Well first of all - what do you mean by "able to sustain human life"?
If you are thinking of Star Trek like planets with food we can eat, air we can breath, and nothing immediately hazardous or poisonous to humans - chances of this is minute.
Particularly - the chance that the life there uses DNA and the same biochemical pathways as Earth life in all its detail must surely be almost infinitesimal.
That is - unless the life has a common origin - and even then - it's likely that Earth life at some point in the last 4.5 billion years took enough of a different direction so that the life is incompatible.
This means - that at the very least, surely we can't eat their food. But - it also means - that if they have microbes like us - then their microbes have a different metabolism, different photosynthesis etc.
So - then the question is -
Depending on the answers their, then
Only the first of these is reasonably safe for Earth life. But the other two possibilities are surely at least as likely. And first possibility is bad news for ET life.
Our bodies have defenses against foreign microbes. But - these defenses are designed to combat Earth life. They might simply not see the ET life as a threat.
Particularly - they could live on our skin, in our lungs, in our stomachs, in various spaces inside our body e.g. sinuses etc - where they don't have to take part in the intricate dance of the biochemistry of a human body - but live on top / outside of the blood and skin - and still could be a major nuisance to the extent that humans die quickly as a result of the infection.
For instance, ET life could also produce biproducts that are toxic to humans - not as a poison targeting humans - but just by accident - that some of their biochemistry closely resembles ours - but not enough to be useful to us as food - just enough to interfere with the functioning of our bodies so that we die when we breath or ingest them.
They could also be toxic to our crops, our animals, create toxic chemicals in the air we breath etc - that is toxic for Earth based life - fine for ET life.
I think there is a pretty good chance that humans simply wouldn't survive the encounter and would die. There's also a reasonable chance that the encounter would be disastrous for their ecosystem.
And the chance that we can grow crops on their worlds or eat their own food - must surely be infinitesimal - unless we sterilize their worlds of the native life first.
You might object that none of the Holywood movies to date show these issues - and in Star Trek especially the crew are able to eat food on every world they encounter. Well there are many things in our movies that are not 100% scientifically accurate according to modern understanding. This is one of them.
You do get a similar idea in H.G. Wells War of the Worlds - but the idea there of a disease of humans attacking ETs seems very unlikely that it would be specifically adapted to them. But Earth life being hazardous to ETs because the microbes from Earth behave differently to ET microbes in many ways - and vice versa - that just an ET landing on the Earth could endanger our entire ecosystem - even to the extent it might mean "Goodbye DNA" - can't think of a single movie that explores that idea.
I think it would make an interesting movie, maybe some day someone could give it a go.
So - the only options here for human colonization are
There's also a moral issue here, to return to the main topic of the question.
Should we colonize the galaxy? Assuming we do find a way to do that - which involves maybe demolishing all the asteroids and comets to make into habitats (I suspect the simplest solution for ETs is to just ignore planets and focus on asteroids and comets instead - far easier to deal with and easier to make controlled habitats to live in)
- the problem there is - that what about our descendants a few million years down the road - even a few thousand years? The entire galaxy could be full of humans by then. Some with obscure aims and ideas. And they'd be immensely powerful as they could create self replicating machines - which could - literally - remake the entire galaxy according to their wishes.
So with a hundred billion stars - each colonized by hundreds of trillions of human colonists - and any of those humans might decide to make a self replicating machine - or a cyborg - or new life form - that they might then unleash in order to remake the galaxy according to their wishes - doesn't seem a nice future to me.
We'd be the ET monsters then in the galaxy. I think we need to proceed very carefully. We could be like an infection that could destroy our galaxy.
I don't know what the answer is here. But what is safe is to explore the galaxy rather than colonize it.
If we proceed slowly - and explore and find out more about the galaxy first - and proceed with respect for other life forms that we encounter - and don't destroy their worlds - just as we would not want other ETs to destroy our world - then maybe we can find an answer.
If there are ETs in our galaxy - then they have shown similar restraint. They are not here and have not taken over or destroyed our Earth - which they could have done millions of years ago (chances any ET evolved exactly same time as us is again tiny so if there are any - surely must be at least millions of years old civilization)
So - we should show similar restraint, is my view.
Will be some time anyway before it's an issue. Is no planet in our solar system able to sustain human life. And nearest other star is so far away that with present technology you are talking about a multiple century journey.
But if a point comes where we have the ability to send a human colony to another star - then I think we should consider very carefully indeed, as it is first step towards potentially colonizing entire galaxy.
It may be impossible to reverse or stop that once you have independent human colonies about two stars separated by light years, unless you have some safeguards already in place to prevent it. For the reasons I gave - if you have so many colonies each with almost limitless power to reshape the galaxy to their wishes - that may not be a good outcome long term for humans or for other life in our galaxy. So needs great care.
Similarly also of course if anyone proposes building a 3D self replicating machine and sending it out into the galaxy to reshape it to our wishes - again that needs immense care - we may have that capability sooner than we expect - truly nanoscale 3D printerrs able to print out working computer chips might be all that we need. But self replicating humans sent out into the galaxy can be just as dangerous for us as those self replicating machines (because of the power they will have with future technology) if done without the right precautions.
If you are thinking of Star Trek like planets with food we can eat, air we can breath, and nothing immediately hazardous or poisonous to humans - chances of this is minute.
Particularly - the chance that the life there uses DNA and the same biochemical pathways as Earth life in all its detail must surely be almost infinitesimal.
That is - unless the life has a common origin - and even then - it's likely that Earth life at some point in the last 4.5 billion years took enough of a different direction so that the life is incompatible.
This means - that at the very least, surely we can't eat their food. But - it also means - that if they have microbes like us - then their microbes have a different metabolism, different photosynthesis etc.
So - then the question is -
- are their microbes better at what they do than our microbes - e.g. better at photosynthesis, or better at converting chemicals into energy?
- are our microbes always better than theirs
- Are they about the same
- Are some better than ours and some the other way around?
Depending on the answers their, then
- Earth microbes could totally take over from the ET microbes - "Goodbye ET life"
- ET microbes could totally take over from Earth microbes - out competes DNA based life in all respects - "Goodbye DNA"
- We might end up with a mixture of some of each. So some ET microbe based micro-habitats - some that are totally Earth microbes - and some that have microbes of both types.
Only the first of these is reasonably safe for Earth life. But the other two possibilities are surely at least as likely. And first possibility is bad news for ET life.
Our bodies have defenses against foreign microbes. But - these defenses are designed to combat Earth life. They might simply not see the ET life as a threat.
Particularly - they could live on our skin, in our lungs, in our stomachs, in various spaces inside our body e.g. sinuses etc - where they don't have to take part in the intricate dance of the biochemistry of a human body - but live on top / outside of the blood and skin - and still could be a major nuisance to the extent that humans die quickly as a result of the infection.
For instance, ET life could also produce biproducts that are toxic to humans - not as a poison targeting humans - but just by accident - that some of their biochemistry closely resembles ours - but not enough to be useful to us as food - just enough to interfere with the functioning of our bodies so that we die when we breath or ingest them.
They could also be toxic to our crops, our animals, create toxic chemicals in the air we breath etc - that is toxic for Earth based life - fine for ET life.
I think there is a pretty good chance that humans simply wouldn't survive the encounter and would die. There's also a reasonable chance that the encounter would be disastrous for their ecosystem.
And the chance that we can grow crops on their worlds or eat their own food - must surely be infinitesimal - unless we sterilize their worlds of the native life first.
You might object that none of the Holywood movies to date show these issues - and in Star Trek especially the crew are able to eat food on every world they encounter. Well there are many things in our movies that are not 100% scientifically accurate according to modern understanding. This is one of them.
You do get a similar idea in H.G. Wells War of the Worlds - but the idea there of a disease of humans attacking ETs seems very unlikely that it would be specifically adapted to them. But Earth life being hazardous to ETs because the microbes from Earth behave differently to ET microbes in many ways - and vice versa - that just an ET landing on the Earth could endanger our entire ecosystem - even to the extent it might mean "Goodbye DNA" - can't think of a single movie that explores that idea.
I think it would make an interesting movie, maybe some day someone could give it a go.
So - the only options here for human colonization are
- Only target completely lifeless planets, or
- Target planets with life - and exterminate all the native life in order to make it a planet decent for humans to live in - who would want to do that?
- Don't live on planets at all - but instead build free floating colonies in space - O'Neil cylinders, Stanford Toruses etc
There's also a moral issue here, to return to the main topic of the question.
Should we colonize the galaxy? Assuming we do find a way to do that - which involves maybe demolishing all the asteroids and comets to make into habitats (I suspect the simplest solution for ETs is to just ignore planets and focus on asteroids and comets instead - far easier to deal with and easier to make controlled habitats to live in)
- the problem there is - that what about our descendants a few million years down the road - even a few thousand years? The entire galaxy could be full of humans by then. Some with obscure aims and ideas. And they'd be immensely powerful as they could create self replicating machines - which could - literally - remake the entire galaxy according to their wishes.
So with a hundred billion stars - each colonized by hundreds of trillions of human colonists - and any of those humans might decide to make a self replicating machine - or a cyborg - or new life form - that they might then unleash in order to remake the galaxy according to their wishes - doesn't seem a nice future to me.
We'd be the ET monsters then in the galaxy. I think we need to proceed very carefully. We could be like an infection that could destroy our galaxy.
I don't know what the answer is here. But what is safe is to explore the galaxy rather than colonize it.
If we proceed slowly - and explore and find out more about the galaxy first - and proceed with respect for other life forms that we encounter - and don't destroy their worlds - just as we would not want other ETs to destroy our world - then maybe we can find an answer.
If there are ETs in our galaxy - then they have shown similar restraint. They are not here and have not taken over or destroyed our Earth - which they could have done millions of years ago (chances any ET evolved exactly same time as us is again tiny so if there are any - surely must be at least millions of years old civilization)
So - we should show similar restraint, is my view.
Will be some time anyway before it's an issue. Is no planet in our solar system able to sustain human life. And nearest other star is so far away that with present technology you are talking about a multiple century journey.
But if a point comes where we have the ability to send a human colony to another star - then I think we should consider very carefully indeed, as it is first step towards potentially colonizing entire galaxy.
It may be impossible to reverse or stop that once you have independent human colonies about two stars separated by light years, unless you have some safeguards already in place to prevent it. For the reasons I gave - if you have so many colonies each with almost limitless power to reshape the galaxy to their wishes - that may not be a good outcome long term for humans or for other life in our galaxy. So needs great care.
Similarly also of course if anyone proposes building a 3D self replicating machine and sending it out into the galaxy to reshape it to our wishes - again that needs immense care - we may have that capability sooner than we expect - truly nanoscale 3D printerrs able to print out working computer chips might be all that we need. But self replicating humans sent out into the galaxy can be just as dangerous for us as those self replicating machines (because of the power they will have with future technology) if done without the right precautions.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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