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The Background:

I am going to be asking a series of questions that will be relevant to forming some sort of a picture of human space commerce.

Let's say that Earth-based human civilizations have discovered a series of ancient jump-gates that allow them to travel within a large and varied interstellar network.

There are not many clues, apart from the jumpgates, as to who left this system behind. For the moment, I am assuming that there is no bias to the kind of systems included in the network: i.e. its not like systems with earth like planets make up the majority of the planets in the network. So, "system types" have roughly the same probability of occurrence as if one were just taking a cross-section of space and scanning it.

Put another way, the gates simply connect a large number of close-by star systems, rather than a large number of only useful star systems.

While genetically-engineered humans exist in this "universe", no sentient non-human aliens have yet been encountered.

The Question:

If a space faring civilization can build space stations, why would they ever bother with planets?

For resources, space stations could be built around asteroid belts. Otherwise, space stations may be placed in almost any orbit or other stable configuration.

So, what might planets offer, that stations simply cannot? If terraforming technology exists, assume that it is time consuming (centuries).

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    $\begingroup$ They would bother with (Earthlike & terraformable) planets because they are nice places to live. Why do people with sufficient wealth to do so purchase country estates, rather than choosing to live in urban tenements? $\endgroup$ – jamesqf Feb 24 '15 at 4:23
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    $\begingroup$ Just for the record, someone already used that idea.....whoever wrote stargate. $\endgroup$ – JDSweetBeat Feb 24 '15 at 15:43
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    $\begingroup$ @Dustin Jackson - it is a fairly common scifi trope. The tabletop game Eclipse Phase also has jump gates. $\endgroup$ – Brad Feb 24 '15 at 20:52
  • $\begingroup$ @Brad Mass Effect (the game) has Relays - that are jump gates and Freelancer (the game) has jump gates. $\endgroup$ – SovereignSun Oct 15 '17 at 4:17
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There's a couple of upsides, plus one obvious large downside.

First, planets are much more robust than space stations. A full ecology has a lot of depth, layers and layers of life and interactions. That depth allows it to absorb disasters and keep ticking - it doesn't negate the bad effects, but it means they are less likely to wipe you out. A disease that eats your corn crop means you go hungry, but you survive on potatoes. On a space station you have a smaller, technology-enabled ecology that's rather shallow, which means it's going to have a lot more points where only 1-2 things can fail and your entire space station becomes unhabitable. If your algae vats get corrupted, you're out of food and air, and you might not have a quick enough solution to keep people alive.

Second, planets are already there. Building a city on a planet has start up costs, sure. But building that same city has the same start up costs, plus you need to build everything the planet gives you for free - air, water, food, gravity, and the space itself. Space stations are also 3-dimensional builds, which requires more material than a 2-dimensional city for things like infrastructure. If this doesn't seem important for a city, think about the cost of creating a cattle ranch in space vs on a planet.

Finally, the counter - a planet is deep inside a gravity well, which negates some of the benefits listed above. Getting in and out of gravity wells can be incredibly difficult unless you have something like a Space Elevator to help you, which increases those expenses greatly.

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  • $\begingroup$ #2 is dubious, depending on your assumptions going into it. I'd argue a station is generally more likely to be already there because the ship you rode in can be converted to a station by just putting it in park! Maybe the OP's jump gates mean ships can't do the longer term sustainability required to be a viable station, but it also says terraforming is an enormously difficult process, so while the planet might physically be there, and might even be close to Earth like, it isn't likely to be ready to sustain humans long term either. $\endgroup$ – Adam D. Ruppe Feb 24 '15 at 18:55
  • $\begingroup$ yhep main problem with that question - before build ranch on planet you have to make it habitable. Before relay on those layers of biosphere you have to create this biosphere, and we are very picky in therms what we find as good biosphere, so just stable is not good enough for us. In therms of biological hazards - building robust system in space is easier then on planet, specially when used free planet resources. $\endgroup$ – MolbOrg Sep 5 '16 at 22:42
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If humans can build condominiums, then why would anyone live in houses?

Angular Velocity

We know only two ways to simulate gravity. One uses the mass of a planet. The other uses spin. It has been speculated that humans might find some aspects of spin-based gravity uncomfortable. For example, certain kinds of motion (nodding or shaking one's head) may cause odd sensations in the inner ear.

Natural Geography

Some people may simply prefer natural geography to artificial geography. For example, a mountain climber may prefer a planet because the mountains are taller. Or because the mountains are more variable. Or because there's simply a greater variety of mountains on a planet.

Size

Compared to a planet, space stations are small. If one likes to see new sights, one may well prefer the sheer variety available on a planet. Or perhaps flora is your thing. Or fauna. Maybe you like undersea exploring (how deep are the seas on a space station?).

Stubbornness

"My parents lived and died on a planet and so will I." Even if energy and raw materials are cheaper on space stations, some people may still prefer planets. Just because.

Anti-technology

A space station is built with technology and runs on technology. It's hard to envision an Amish lifestyle on a space station.

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    $\begingroup$ I often like to imagine that in the 28th century there will be a set of people who insist on living only on 25th century technology, because that was exactly the right amount of technology. Life was so much simpler in the 25th century... $\endgroup$ – BrianH Feb 24 '15 at 3:04
  • $\begingroup$ "Or because there's simply a greater variety of mountains on a planet." - Or because there's not. You can make artificial or simulated mountains indefinitely taller/harder, but there's only one Everest. $\endgroup$ – Random832 Feb 24 '15 at 18:21
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While many of these answers are pretty good, I think there's one very simple reason people would use planets over space stations. It's been said before, but I don't think enough emphasis is placed on it yet.

Planets are Incredibly Big

Seriously; there's 7 billion of us living on a fairly moderate sized little hunk of rock, which consists of 70% uninhabitable ocean, and even of the 30% of land that it has, most is uninhabited.

Doing a quick conversion of the Earth's land area (estimated at 150,000,000 square kilometers) divided by the number of people on earth (estimated at 7 billion) shows us that the average amount of land available to each human currently alive is 21400 square meters. (Which is about 5 acres)

That means if the Earth's land area were fairly divided amongst the people living today, every person would have an area of land big enough to build about 100 houses with gardens on.

This answer to a question about overpopulation says that the Earth could house as many as 2.7 trillion people if we were to convert most of it into cities. You're not going to be able to build enough space stations to ever compare to the kind of ground area a planet supplies.

When you hear "a planet is larger than a space station" it's easy to miss by just how much the planet is bigger.

Although technically, if you have so much energy you run out of other stuff to do with it, you could consider taking apart the Earth entirely and converting it into space stations. If we take a look at the weight of the Earth (5.97219 × 10^24kg) vs the weight of the International Space Station (450.000kg), then the Earth would provide enough building materials to build 1,327,150,000,000,000,000 ISS sized space stations.

That's roughly 1,895,000,000 space stations (almost 2 billion) per person alive today. Of course, we'll not go into the cost in terms of energy or labor for doing so, considering even a single ISS costs upwards of $150 billion dollars.

But yeah. Planets are big. And they're just floating there. I don't see why you would not use them.

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  • $\begingroup$ Why do you think you can build 100 houses with gardens on 5 acres? Unless your idea of garden is a few potted plants on a windowsill. $\endgroup$ – jamesqf Feb 24 '15 at 18:31
  • $\begingroup$ Using standards from my own country, I guess. The plot I live on has a house and garden and is only about 140m2 or so, so it'd fit easy. By US standards it would be a bit more cramped, because the US is very sparsely populated. (Or very big; depending on how you look at it) $\endgroup$ – Erik Feb 24 '15 at 21:21
  • $\begingroup$ maybe because, specially if we are flexible enough to think about taking earth apart, that artificial object with 10m high ceilings and with radius (as sphere) of 107km will have usable (not for all goals but for those which are mention) area just about earth surface area(oceans etc included). Orders of magnitude, but mass of that object is expected to be 0.000026% of earth mass. Not all material will be good for such station, but if 0.5%(carbon abundance) will be usable for it to build, 19530 stations from one planet. If we are in demand of surface, it might be reason why not to use. $\endgroup$ – MolbOrg Sep 7 '16 at 13:33
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Valuable to whom exacly?

Sure, to the ruling elite planets are deep holes; and even if it was covered with pure gold it wouldn't be worth the fuel required to fetch it.

All that changes when you consider moving there. A couple of robo-miners, a fabrication box and you can live out your life in luxury. Throw in a genetic variation set and your family can last forever.

And when you have a population of 5 billion: it's still pointless trying to export steel; but information doesn't care about gravitywells. Just imagine the scientific experiments you can perform when you have free gravity and a massive neutrino-filter just below your feet.

Software is also exportable; the latest mobile phone has all the "Angry birds"-clones of a thousand planets at your fingertips.

Bitcoin; and other computation time is also easily exported.

All of this is mostly true for the interstellar empire as well. There just isn't that much to expect from other planets/solar systems than more of the same. Exporting steel from one solar system to another; why bother?

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    $\begingroup$ You lost me at Bitcoin. $\endgroup$ – Mordred Feb 24 '15 at 15:51
  • $\begingroup$ @Mordred just think of it as a particularly shallow example of computing capacity as a commodity. $\endgroup$ – Random832 Feb 24 '15 at 18:24
  • $\begingroup$ you should answer more question, good answer. $\endgroup$ – MolbOrg Sep 7 '16 at 12:57
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Just to complement the good answers already here, let me bring up defense. Specifically, camouflage.

Suppose a hostile environment in which your enemy might track your gates or your movement through them.

If at the end of the gate there's a space station, the enemy can just shoot away. You may have to place your base astronomically far from the gate to give them a search problem.

On the other hand, if there's a planet with permanent cloud coverage, and your base is over a few dozen miles from the gate, the enemy is now lost, regardless of whether the gate leaves them out of or in the planet (you didn't specify).

Of course they can explore, but likely they were detected when exiting the gate, and your own defenses can do their thing.

Additional precautions may be having false tracks in different directions if the gate is on land, or having supply ships change course below the clouds if the gate is in space.

As a downside, though, a base in a planet is very vulnerable to bombardment from space once the enemy knows where it is. No warheads needed, just throw rocks at exactly right angles.

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  • $\begingroup$ Depending on the enemy's morality and the inhabited-ness of the planet, they might not even need to know where the base is before they start throwing rocks. The only realistic defense would probably be to build something like Battleplates, if you have the resources to pull off such a project. $\endgroup$ – Mason Wheeler Feb 24 '15 at 15:56
  • $\begingroup$ @MasonWheeler: true, but destroying a whole planet would require time to accelerate the missile. Longer time if the gate leaves you at the surface. In this time, the defences could act. $\endgroup$ – Emilio M Bumachar Feb 26 '15 at 4:50
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Others have covered some very good points. I'd just like to add the following:

Rare Resources - organics

While it is highly probable mineral resources could be extracted from asteroids or other small bodies the complex organics created by life are unlikely to be present in any quantity off of significant planets. While we are slowly mastering genetic engineering there are now many plants that cannot be grown outside of their original habitats and we understand the ecology of Earth from thousands of years of study. A plant, fungus or something entirely new that has a medication like effect on people might not be easily created outside it's home environment on an alien planet.

Obviously trade in something like that depends on very low cost to planetary travel but the neat thing about hypothesizing about things like this is that costs are likely to be high at first but decline over years until they are trivial. Rockets are a resource expensive way into space. Self replicating, self assembling nano built beanstalks could be much less so and given a few thousand years such imaginary technology looks less and less like magic and more and more like something you could do with your phone. Will our understanding of medicine or life on other worlds and how to farm it grow as fast as our travel technology and make it so everything we ever find could be grown everywhere? There is no way to know.

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Gravity

Once your space-faring civilization enters a new star system, they still have to get around on their own, and can use (particularly massive) planets for gravity assists. The other benefit to gravity is that a planet with similar mass as Earth can offer a lot of space to stretch their legs.

Resources

We only speculate what resources are available on exoplanets, but they vary greatly in size and probable composition. If a particular resource (water/ice, oxygen, etc.) it's possible that at least one planet in each candidate star systems has what your civilization needs. And LOTS more of it than a comet or asteroid can provide.

Protection

If any of the exoplanets have an atmosphere to protect from space debris, or can protect from harmful radiation, then it would be a great place to make a cheaper 'space station'.

Geology / Morphology

Rocky and gassy, all-water, and acid-raining planets would be so cool to explore to understand processes on our own planet, as well as provide evidence of potential life.

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Mining Camps

Tectonic processes concentrate ores. We don't know the mineral distribution of the asteroids, but it is likely that they did not have tectonic processes to concentrate ores (there may be other things which cause concentrations... but...)

Gold may be rare, but it comes in mother-lodes in tectonically active planets. Ditto for rare earths, etc.

Planets may end up as nothing more than mining camps, and the odd-ball or two who likes natural settings.

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  • $\begingroup$ Actually, this is the other way around. Asteroids seem to mostly be made from a few different materials, and there's not enough mass to force them to make the stuff you want hard to access. If you want an iron asteroid, it's likely a mass of 50% iron, for example. On a planet, by far most of the iron has dropped down into the core, a lot harder to access. There's some stuff that's comparatively easy to find (and concentrate) on a planet - like uranium. But gold is a prime example of the opposite - Earth's primordial gold is in the core. The surface stuff is (probably) from asteroids. $\endgroup$ – Luaan Feb 24 '15 at 8:37
  • $\begingroup$ @Luaan: But the gold & other rare elements are probably dispersed fairly evenly through the mass of asteroids. On Earth (or other geologically active planets), geological processes have concentrated some of the interesting minerals. Even in a fairly rich gold ore body, you need to process 4-5 tons of ore to produce one ounce of gold: en.wikipedia.org/wiki/Carlin–type_gold_deposit $\endgroup$ – jamesqf Feb 24 '15 at 18:40
  • $\begingroup$ @jamesqf Actually, it doesn't seem so :) en.wikipedia.org/wiki/Asteroid_mining In fact, all the gold, cobalt, iron, manganese, molybdenum, nickel, osmium, palladium, platinum, rhenium, rhodium, ruthenium, and tungsten mined from Earth's crust, and that are essential for economic and technological progress, came originally from the rain of asteroids that hit Earth after the crust cooled. They were concentrated in the asteroid, dispersed by impact and tectonics, and then concentrated again (e.g. by rivers). And they tend to be rather pure. $\endgroup$ – Luaan Feb 25 '15 at 8:35
  • $\begingroup$ @jasqf Since there's little free oxygen, it's a lot easier to use the materials right away - it seems that a lot of the steel humans originally used came as is from asteroid impacts (iron + nickel, natural stainless steel, without the pesky oxygen and sulphur). Refining is trivial compared to planetary metals. A small 10-meter S-type asteroid contains about 650,000 kg (1,433,000 lb) of metal with 50 kg (110 lb) in the form of rare metals like platinum and gold. M-type asteroids have a much bigger ratio. $\endgroup$ – Luaan Feb 25 '15 at 8:38
  • $\begingroup$ @Luaan: Where is the evidence that the rare metals were in any way concentrated in the asteroids, rather than being evenly dispersed throughout the volume? That is, any particular chunk of asteroid (say the parent of a iron/nickel meteorite) you'd have lots of iron & nickel, some cobalt, but maybe parts per million/billion concentrations of the rarer elements. Useful if you want steel, much less so if you want gold & sliver, rare earths, fissionables, etc. $\endgroup$ – jamesqf Feb 26 '15 at 6:00

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