This question about eating on a alien world got me thinking.... Would it be easier for humans to colonise a world with existing life, or one which was biologically dead?

For the living world, let us assume a world with no technically proficient lifeforms, but one in which living material (i.e. forms of matter capable of using available energy sources to grow and reproduce) is abundant.

For the dead world, let us assume a 'goldilocks zone' world with an active hydrosphere (or at least sufficient water to create one given enough heat) and a G comfortable for humans.

On the one hand, the living world seems like the best choice, colonisation might be analogous to human groups moving into a new, unpopulated (by humans) environment on Earth in which the colonists learn to make use of the natural resources available. On the other hand, some of the answers to the linked question suggest that we wouldn't even be able to produce food on the new world as there is no reason to think that the alien biochemistry would be usable in any way by Earth-evolved life. In this case, the colonists would have to basically eradicate any existing life-forms and replace them entirely with Earth stock. This strikes me as a harder task than just starting from scratch on a suitable rock.

So, should future space colonists be aiming for that exciting distant world with the traces of life detected by atmospheric spectroscopy, or should they set course for the god-forsaken rock with nothing but the cold, lonely isolation of space for company?

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    $\begingroup$ Back in the 1970s, Larry Niven pointed out that interstellar colonists, having solved all the problems of living in space for generations, need not travel from planet-to-planet. Space habitats are their home, and the environment they grow up in and are familar with. You may see a "living world" as welcoming and preferable, but they will likely see it quite differently. $\endgroup$ – user535733 Sep 29 '19 at 21:42
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    $\begingroup$ @user535733 Space-dwellers might even look at a big deep gravity well like that of the Earth and actually find it a terrifying prospect, akin to being trapped. $\endgroup$ – Arkenstein XII Sep 29 '19 at 22:47
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    $\begingroup$ Also relevant, The War of The Worlds, 1897. The Martians chose a living world, with consequences for everybody. $\endgroup$ – user535733 Sep 29 '19 at 23:39
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    $\begingroup$ And then there's The Jupiter Theft by Donald Moffitt. $\endgroup$ – jwenting Sep 30 '19 at 9:07
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    $\begingroup$ Of note is that a dead world is much less likely to have an atmosphere that contains oxygen. This is far from a show-stopper for colonization (spacesuits exist, and science fiction is quite fond of climate-controlled habitat domes), but oxygen is a sufficiently reactive gas that its presence in the atmosphere of a planet is a sure sign of some kind of ongoing chemical cycle, and that chemical cycle may well be sentient. $\endgroup$ – Hearth Sep 30 '19 at 15:27

Future space colonists should aim for the lifeless rock. Problem with alien biochemistries is that they might be immediately hazardous (anaphylactic shock after a lungful of the local air) or maybe its just filled with stuff that causes horrible birth defects and chronic brain damage and your brave new world is going to turn into a hideous and drawn-out deathtrap at some point in the next few decades. It'll take you so long to work it out, one way or another, that you'll have to set up camp on the next nearest lifeless ball of rock (or build a nice orbital habitat) anyway, so you may as well make yourself at home there.

(also, a world with an active hydrosphere and an earth-like gravity sounds positively welcoming compared to the lifeless rocks in our own solar system. definitely not a place to complain about!)

More generally though, earth-like planets appear to be unusual, to say the least. Although we won't know until we manage a better way of surveying exoplanets, it also seems likely that life is pretty rare, too. Finding a world that would be hospitable for terrestrial life and has its own autochthons already? That's a pretty amazing find and one worthy of an awful lot of study. Wrecking it by contaminating it with your filthy microbiome greatly reduces its value and puts it at risk (because we might be the ones from the horrible all-consuming deathworld ecosystem) so it should be left pristine. That won't go down well with a certain kind of colonialist mindset, but you can always play them some videos of other manifest-destiny types dying of horrible xenobotanical allergies and hope it gives them pause for thought.


Breeding rats and monkeys and exposing them to the xenobiome might help prove safety, but rats and monkeys ain't human, as people reading (and doing) science often need reminding. Until you've had a statistically relevant number of people produce and raise a statistically relevant number of children to adulthood (and ideally to the point where they start their own families) you can't be sure you brain and fun bits won't melt horribly next week.

Just stay away from the alien replicators, especially if they evolved to live in environments a bit like you.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Oct 1 '19 at 19:12
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    $\begingroup$ In regards to your addendum, rats and monkeys aren't human be we are also equally alien to a new world. In tests on Earth we are looking at very specific things that might target humans, but on another planet we are looking at very broad things which target 'Earth biota', so a monkey probably is sufficiently close for a reasonable test. $\endgroup$ – Turksarama Oct 1 '19 at 23:24
  • $\begingroup$ Earth-like Planets (mass, size, habitable zone) may appear to be rare because they are still fairly hard to detect by current search methods. $\endgroup$ – collapsar Jan 16 at 0:13

Unfortunately, I suspect the best choice is

Start with a living world, then kill it

The dead worlds are unlikely to have water and breathable atmosphere. The living worlds have, well, life — at its worst, deadly on contact, at its best, kudzu and mosquitoes and funguses that eat rubber seals. But if you can hit the planet with neutron bombs, you can kill off even microbial life but leave the atmosphere and water usable almost immediately.


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    $\begingroup$ .... and the award for the darkest vision of mankind's future goes to..... $\endgroup$ – DrMcCleod Oct 1 '19 at 8:23
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    $\begingroup$ Peter F. Hamilton's "Fallen Dragon" has the exploration ships equipped with massive gamma ray emitters (iirc) for sterilising the biosphere. By the time of the book, they're used for an interstellar protection racket. $\endgroup$ – Roger Lipscombe Oct 1 '19 at 15:53
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    $\begingroup$ You can absolutely not guarantee that your sterilisation option will kill everything. How are you intending to irradiate every cubic metre of a deep ocean? Guess you're hoping that stuff can't survive undergound and spread back to the surface, too. Unless you're boiling the ocean and glassing the land you can't be sure, and if you are doing that, it seems like it'd be easier to just live somewhere else instead... $\endgroup$ – Starfish Prime Oct 1 '19 at 16:57
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    $\begingroup$ You don't need to sterilize everything, only enough that earth lifeforms have an advantage when it comes to eating the planet's resources. $\endgroup$ – Geronimo Oct 1 '19 at 19:30
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    $\begingroup$ @StarfishPrime I was under the impression that a neutron stream from space could penetrate pretty deep. I was figuring by the time they delved deeper than the sterilization, they'd be into "dwarves meet balrog" territory. Maybe I'm incorrect? Perhaps a good question to ask on the site: "What would it take to sterilize a biosphere?" In fact... come to think of it... I think I asked a question like that not too long ago, about Mars. $\endgroup$ – SRM Oct 1 '19 at 20:10

You will have to make assumptions about the likelihood and severity of problems.

  • The best case would be a highly compatible biochemistry. You can assume that the choice of amino acids and the chirality of organic molecules in terrestrial life is not random, but rather a subtly optimal combination.

  • The worst case would be an incompatible biochemistry. Starfish Prime explained that in this answer.

  • A lifeless rock falls somewhere in between.

You will also have to make assumptions how likely a world with the right gravity, temperature, etc. is going to develop life. If life comes up easily, then any suitable world will have a biosphere. There must be something wrong with the lifeless world -- too small, to big, too little atmosphere, too atmosphere ...

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    $\begingroup$ Chances of compatible amino acid structures are... lottery-level low. However, you can handwave that if you want to, as it's not Impossible. $\endgroup$ – Gloweye Sep 30 '19 at 7:12
  • $\begingroup$ @Gloweye: I don't think we have any insight yet in degrees of compatibility. Our digestive system is first going to soak food in a acid bath, after which peptases are going to make an effort to cut the proteins into amino acids. While we can directly synthesise 20, and 2 more with an extra step, we already know of 900 amino acids that occur naturally on earth. And that already gives us some interesting non-compatibilities - some are downright toxic if eaten. It seems we can live with that, literally. $\endgroup$ – MSalters Sep 30 '19 at 17:11
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    $\begingroup$ Not claiming it should kill us. But all our amino acids have a particular structure, and interact with our DNA is an extremely specific way. If for some reason there is even a minor difference of structure in DNA and animo acids, our entire method of synthesizing proteins will just not work with their amino acids. Best case it also implies we can't digest them and just poop them out, worst case it floods our transport systems and kills us that way. We might lack the comparison material for true insight, but there's so much fine tuned machinery that I don't believe there's much chance. $\endgroup$ – Gloweye Sep 30 '19 at 19:14
  • $\begingroup$ There's another possibility with the lifeless world, where there's something in the ground itself that prevents plants from growing. It could be high chlorine or salt content, or something else that makes growing things really difficult-to-impossible for plants to take root. It could be something as "simple" as a granite hard surface that has no cracks or "dirt" for plants to grow into. I'm not saying any of this is likely, but with endless possibilities in endless worlds, it's possible. Earth's mountains are granite, but have a covering of dirt to grow on. $\endgroup$ – computercarguy Sep 30 '19 at 20:23
  • $\begingroup$ Taking the mirror image of literally everything in biology changes nothing, so it isn't really scientifically sound to assume that the enantiomers would line up for some as-yet-poorly-understood reason. It is sound for fiction's sake (you just assume that your coin flip worked out, more or less, possibly after having failed on a previous planet). $\endgroup$ – Ian Oct 1 '19 at 14:49

Given the random element driving evolution and the multiplicity of possible chemical out comes it must be highly unlikely that an alien biosphere would be hospitable to life from Earth. There is every possibility that different bases, sugars, amino acids and many other novel unfamiliar compounds would have evolved. Some probably harmless, some less so and some toxic.

Life would adapt to its environment so the locals would be well adapted to live in their alien biosphere and would be difficult to eradicate. Earth based plants would be out competed and probably poisoned. So to answer the question, best to find a barren planet with suitable materials rather than an alien biosphere.

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  • $\begingroup$ Given the correct facilities, a spacecraft could come up with a hybrid (GMO or otherwise) that cross pollinates with the local varieties and "converts" the local flora into Earth compatible foodstuffs. We've already done some of that with weeds, making harsh/tough varieties of weeds more susceptible to herbicides. We've also accidentally done the opposite, where herbicide resistant crops cross pollinated with local weeds to create "superweeds". ucsusa.org/sites/default/files/legacy/assets/documents/… $\endgroup$ – computercarguy Sep 30 '19 at 20:34
  • $\begingroup$ @computercarguy Well that cannot be absolutely ruled out as we don’t know the details of the biochemistry however the challenge would be immense if not impossible. We have modified living organisms by manipulating their DNA. But this is just using nature’s genetic code and the molecular machinery used throughout life on Earth, transferring DNA to RNA and coding the RNA into amino acid based proteins. What happens if the alien planet doesn’t use DNA but another sugar or something else? It might use different amino acids or none at all and all of the metabolic pathways are likely to be different $\endgroup$ – Slarty Sep 30 '19 at 21:07

It would be much easier to colonise a living world.

Non-living worlds do not contain much oxygen or products of a long chain and a long time mulching through various substances. Living worlds mostly got that covered, although different products might be toxic cross-ecosystem.


  1. Scan for biocompatibility and toxins,
  2. Prepare substances to bind/remove the toxins,
  3. Sterilize the living planet,
  4. Reseed with your own bio system,
  5. ??? (probably do some gardening)
  6. Profit! (name the planet Eden or something and rent the space to the rich space people :D)
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    $\begingroup$ "Scan for biocompatibility and toxins" - it is not that simple. For example, after decades of research we only think that Alzheimer's disease is caused by toxin created by specific bacteria. Maybe. We still aren't sure if it is a toxin and if it is the real cause. What you wrote is far, far from reality. $\endgroup$ – Mołot Sep 30 '19 at 10:16
  • $\begingroup$ So is terraforming planets, but once we automate the process, we would be able to create a neural network to figure out how to get from A to B relatively quickly. Just identify all compaunds that aren't present at home and remove them from the environment. Say build a drone factory that mines, refines, builds and deploys drones for various tasks over time and you're golden. $\endgroup$ – Gensys LTD Sep 30 '19 at 11:33
  • $\begingroup$ I made a comment on Slarty's answer that could also pertain to this answer so that you don't have to completely raze the existing flora, and potentially fauna, but you can make a in-place conversion. The fauna might not adjust to it immediately, or ever, but that's something else to tackle. It might take many generations for the hybridization to take full effect anyway, so there might be enough time for the "rabbits and squirrels" to evolve and adjust. $\endgroup$ – computercarguy Sep 30 '19 at 20:39
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    $\begingroup$ @computercarguy Biochemically it would probably be easier to cross an oak tree with a lion. At least both share the same genetic code. Alien biochemistries would have developed completely independently and would have very little in common. The alien metabolic pathways would be wired differently using different chemicals for different purposes. To get an idea of how complex that is look here: metabolic-pathway.com/fullMap.html And that’s just a small part of the total. $\endgroup$ – Slarty Sep 30 '19 at 21:16
  • $\begingroup$ A dead world won't have oxygen (because elemental oxygen is eventually unstable in the presence of "most" substances), but if it has oxides and good energy flux from its associated star (comparable to Mars or better) then you can make atmosphere with less difficulty than you'd have in the ship you took to get there. (Assuming you didn't just make the whole trip in cryo or something.) $\endgroup$ – Ian Oct 1 '19 at 14:52

How Goldilocks?

If a planet lay in the Goldilocks zone but doesn't support life, why is that? Lack of water? No magnetosphere? Too many asteroid impacts? Metal poor? If life isn't there already, the conditions may make it hostile to permanent settlement. If life is present, then at least something was able to adapt to the conditions. If you have significant amounts of liquid water, good partial pressure of O2, a water cycle and a functioning magnetosphere, then you're already a pretty big leg up on all the other planets we currently know about. That's a heck of a lot of terraforming already done for you, possibly some or much of it due to your new organic friends.

Strange Biology

We don't know what life could look like elsewhere in the universe. But we do know a few things: the laws of chemistry and thermodynamics appear to hold for other solar systems, to the extent that we can infer them (from spectroscopy, redshift, etc.). Since life is an information-dense chemical system, we know that it requires fairly sophisticated molecules to achieve its effects. While simple crystals "grow", they don't do so in the kind of entropy-lowering way that cells do. That means it will need molecules that can become large and differentiated.

Carbon is a flexible molecule because of its +/-4 oxidation state. Silicon is in the same group, and should have roughly the same flexibility. But the difference in atomic weight causes the chemistry to differ significantly. So while we can make silicon polymers, we don't know how to make silicon amino acids/enzymes, and have reasons to believe that silicon is not capable of supporting these more elaborate molecules. Of course, the rest of the Group 14 elements (Germanium, Tin, Lead) are hopelessly useless for biology.

Ok, Carbon

So if we have convinced ourselves that life is actually pretty likely to be carbon-based, why should we assume that it's anywhere compatible with Terran biology? Well, for starters, it turns out that our genetic code is highly optimized, from an information-theoretical perspective. So while you could have an equivalent code with a different assignment of codons to amino acids, the 3-base codons, 4 base DNA/RNA code appears to be at least a local optimum. For this reason, other biological systems are likely to have at least a similar configuration. Of course, the chirality choice appears to be free, so you could get unlucky and run into life with incompatible chirality.

But we know that amino acids can be created spontaneously under adequate conditions, and thus seem likely to form a good building block on other planets. We have detected amino acids in space. Yet another reason to believe that exobiology is likely to not only be carbon-based, but amino acid-based. RNA and DNA are not a terribly big step up from amino acids themselves, so they represent one of the simplest viable molecular information encodings.

So if the exobiology produces a similar/compatible set of amino acids, then it is possible that your colonists can eat them. After all, digestive enzymes can't and don't target particular proteins and peptides, but rather just particular amino acid bond types. If earth animals can also eat the local flora and fauna, then the colonists should be able to eat a mix of both local- and xeno-food. Terran plants will likely not be able to participate in the local symbiotic networks, and may compete unfavorably with the local flora. But if the other conditions are suitable (light, water, temp), then the colonists should be able to grow Terran plants in isolated greenhouses.


We like to hedge our bets and cover our bases and assume that life can take on some mysterious form that we cannot even imagine. But the reality is that chemistry and thermodynamics significantly restrict the kinds of magic that any plausible xenobiology could employ. I am willing to go out on a limb and claim that if life exists elsewhere, there is more than 50% chance it is carbon based, and that if it is carbon-based, there is more than 90% chance it uses amino acids, and that if it uses amino acids, that there is more than 90% chance that it uses some to many of the same amino acids that we do.

Note that there is some tolerance in amino acid utilization. If a protein calls for one amino acid in a particular location, but ends up with a different but similar one instead, it can still be mostly functional. Obviously, this is not accidental. When we say that the Terran genetic code is universal and "optimal", we mean that it is maximally robust to the most frequent kinds of noise/errors in transcription because similar codons are assigned to the same amino acid, and codons for different amino acids that are likely to be mistaken for one another code for functionally similar amino acids. Thus, even if xenobiology uses a different but overlapping amino acid set, there is a possibility that some of the proteins used by xeno-life is still partially compatible with Terran life.

Writers often cast biology as a binary compatible or incompatible, but the scale is much fuzzier than that. Even on earth, members of different species are supposed to be infertile together, and yet we have regular inter-species hybrids like mules and ligers. Such hybrids are not infinitely far-fetched between Terran and xeno-species, though the success rate is likely proportional to how large/complex the individuals are (and thus, most likely limited to unicellular/bacterial colonies).

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A living world is strictly better. But you will still need to live in airtight habitats.

You have an atmosphere that has a reasonable pressure. This means your dome doesn't need to be all that strong; you're not trying to keep the dome from collapsing or popping.

You probably have a reasonable temperature, so your habitat will likely be easy to heat and cool.

The atmosphere protects your habitat from space threats; cosmic radiation, solar storms, and micrometeorites specifically. You're still at risk from big asteroids, but considering this planet has life, the solar system is probably well past its late heavy bombardment.

Since the world has an atmosphere, it must have a magnetosphere. So your orbital infrastructure can use electrodynamic tethers for stationkeeping, which is really convenient.

Since the world has an atmosphere, it also alleviates some of the dust problems Apollo astronauts had. Lunar dust is jagged and sharp, which makes it extremely irritating to the eyes, nose, and throat, and it's extremely difficult to clean up. It's jagged because, without an atmosphere, there is essentially no weathering. Each grain of dust is the same as it was when it was first chipped off its parent rock. Your alien world doesn't have that problem, thanks to the atmosphere.

The world also probably has oxygen and water, so you can use that instead of carefully recycling your own forever.

But even given all these advantages, Larry Niven was right. Rotating orbital habitats are better yet.

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  • $\begingroup$ Larry Niven didn't live in the Age of Terrorism when he wrote that. All of those fanci sci-fi megastructures like space elevators and O'Neill cylinders are terribly vulnerable. $\endgroup$ – ths Oct 1 '19 at 9:26
  • $\begingroup$ All human structures are terribly vulnerable. $\endgroup$ – Ryan_L Oct 1 '19 at 15:07
  • $\begingroup$ that's why you distribute your population across a continent instead of stuffing them in a single can. $\endgroup$ – ths Oct 1 '19 at 15:12
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    $\begingroup$ What makes you think we'd have just one O'Neill cylinder? What makes you think each O'Neill cylinder wouldn't have multiple air-tight sections? $\endgroup$ – Ryan_L Oct 1 '19 at 15:24
  • $\begingroup$ If a society manages to build megastructures like space elevators, they very likely would not live in an 'Age of Terrorism'. A misnomer btw, when comparing against historical precedents of high times of terrorism. $\endgroup$ – collapsar Feb 23 at 19:20

If we have biological humans traveling between stars, we are a K2+ civilization, and are capable of disassembling solar systems.

If we have star wisps, we are K1+, and we'll arrive and have to build a functioning industrial economy from a tiny object in order to get to the point of being able to "print" biological humans.

Current Human industrial civilization exists as a parasite on the Earth's biosphere, but by the time we are launching star wisps we'll have substantial off-world industry (which presumably mostly bootstrapped). It is plausible that the star wisp will want to bootstrap in an asteroid belt during one phase of development, then lower itself down to a living biosphere in order to exist as a parasite on that while it continues to grow.

Odds are that biospheres on alien planets won't be compatible with human life, so any printed humans will have to exist within a constructed biosphere. If we want humans to live on the planet, we'll have to terraform the planet, which is a K1+ civilization effort; so you'll have billions of humans living in space habitats long before they are walking around on a world.

They might choose to defer printing biological humans until the planet is terraformed; after the 100s or 1000s or 10000s of years of terraforming effort, a human-compatible biosphere is complete, and some printed humans step out onto a green field with a blue sky above.

Little if anything will be left of the original biosphere.

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