly enough time for natural terraforming to take place. Also, I doubt human poop would have any photosynthetic bacteria in it. $\endgroup$
    – Rob Watts
    Jan 5 at 19:29
  • $\begingroup$ @RobWatts - you are right about photosynthetic bacteria in poop. But this particular poop had a lot of chia seeds given that the pooper was living largely on Chia Chewy energy bars. Chia seeds can sprout after passage thru the gut, can self pollinate and self sow. Aggressively self sow. The planet looks like a rolling green lawn. $\endgroup$
    – Willk
    Jan 6 at 0:15
  • $\begingroup$ Admittedly I am on the "low expertise" end of the Dunning-Kruger scale here, but it would seem plants do need soil microbes to be able to extract nutrients from otherwise pure mineral soil. The good news is that this quite resembles the population in (healthy) human poop (presumably it got there from "earth" soil in the first place). How the gut microbiome of a denizen of a post-scarcity society, living on highly processed/preserved rations from ship's stores, will look like, is a bit of a guess (not good by current research, but perhaps they will catch on?) Maybe also e-mail Mark Watney? $\endgroup$
    – frIT
    Jan 6 at 10:52
  • $\begingroup$ @frIT - I am sure those future folks will have all sorts of probiotics to eat. A microbiome to produce macroBMs! $\endgroup$
    – Willk
    Jan 6 at 19:01
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You've already answered your own question with "prebiotic Earth". Already has a reasonable atmosphere and magnetosphere, mineral rich crust, substantial water inventory.

If you don't have a decent neutral atmosphere already (eg. plenty of nitrogen) importing it from elsewhere will be near-impossible unless you can shift around small moons on a whim, which more or less requires soft scifi super tech. Similarly, if there isn't a decent amount of water or at least ice available, importing an ocean and biosphere's worth will take you forever.

You don't necessarily need a nice mineral supply, but if you don't have at least some rocky surface it'll be difficult to prevent nutrient and biological matter loss, eg. on a deep ocean world you can seed the surface with plankton friendly nutrients all you like, but once the dead stuff and poop sinks you'll probably never cycle it back to the surface through hundreds of kilometres of water.

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    $\begingroup$ As an additional point to this, if you can transport enough Nitrogen at interplanetary distances, then Venus may also become a viable target, as you should also be able to transport enough CO2 away from Venus to aid in terraforming it. $\endgroup$ Jan 5 at 14:45
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A planet that is slightly too cold for liquid water, but has water in the form of ice, a nitrogen-oxygen atmosphere like Earth and possesses large deposits of fossil fuels or other flammable carbon compounds. Fossil fuels imply that the planet once had an ecosystem, but for some reason the planet got a lot colder and the ecosystem collapsed. A possible explanation could be that its star went into a phase of lower luminosity or that the planets orbit got changed somehow.

Humanity could terraform such a planet just like it is currently in the process of terraforming Earth: By burning those carbon compounds, blowing lots of carbon-dioxide into the atmosphere and thus creating a greenhouse effect that increases the temperature on the planet. The ice on the planet would melt into water, create oceans and a water cycle and thus create the basis for a new ecosystem.

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    $\begingroup$ Use SF6, haloalkanes, or methane: tens of thousands, hundreds, or twenty times as warming as CO2. Definitely don't burn the volatile alkanes! $\endgroup$
    – user86462
    Jan 5 at 7:08
  • $\begingroup$ @SeanOConner but how will we power our steam engines? $\endgroup$
    – user253751
    Jan 5 at 17:58
  • $\begingroup$ @SeanOConnor The problem is that these require a lot more effort to manufacture than just starting a fire. They also don't stay in the atmosphere as long as CO2. $\endgroup$
    – Philipp
    Jan 6 at 22:42
  • $\begingroup$ "The problem is that these require a lot more effort to manufacture than just starting a fire." On the contrary, if there's huge deposits of releasable methane, it's easier to release it than burn it. SF6 and CFCs, yes, more work although with warming potential of 30000 to 1, it might still be viable. $\endgroup$
    – user86462
    Jan 7 at 5:12
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A metallic astroid near the systems frost line

Defining "easy" as the shortest time until it is habitable with the smallest resource investment, planets are all together a bad idea. What is a much quicker, safer and more efficient solution are large habitable space stations using the centrifugal force to stimulate gravity.

Using steel or silumin (aluminium-silizium alloy), you get radi of up to 25 km which means you can get livable areas the between the size of the Brunei or Cyprus and up to Panama or Austria. Using carbon based nano materials, radi of roughly 1000km become possible. Those materials get you the land area of India or Australia on the low end and slightly less than the land area of Earth or the surface of Mars on the high end. on the high end. This assumes you go for small radi and 5x the diameter in length on the low end or for the maximum radi and 10x diameter in length on the high end. Of cause, there is no real length limit. You can have a tube of those looping around the local star as many times as you want. This is then called a Topopolis.

Furthermore, those constructs like coming in pairs, as that helps them with gyroscopic stability. Either add a second cylinder inside or connect the second one using an external superstructure. All it has to do us rotate counterclockwise.

The Advantages

Choice of location those stations can be built wherever you want/need them. As your fust step in settling a solar system should be the establishment of space based industry anyways, those habitats benefit from the low launch cost. Just get an icy, rocky, carboniferous and metallic astroid and equip them with mass drivers, laser launchers or launch tethers. Then, set up solar power satellites in low solar orbits, which beam you the energy via laser and your industry is set.

Build to your preferences. The habitats are vastly more flexible when it comes to the environments you can build in them. This goes for environmental and social systems. Smaller, more independent habitats can allow more flexible designs and reduce interdependence. If groups are discontent with the way a state is run, they can get their own country.

Planetary protection. You don't antagonize conservationist groups doing this. Just leave the natives, be they bacteria or blue skinned native American stand-ins alone.

Safety. Planets are dangerous. Earthquakes, Tsunamis, Volcanos, Hurricanes, Blizzards, ... What sort of space ork would want to live on a thin layer of rock above a hellish magma-pit? And that's not even considering how fast and easy pathogens, nanotechnology weapons or missiles can be sent from one polity to another. And that's not to mention the radiation issue. Radioactive materials are just strewn about in the environment, only a magnetic field protects from particle radiation and the thing gas layer above you is just transparent to some parts of the UV spectrum. Inhabiting planets is sheer madness!

Ressource efficiency. Per square meter of habitat area you need a few tons of material. A planet is using orders of magnitude more mass per livable area. This is just a waste of resources.

Scalability. A full on habitat smarm can use the entire energy of its star. Terraforming is, even one one uses Birch Worlds to terraform gas giants, stars and black holes, a dead end in that regard. If you want a planet, you can just construct it using hydrogen and a shell. That is the only way your resource efficiency could come close.

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I completely agree with @Pelinore, but to be more verbose...

Assuming Clarkean Magic technology, all planets can be terraformed

Why can I say this? Because just as we can keep a body alive almost indefinitely using machinery, given the existence of appropriate machinery and the resources to run those machines, a planet can be forcefully terraformed and kept that way.

So, given that any planet can be terraformed, what would be the easiest to terraform?

Well... if you'll excuse me... but, um... duh, a planet that already hosts life and only needs the introduction of terrestrial lifeforms to make it perfect for Terrans.

Yeah... that was probably a bit rude, but please think about it. On the one side of the spectrum are:

  • planets like barren, rogue planets where a whole sun must be provided or planets so toxic that serious industrial-grade filtration is needed or gas giants where someone would do it only because they're so honking rich they can stroke their ego with it. Planets that would be impossible were it not for the Clarkean Magic technology that permits it,

and the other side of the spectrum, which happens to be...

  • Earth.

By definition, whatever planet is closest to Earth is the easiest one to terraform.

If this doesn't answer your question, then you weren't ready to ask it. May I suggest using the Question Sandbox, where you can perfect your question before introducing it to Main?

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  • $\begingroup$ I don't think this answer is as obvious as you say. Extant life on a planet will already be adapted to that planet's environment, so Earth life might not be able to out-compete it. The extant life also might turn out to be poisonous to Earth life or dangerous in some other way. $\endgroup$
    – N. Virgo
    Jan 5 at 4:52
  • $\begingroup$ @N.Virgo human life drives out non-human life, just as we see on Earth. We can murder it all, no problem. $\endgroup$
    – user253751
    Jan 5 at 17:59
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    $\begingroup$ @N.Virgo sealing out and/or killing indigenous lifeforms is probably still a lot easier than completely reforming the atmospheric chemistry and shifting the planet's mean temperature. $\endgroup$ Jan 5 at 17:59
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    $\begingroup$ @SarahMesser I don't know - I mean, humanity has a long history of wiping out big animals, but when it comes to making microbes extinct we've generally found that an awful lot harder. $\endgroup$
    – N. Virgo
    Jan 6 at 1:27
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    $\begingroup$ @N.Virgo I'd argue that we've only tried killing microbes without sterilizing the rest of the biosphere, but ... I get your point. My favored ways of killing microbes at planetary scale all involve radically changing the planet's chemistry and/or temperature: A few thousand nukes + a century of nuclear winter would probably kill off most of the megafauna and leave the natives' diseases without suitable hosts. That's probably *most*/all of the dangerous stuff, since the surviving extremophiles would probably be bad at infecting humans, but if you want the place thoroughly sterile ... $\endgroup$ Jan 6 at 17:41
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There is actually a quite fitting example in reality. In JRE #1609 Elon Musk talks about the possibility of terraforming Mars.

Atmosphere:

There is a lot of frozen CO2 beyond which could help build an atomsphere on Mars if we somehow managed to release it. So a suitable amount of CO2 residing on a planet without an atmosphere sure helps with terraforming. But NASA claims that even when releasing all CO2 on mars NASA we could only reach 60% of the atmospheric pressure of earth. Although it is impossible to do with current technology (Musk suggests firing atom bombs on mars btw) it may be possible some day.

Light:

Lets stick to Mars. Light is also a deciding factor when it comes to growing food in order to make a planet habitable.

Gravity/Size:

The human body is optimized for earths gravity. Living in an environment with different gravity has subsequential consequences. A planet optimal for terraforming would of course have a similar size to earth. I guess you could reduce gravity by removing a large mass of the planet but this isn't easy to do, eh?

Of course there are a lot of other factors to be considered. I found the wikipedia page about terraforming mars quite interesting if you want to dive deeper into that topic.

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A planet that has already had a Great Oxidization Event

You should take a look at the Wikipedia article for Earth's Great Oxidization Event. In particular, if you look at the graph at the top of the article you'll see that it took over a billion years of oxygen production for oxygen levels to reach 0.1 (which corresponds to the atmosphere being 10% oxygen). Stages 2 and 3 in that graph are important to note - those are the periods during which the sea and then the land are absorbing oxygen. It is only once both the oceans and the land are fully saturated that oxygen levels are really able to increase.

Here's something else that's interesting - plants need oxygen to live. If you took an absurd amount of seeds (of grasses, trees, and everything really), and planted them on a barren planet that's in a star's habitable zone, they would die due to a lack of oxygen. Same thing if you took the effort to place already-grown plants. They are net producers of oxygen, but at times (such as during the night), they consume more oxygen than they produce. The oxygen they produce would be absorbed by the land around them, leaving none for them at night when they will need it to survive.

Don't forget that most plants also need soil, which a barren planet doesn't have. Soil is a mixture of minerals and organic material, and that organic material has to come from somewhere. It's not unreasonable to assume that a civilization capable of travelling to a new planet would have enough bio-engineering capability to produce plants that can start the process of soil production, but it's also likely that those plants aren't what they would want to have as the final product for growth on the planet.

So the easiest planet to terraform would be one that already has plenty of oxygen and soil. It's probably what Earth looked like right around the time that life started to leave the oceans.

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    $\begingroup$ you might want an earth similar to the Ordovician, so it has no land animals or complex land plants your earth life has to compete with. There are plenty of plants on earth specialized in colonizing sterilized soil, usually around volcanoes. $\endgroup$
    – John
    Jan 5 at 21:54
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Depending on the amount of earth creatures you intend to import to the planet later, there should be seasonal weather changes, a slowly orbiting moon close enough to induce tides, a magnetic core, lots of water to moderate the days, a yellow sun like our own sun ready to power chloroplast filled life.

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