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In Science Fiction, when civilizations grow powerful enough to conquer the planet and are in need for room, they turn to the planets in outer space. Obviously, the other planets have very different conditions, some are hot, some are cold, and all don’t have proper air that we can breathe. At this point, there are two common options:

  1. Terraform the planet such that it becomes sustainable for us humans. Reheat the core, add some plants and gas, you name it.

  2. Stripmine the planet for resources to build artificial worlds, such as Dyson Spheres, Bubble Worlds, Ring Worlds and other planet-like environments.

It can be assumed in this case, the given civilization has more than enough resources and time to do both. However, they still have their limits, and need to decide which planet is better off being terraformed, and which planet is better off being stripmined. Because, if they are not careful, a terraformed planet would be dangerous, while building bubble worlds might be a waste of time.

So, what are the factors that can help civilizations decide which option better suits which planet. Use our planets in our Solar System as the easiest example.

Also, let us assume that FTL doesn’t work properly yet, so going to far away planets similar to Earth is a no-no.

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    $\begingroup$ If you can cross space, then why bother doing either activity at the bottom of a gravity well? Lots of room for billions of orbital habitats, and plenty of asteroidal material floating around. Might vacation by a "beach" or a "mountain" or a "desert" just for the novelty value, maybe try non-zero-gee sex for the same reason. But live or work there? Bah. $\endgroup$ – user535733 Jun 24 at 3:10
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    $\begingroup$ Have to agree with user535733. A space faring civilization would tend to ignore bodies with large gravity wells for mining purposes. If someone arrived in our solar system and found a dead earth it would not be their first pick for mining - the moon or asteroids yes. Earth no. The exception would be if the planet in question had a lot a rare elements in its crust. If they really wanted to mine planets the best option would be to literally blow them up via a large kinetic impacts then harvest the smaller pieces (planetary cores would be useful - lots of metals). Needs careful planning though. $\endgroup$ – Mon Jun 24 at 4:04
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    $\begingroup$ Why not both? No, really, Why Not Both? <-- there, minimum comment length handled $\endgroup$ – Carl Witthoft Jun 24 at 13:01
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    $\begingroup$ Why can't you stripmine it then terraform it? $\endgroup$ – DKNguyen Jun 24 at 15:15
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    $\begingroup$ Your civilization's need for materials is likely to grow much faster than it's need for habitable living space. Most planets will be strip-mined, only a few will be terraformed. $\endgroup$ – cowlinator Jun 24 at 22:33

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Terraforming is going to be a far greater investment than strip-mining.

However, the cost will still vary wildly based on what the planet is like. Planets which are close to the target for terraforming will cost less to terraform, but will be in short supply.

So the logic will likely go:

  1. Are we really low on a particular resource this planet has? Strip mine it.
  2. Is it vaguely habitable? If so, terraform it.
    • not toxic
    • low radiation
    • sensible day/night cycle
    • gravity is reasonable
    • temperature within a few 100C of the target?
  3. Has it got materials worth the investment of mining? Mine it.
  4. Has it got some strategic importance (refuelling, defence, etc?)? Build an outpost.
  5. Otherwise, survey it for future reference, then move on.
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  • $\begingroup$ Not only that, but any technology that makes terraforming feasible in general probably also makes strip-mining unnecessary. You can transmute and create oxygen-filled skies from those poisonous clouds? You no longer need to dig in the ground for traces of neodymium. $\endgroup$ – John O Jun 26 at 17:30
  • $\begingroup$ @JohnO yeah, I have no idea what technology would be needed to terraform in a meaningful timescale. But it might require stripmining huge amounts of resources from other planets to drive it? I imagine 100 worlds mines to terraform one. $\endgroup$ – Dan W Jun 26 at 19:49
  • $\begingroup$ Generally, there are only four aspects to terraforming. Atmospheric modification, weather modification, climate (temperature) modification, and biosphere modification. The first two are about volatiles... not easily strip-mined, the third is about orbital engineering, and the last is about removing biologicals that are incompatible/dangerous with the terraformers. I just don't think mining can help much. But maybe I'm missing something $\endgroup$ – John O Jun 26 at 20:04
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    $\begingroup$ @JohnO I’m imagining mining huge amounts of uranium/tritium for the reactors required to power terraforming (They probably don’t want to use a Dyson sphere as they want the system’s star to keep shining), or large amounts of catalysts for reactions of volatiles, or lots of acidic/basic compounds to pH balance a planet... a project of that scale feels like it should need a lot of resources? We don’t have any tech to do it at the moment, but if the OP needs a motivation to strip mine planets for their plot, it doesn’t seem unreasonable. $\endgroup$ – Dan W Jun 26 at 20:12
  • $\begingroup$ @DanW Why use nuclear power? For gross atmospheric changes, your best bet is wide dispersal of distributed, self-replicating, solar-powered chemical processors - or, as you may prefer to know them, plants. (Well, bacteria and algae mainly. Plants come later to transform the soil by leaching harmful materials and reducing their bioavailability.) $\endgroup$ – Cadence Jun 26 at 22:43
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  • If it is a planet, terraform.
    Sure, hollow habitats are neat, but if you have the tech and energy budget that there is a genuine choice to strip-mine a planet, you are rich enough to afford the luxury of a planetary habitat. Wind and sun in your face, hiking the hills and sailing the seas, that just doesn't feel the same in an artificial habitat. People are silly that way.
    (Trick question: Would you prefer a genuine lemon or synthetic lemon acid?)
  • If it is a minor planet/KBO, strip-mine.
    Even if energy is plentiful, it will probably not be entirely free. So it makes a difference if you crack many little rocks or one big rock. Take the little ones.
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    $\begingroup$ I would amend that to "rocky planet". There's no point in trying to terraform a gas-giant like Jupiter or Saturn. Of course strip-mining doesn't necessarily apply there either, but you could still extract useful materials by other means.. $\endgroup$ – Darrel Hoffman Jun 24 at 13:41
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    $\begingroup$ Also keep in mind that the energy cost of strip mining will increase with a larger planet due to the higher gravity. All of that material has to be lifted from the surface which can be quite costly for a reasonably sized planet with even earth-like gravity. Atmosphere also will play a role in the process. In the end the smaller the planet(oid) the easier it will be to strip mine. Terraforming doesn't suffer this setback $\endgroup$ – Kai Jun 24 at 17:17
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    $\begingroup$ @DarrelHoffman No point trying to terraform a gas giant?! Sounds like the words of a Type 1 civilization :P $\endgroup$ – DKNguyen Jun 25 at 2:02
  • $\begingroup$ @Kai Terraforming does suffer it, but in a round-about way. Any gases you need to add to the atmosphere from off-planet will add heat, roughly proportional to the mass of the planet. Bigger planets make the gases pick up a lot of velocity on their way down, which ends up becoming heat. The only way to reduce this heat is to let it slowly radiate away, which can take millennia. Planets just plain suck. $\endgroup$ – Ryan_L Jun 25 at 3:34
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I don't think terraforming really ever makes sense. It just takes so incredibly long that it's just much more efficient to make orbital habitats by stripmining asteroids. In our own solar system, there is enough matter floating around in asteroids and minor planets to make many tens of thousands of habitats providing thousands of times the surface area of the Earth. The only time I could see terraforming being worth it is if a planet is nearly habitable and just needs tweaked a little, and also your propulsion technology makes gravity wells negligible.

If gravity is a real concern, life on a terraformed world will always be more expensive than life in an orbital habitat; transiting the gravity well will incur an added cost that will increase the price of any good or service involving the planet, which hab-dwellers will not have to pay. Consider if you want to buy some new widget on Space Amazon. If you live on a planet, the Amazon delivery shuttle has to be strong enough to survive re-entry, has to have enough thrust to make a soft landing, and it has to be able to carry enough fuel to make it back to orbit on the way back. Now lets say you live in a habitat in interplanetary space. The Amazon delivery shuttle only needs enough fuel to reach you. It doesn't need a heat shield, and it doesn't need expensive, high thrust engines. It can also skimp on fuel by using a low-thrust, high specific impulse engine.

I would go further and say stripmining planets doesn't make sense either, because these same launch cost concerns apply just as much to mining as they do to habitation. Planets are inefficient, O'Neill cylinders are the best bang for your buck.

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    $\begingroup$ Terraforming planets has several advantages: it's a lot more stable than a habitat, because once it's set up it sustains itself without any technological intervention, and also it's likely to be more preferable by those who can afford it to live on a habitable planet over a space station. $\endgroup$ – Starsong67 Jun 24 at 11:02
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    $\begingroup$ Depends how long-lived and how inately long-thinking your species is. Humans aren't. $\endgroup$ – DKNguyen Jun 24 at 15:11
  • $\begingroup$ Agree that the timeframe matters. With enough time, inhabitants of the surface won't need to order from Space Amazon, since they'll have local infrastructure. On the other hand, strip mining doesn't get any easier over time, since you're always just shipping raw materials offworld. In the long run, it'd be better to process raw materials on the planet, and only ship finished products offworld. $\endgroup$ – Nuclear Wang Jun 24 at 15:43
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    $\begingroup$ Given a planetary mass worth of material, the amount of livable space you can create in habitats is orders of magnitude greater than a planet. You also avoid dealing with the hugely inconvenient gravity well. A large enough habitat (the only ones worth comparing to a planet) will also be largely self sufficient in resource recycling. They also have the benefit of using their mass as shielding instead of trying to get a planetary magnetic field powerful enough to do the job. On the other hand, terraforming an entire planet to be livable will take thousands (maybe just hundreds) of years. $\endgroup$ – Michael Richardson Jun 24 at 17:28
  • $\begingroup$ @MichaelRichardson space habitats require exotic materials which mean orders of magnitude worth of a planet's mass is wasted just looking for those materials in proper proportion to what you are building. Also, building a space habitat that can house a planet's population would also take hundreds if not thousands of years. The difference being that terraforming can more or less do itself just by introducing the right kinds of lifeforms whereas habitats take tons of labor to build. $\endgroup$ – Nosajimiki Jun 24 at 22:07
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No FTL? Terraforming is strongly preferred.

Consider this - terraforming takes a long time. Do you know what else takes a really long time? Getting anywhere without FTL. There are 33 stars within 12.5 light years of Earth. Travelling at 0.5c requires having about 1.4e16 J kinetic energy per kg, and via e=mc^2 we know 1 kg could be converted into about 8.9e16 J kinetic energy. So if your power source converts mass into energy at 100% efficiency you need about 15kg of fuel per 100kg of ship to slow it up, and then that much again to speed up the ship + fuel in the first place. That lets you get to 33 stars within 25 years.

That's a long trip with a very high energy cost (even for an advanced civilization). It's much more likely that you'd have ships travelling slower and using some form of stasis or cryosleep. That means that between scouting a planet and having the next group of people landing on it, you'd likely have decades or centuries in which to terraform the planet, and even if the terraforming is only partly done the planet will be more survivable than an airless rock.

Consider also the consequences of an accident. If you crash into an airless rock you're likely dead - no FTL means no quick rescues, and you have to rely completely on your (potentially damaged) life support systems. If you crash into a habitable planet it significantly extends the amount of time you have for someone to rescue you, possibly indefinitely.

Having a habitable planet in a system also can serve as a stable base of operations for everything else. Again, with no FTL any help from outside the star system will take years to know that you need it, then years or decades to arrive.

Another thing to consider is using the resources. If you strip mine a plane, sure you've got a lot of resources but what are you going to do with them? Shipping them to another star system is going to run into the same problems of prohibitive energy costs or long shipping times. A planet also acts as a place where resources can be used.

All in all, star systems with a planet that can be terraformed will be much more useful than those without one.

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  • $\begingroup$ the chances of crashing into a habitable planet unless you already terraforming planet are too small to reasonably include in a risk assessment. in all likelyhood the ship doing the traveling IS a megastructure. $\endgroup$ – John Jun 25 at 4:26
  • $\begingroup$ An interstellar journey is much more likely to be some kind of generation ship (large space habitat with engines) rather than a sleeper ship. Any such trip will not be to send back resources to the home system; it will be to build additional habitats and gather fuel and resources. Planets will likely only be used once an orbital ring has been set up with tethers to allow easy access to the surface. $\endgroup$ – Michael Richardson Jun 25 at 19:52
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Disassemble everything: asteroids, planets, stars.

A constantly-growing civilization which can think in the long-term will inevitably realize that it will hit constraints on population density based primarily off the number of people its resources can support, and once that happens, demographic crisis looms. Its goal, then, will be to obtain as much territory as physically possible to stall its collapse. Planets are positively useless at this compared to the thousands or millions of times the surface area one could get by breaking them down into cylinder habitats; stars are incredibly inefficient compared to starlifting them and shepherding the fusion reactions in a more distributed, long-lasting manner.

In some settings, a universal government will be able to rein people in, imposing hard birth limits or trying its best to disincentivize births. In a setting without FTL, as proposed here, splinter groups intent on growing become impossible to coordinate due to light lag, and they'll realize that they need all the living space they can get. Yes, birth rates might become lower because of economic development, but you only need one fringe culture who doesn't like that idea for this problem to arise.

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They will terraform any planet that they can and ignore most of the rest

As much as I love theorizing about mega-structures, the cost of pulling matter out of a planet's gravity well is immense; so, unless your civilization is using some manner of Non-Newtonian, free energy method of strip mining a planet, chances are that what ever fuel they are using to strip mine it is thousands of times more valuable than the materials they can hope to gain for thier efforts.

Also, once you build a space habitat, it has a very limited cycle of natural resources at its disposal. The trade-off for efficiency in any system is always resiliency. Inefficient systems like planets always have ways you can optimize your use of resources to overcome obstacles, but when an efficient system is hit by the unexpected, there is no compensating. This makes space habitats short term solutions, long term problems compared to planets.

There is also the issue of how economies might introduce new technologies into societies. On a habitat, your resource cycle is all spoken for. This means if you want people to have a new luxury item, then you will need to manufacture everything somewhere away from your habitat, then ship it in which is both very expensive and takes a very long time considering that you are a pre-FTL civilization. So, by the time you get your "new" products they are already decades old technology. In contrast, a planet would be able to pay a royalty fee to the engineering firm on another world which designed the new product, and then manufacture it themselves. This means you would be able to propagate new technologies at the speed of light instead of sub-luminal speeds.

What about low mass planets and planetoids?

Comments raise a good point though about planets with low enough of gravity to get mass off world using rail guns. If you live in a star system where you have a highly populated planet which lacks certain key resources, mining smaller planets this way might be useful, but you would not be "strip mining" them. You would be selectively taking just the ores you need and blasting them off to thier destination. This is because common elements you would normally strip mine such as iron, nickel, or silicon are way more abundant than other useful elements you may need like lithium to the point that you just won't have a use for most of what is in a planet. It would be more economical for an inhabited planet to continue mining it's own common elements and only import the scarce ones.

For example: Let's say building a mega-structure requires the same amount of lithium per kg as the international space station. Some rough estimations based on what is published about its specs tell me that the space station probably uses 300kg of lithium which would constitute about .07% of the station's mass.

However, when you look at the Earth, we have about 2e18 cubic meters of material that can be effectively mined and about 5.7e7 of those cubic meters are believed to be economically viable lithium. That means that only 0.00000000285% of the Earth that can be mined is worth mining for lithium. So if you were to strip mine the entire top few kilometres of Earth's crust to make a mega structure, 99.9996% of what you mine would be useless because you would not have enough lithium for a space habitat that uses more than 0.0003% of the total material you have at your disposal.

What if a low mass planet or planetoid does not share a system with a populated world.

In this case, a better approach to small planets may be a system of hybrid terriforming / mega-structures. Because the gravity is already so low, you can build things up MUCH higher. Instead of spending tons of power shooting the planetoid's mass off into space one chunk at a time, you could simply leave the mass on the planet where it is easily accessible to you and build up (or down). The lower gravity means you could slowly sculpt the planet into a mega structure all while still living on it in habitats. The end result would be the outer "crust" of your planetoid builds up into a Ecumenopolis, while your mines go deeper and deeper in search of more ores.

There is no real reason to endanger your survivability by separating your habitat(s) from readily available sources of raw materials.

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  • $\begingroup$ Or if he planet is small enough to build rail launchers. A planet just a bit smaller than mars would be minable using only electricity. $\endgroup$ – John Jun 24 at 16:13
  • $\begingroup$ Yes, but can you vs should you remains an issue. If you need many square miles of solar panels to build just a few cubic meters worth of space habitat per year, then your efforts are probably better spent just building habitats on the miniature world. By the time gravity gets low enough to make rail launchers trivial enough to be better than just building self contained habitats, you're generally no longer really looking at "planets". $\endgroup$ – Nosajimiki Jun 24 at 16:45
  • $\begingroup$ I don't understand what you mean, why would you be using the solar to build space habitats, I am just talking about economical mining. there are things in much higher concentrations on planets, like hydrates or some light gasses, which could make them economical. There is no fuel being consumed, just energy. $\endgroup$ – John Jun 24 at 17:21
  • $\begingroup$ You need electricity to get the material off world. Mega structures and the planets they are made from are extraordinarily massive meaning even if the planet has a low gravity, it will still take a really large amount of power to lift any significant portion of it into space. $\endgroup$ – Nosajimiki Jun 24 at 21:18
  • $\begingroup$ That said, the issue of exotic elements is probably an even bigger limitation. Space habitats are technologically advanced systems meaning they need certain amounts of certain elements that you can't really find in appropriate ratios; so, even if you were to mine a whole planetoid, the vast majority of what you dig through would be unusable for a mega structure. $\endgroup$ – Nosajimiki Jun 24 at 21:21
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If the planets are unable to be terraformed, like those seen around red dwarfs, then strip mine them. This is because planets around class M stars usually have the star defeat all attempts to terraform the planet, since the solar radiation will strip away any atmosphere you may add. In fact, it's now theorized that planets like the Trappist 1 planets may all be Venusian in nature, and as such would be impossible to terraform.

Also, planets not in the ecosphere of the star in question cannot be terraformed, so strip mine those as well.

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  • $\begingroup$ If the planet has a high magnetic field, it could be protected from the solar radiation. Also, we do have some theorised schemes for terraforming Venus and other venus - like worlds. However, I do agree with the general statement of "terraform anything we can terraform, then strip-mine anything we can't" $\endgroup$ – Starsong67 Jun 24 at 11:00
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The game Spaceward Ho! demonstrates a key criterion for this decision. Some parts of a world can be modified by terraforming (which the game represents as "temperature") and some cannot ("gravity"). Planets with good gravity (the range of 3/5 to 5/3 that of your homeworld) are good for terraforming. Planets with bad gravity (less than 2/5 or more than 5/2) are suitable only for strip-mining.

Ho! includes a category I haven't seen addressed very much: The intermediate case between "good" and "bad". These planets can be terraformed and turned into decent colonies, but it takes more resources (time and money) than good planets.

The general principle applies: All planets will have some characteristics that cannot be modified through terraforming. So colonize and modify those planets that are closest to the homeworld specs, and do a cost-benefit analysis on intermediate cases. Sometimes the answer is "strip mine now, abandon the planet, and we'll come back if constraints permit terraforming the world."

(On review, my answer provides a decision path for "vaguely habitable" as Dan W discusses in their answer. My answer is not a duplicate, as I address the "maybe" case.)

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Neither option would be acceptable for any advanced civilisation, which would by definition be aware of the ecological consequences of its actions. They would take a 'tread lightly' approach and ensure that planetary life, archaeology etc is nurtured and encouraged, not exterminated for gain.

They're not idiotic, genocidal 18th-century colonialists for goodness' sake.

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Magnetic field or no.

A planet without a magnetic field gets harsh treatment from its star. The solar wind strips away any atmosphere, as happened to Mars when its internal dynamo faded. Those charged particles in the wind are also brutal for life (and electronics) on the surface. The only prospect for life on a planet with no magnetic field is in deep bunkers under the surface, with overlying rock or liquid, or superfat Venusian atmosphere serving as shielding. No amber fields of grain on these planets.

Getting a magnetic field for a planet that has none is a feat even more daunting than terraforming. Terraforming is turning Eliza Doolittle into a proper lady; generation of a magnetic field is turning her into a silver dragon. Where to start?

That is a simple calculus that could be used to decide. No magnetic field - get what you can and go,. Yes magnetic field - work with it and see if you can make it a nicer place for life.

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Why can't you just stripmine the planet and then terraform it afterwards?

There's no ecology so you aren't harming anything strip mining it. Presumably you are only collecting valuable minerals and elements from concentrated deposits which still leaves behind all the more mundane stuff that biology (as we know it anyways) is mostly composed of plus traces of more valuable elements (which are inaccessible to biology anyways if they are stuck in a concentrated deposit).

Then you can terraform after you strip mine and biology wouldn't be affected. And if the gravity well is an issue you could just mine everything and leave it on the planet, terraform it then use those resources to build on the planet. Very long term planners.

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Strip mining doesn't automatically placate the planet is uninhabitable. What are you extracting you have to ruin the entire biosphere?

Strip mining is a cheap method of mining, but depending on what mineral resource your extracting; that doesn't bode well for strip mines. China is full of them for rare earths and they're environmental nightmares. Mining certain metals either requires the metal be valuable enough to extract it in low parts per million concentration or in a high enough concentration the mine will be small and produce ore for the foreseeable future. Rare earth mines are notoriously uneconomical in the industrialized world, China does it with smelting heavy rocks and no regard for the industrial waste or toxic health effects. Africa does it with slave and child labor. And even if the site is strip mined, it can be....reformed.

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Terraforming is valuable for 2 reasons:

  1. shortage of living space for the evergrowing human population
  2. human luxury - having the option to live in and experience different worlds

Point 1 will get solved when people learn to live in free space. Which isn't that hard considering we can more or less do it right now - but won't have near-light speed travel or terraforming capabilities anytime soon.

Point 2 will be initially highly attractive - but eventually undergo a point of diminishing returns. Having 100 options is great, having 10^10 not so much considering many of them would be quite alike to each other.

At some point we'll also figure out how to recreate many pleasurable and aesthetic experiences on home planets and free space too.

The only reason point 2 might continue to hold importance even with 10^10 options is if the human population too grows at such an exponential level - and there is sufficient demand for the experience of exotic living that these many planets need to be entirely occupied to fulfill this demand. But to me it seems unrealistic to even attempt predicting human nature or desires that far into the future (when we have say 10^16 humans in existence).

Strip mining is not particularly valuable when compared to asteroid mining as mentioned in the other answers.

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