Mdthbs

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?

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.

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.

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.

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 ...