Would it be easier to colonise a living world or a dead world?

Question

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?

Answer 1

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.

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addendum

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.

Answer 2

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.

https://en.wikipedia.org/wiki/Neutron_bomb

Answer 3

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 ...

Answer 4

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.

Answer 5

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.

So.

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)

Answer 6

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.

Conclusion

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).

Answer 7

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.

Answer 8

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.