Okay - well first - their simplest solution is to use their advantage of time - to invade Earth some tens or hundreds of millions of years ago - before humans evolved.
That's because the chances are tiny that an ET with an interest in invading Earth gets here at exactly the same time as Earth reaches technological capability.
Our Earth is 4.5 billion years old. and - probably was possible for life to start to evolve for another four or five billions of years before the Earth formed.
So an ET could have developed ability to reach our solar system any time in the last nine or ten billion years or so.
Even if they reached technology, say, in the last billion years - then chances that they get to Earth exactly in this century is about 1 in ten million.
So surely, if any ET is searching the galaxy for habitable worlds - they got here a few million years ago at least.
So why wait for humans to evolve?
So - I think we can rule out the idea that they have come here just to find a place to live, because if so they would be here already.
In any case a civilization just a short way into the future would have the ability to build giant habitats in space - easily - just set a few machines going to build them for them.
If they have technology to get here over the light years distance, this is trivial for them surely, so easy it's not worth spending the effort to try to change the Earth to their liking. Especially since a planet, with its volcanoes, earthquakes, storms etc - is a more hazardous place and harder to control than a habitat - and can't be moved out of the way easily if there is any incoming danger.
Doesn't really make too much sense for an advanced et with ability to make habitats in space to transform planets to make new habitats.
Surprisingly perhaps - there is enough material in the asteroid belt to build many times the surface area of the Earth as habitats like this just in the asteroids, never mind other resources in our solar system.
Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths
So if they wanted to live in our solar system - rather than remodel a world to suit them - just build millions of space habitats using resources of our asteroid belt.
And if they have got here already and this was their objective, then we should just look up and see a night sky filled with space habitats.
They might eventually want to build a Dyson sphere (as perhaps might we later on) - and if you went all out on that project - that could involve demolishing all the planets of our solar system.
Or a Dyson swarm of habitats and solar satellites around our sun:
If that was their objective they'd be interested in the materials that the Earth is made of - as just one more lump of construction material - along with all the other planets.
But again - why wait for humans to evolve before doing this?
If they just build one in the galaxy no problem. Likely to be hard to detect. Or even build a dozen and then stop, and don't build any more for hundreds of millions, or billions of years, again not likely to spot them yet unless close to us.
But if there are ETs out there who build Dyson spheres - and they start to expand and build 2 spheres, then 4, then 8 - then before long every star in the galaxy would be surrounded by Dyson spheres.
That would happen within a few tens of millions of years most likely. Since we don't see that, again doesn't seem too likely that anyone wants to make our solar system into a Dyson sphere any time soon.
I'm sure they don't need our radio signals to detect habitable planets - in near future we'll be able to detect habitable planets around nearby stars, including detecting life in their atmosphere. So they could do that also - but at far greater distances.
Indeed if an ET wanted to do a preliminary survey of our galaxy - just send out a whole bunch of probes. A technology not far ahead of us could send carefully programmed safe self replicating probes one to every star system in the galaxy to monitor it from close up to see if there is anything interesting there.
So any ET in our galaxy with an interest in habitable planets has known that Earth is habitable for some time, probably for tens or hundreds of millions of years, maybe for billions of years. But have left us alone.
So - only reason for coming now is because they are interested in humans. Nothing else makes sense.
So - unless someone can find a flaw in this reasoning - I think we can rule out any scenarios involving killing humans or destroying all life on the planet - doesn't make sense, It just makes no sense to first wait for us to evolve, watch us evolve and develop our civilization also for several thousand years - and then kill us.
Then, if they are interested in Earth for food, or find some of the creatures here interesting - it will be the easiest of things for them to replicate them - would surely have atom scale replication by then.
We already have 3D printers. They would have them also, but on the atomic scale, able to build almost anything atom by atom.
So whatever they find interesting enough to want it for themselves - why destroy it? If inanimate, replicate it and put it in giant space habitats constructed swiftly using self replicating machines.
If living, then - they may be able to replicate it also, I don't know - but if not - well grow it in giant space habitats constructed using 3D printers and self replicating machines.
As for being able to eat our food - that's not too likely - but is one scenario where it could be possible - if life evolved first somewhere else - and Earth was seeded by life from other star systems - either deliberately - or through a star passing through our cloud as it formed. If so - they might be DNA based.
If anything else though - then they'd be at risk from our DNA, or we would be at risk from their XNA - that one or other might take over our world, or their habitats or both.
So, ETs probably would need to have some kind of protection suit before they can land on our Earth to prevent their microbes or other small reproducing lifeforms from taking over the Earth or vice versa.
So that's another omission from the movies. They almost never show the ETs as concerned about Earth life harming them - or vice versa, that their XNA could be a threat to our DNA or vice versa.
If they wanted to take over the Earth they could surely do it easily. Easiest method - either a life form - or more controlled - a nanoscale replicating machine - programmed to do whatever they want it to do, including building larger machines.
That could literally reform our world, to whatever they want it to be - right up to the capability to take it apart piece by piece and fire the pieces into space to make a Dyson Sphere or more habitats.
With present day technology not much we could do about it. We probably wouldn't even detect it - there are many microbes we can't detect, the "Microbial dark matter" problem.
Microbial dark matter
Their infiltrating spaceship wouldn't just be insect sized. It would be so small you can't see it with an optical microscope, beyond the optical resolution limit so you can only see it with an electron microscope - and it would surely camouflage itself to look like either a microbe, or a nanobe.
So, surely our world could be infiltrated by nano-scale Alien machines and we wouldn't know, perhaps programmed to reproduce until they are present everywhere in small population - and then in a final phase that lasts a few minutes, grow rapidly and ovewhelm all the Earth life.
So - as the other answers have said, if they are just a few centuries ahead of us - the whole thing would be over before we even know it has started chances are. Might just take minutes in its final phase.
But that would be a boring movie :).
And - if the reasoning here is right - it's not going to happen that way because they won't be just a few centuries ahead of us. They will be millions of years ahead of us.
If that is so - they had the capability to do this long before humans evolved and didn't. With their technology they could survey our galaxy long ago - an easy thing to do with self replicating probes - and know exactly where every habitable planet is in the galaxy - and know about Earth -but decided to leave it as it is.
So they must be interested in us if they come here - and won't be coming to invade Earth for its resources or habitats - but for some other reason which is in some way to do with us. They would be coming here to find us.
For these reasons I find Alien invasion type movies, and also things like Star Trek utterly implausible in our universe.
(I know Star Trek does have a back story of early proto humans who created a genetic seed that lead to a whole lot of human like creatures evolving to our level of technology throughout the galaxy, almost simultaneously, several billion years later - but - that's just a sci. fi. idea and I find it utterly implausible in reality - though okay for a fun sci fi. movie).
But I do enjoy them all the same, are great fun to watch, just don't take them seriously as things that could happen in our universe.
If we go into space and start colonizing the galaxy - and take over other "habitable" worlds - then we will become the ET monsters for the rest of the galaxy. And our descendants could come back and invade the Earth - either them - or the biological creatures or self replicating machines or cyborgs created by them.
So far nobody has invaded Earth - and they have had millions, probably billions of years to try if they exist. So - chances are we are safe. And in remote chance that anyone did want to do anything to Earth it would be over before we knew it had started.
We can't do anything about that either way and the chance of it happening is so remote - given that we've gone 4.5 billion years without an invasion - perhaps 0.000001% of such a thing happening in any given century - that we are better off thinking about other things, and taking care we don't become ET monsters ourselves for the rest of the galaxy.
That's because the chances are tiny that an ET with an interest in invading Earth gets here at exactly the same time as Earth reaches technological capability.
IT IS LIKELY THAT ANY ET WHO WANTS EARTH ALREADY GOT HERE MILLIONS OF YEARS AGO
Our Earth is 4.5 billion years old. and - probably was possible for life to start to evolve for another four or five billions of years before the Earth formed.
So an ET could have developed ability to reach our solar system any time in the last nine or ten billion years or so.
Even if they reached technology, say, in the last billion years - then chances that they get to Earth exactly in this century is about 1 in ten million.
So surely, if any ET is searching the galaxy for habitable worlds - they got here a few million years ago at least.
So why wait for humans to evolve?
So - I think we can rule out the idea that they have come here just to find a place to live, because if so they would be here already.
FOR A TECHNOLOGICALLY ADVANCED ET, BE FAR EASIER TO LIVE IN SPACE IN CLIMATE CONTROLLED HABITATS THAN ON A PLANET
In any case a civilization just a short way into the future would have the ability to build giant habitats in space - easily - just set a few machines going to build them for them.
If they have technology to get here over the light years distance, this is trivial for them surely, so easy it's not worth spending the effort to try to change the Earth to their liking. Especially since a planet, with its volcanoes, earthquakes, storms etc - is a more hazardous place and harder to control than a habitat - and can't be moved out of the way easily if there is any incoming danger.
Doesn't really make too much sense for an advanced et with ability to make habitats in space to transform planets to make new habitats.
Surprisingly perhaps - there is enough material in the asteroid belt to build many times the surface area of the Earth as habitats like this just in the asteroids, never mind other resources in our solar system.
Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths
So if they wanted to live in our solar system - rather than remodel a world to suit them - just build millions of space habitats using resources of our asteroid belt.
And if they have got here already and this was their objective, then we should just look up and see a night sky filled with space habitats.
WHAT ABOUT DYSON SPHERES OR SWARMS?
They might eventually want to build a Dyson sphere (as perhaps might we later on) - and if you went all out on that project - that could involve demolishing all the planets of our solar system.
Or a Dyson swarm of habitats and solar satellites around our sun:
But again - why wait for humans to evolve before doing this?
If they just build one in the galaxy no problem. Likely to be hard to detect. Or even build a dozen and then stop, and don't build any more for hundreds of millions, or billions of years, again not likely to spot them yet unless close to us.
But if there are ETs out there who build Dyson spheres - and they start to expand and build 2 spheres, then 4, then 8 - then before long every star in the galaxy would be surrounded by Dyson spheres.
That would happen within a few tens of millions of years most likely. Since we don't see that, again doesn't seem too likely that anyone wants to make our solar system into a Dyson sphere any time soon.
I'm sure they don't need our radio signals to detect habitable planets - in near future we'll be able to detect habitable planets around nearby stars, including detecting life in their atmosphere. So they could do that also - but at far greater distances.
Indeed if an ET wanted to do a preliminary survey of our galaxy - just send out a whole bunch of probes. A technology not far ahead of us could send carefully programmed safe self replicating probes one to every star system in the galaxy to monitor it from close up to see if there is anything interesting there.
So any ET in our galaxy with an interest in habitable planets has known that Earth is habitable for some time, probably for tens or hundreds of millions of years, maybe for billions of years. But have left us alone.
So - only reason for coming now is because they are interested in humans. Nothing else makes sense.
So - unless someone can find a flaw in this reasoning - I think we can rule out any scenarios involving killing humans or destroying all life on the planet - doesn't make sense, It just makes no sense to first wait for us to evolve, watch us evolve and develop our civilization also for several thousand years - and then kill us.
IF INTERESTED IN ANY MINERALS OR ARTEFACTS OR LIFE FORMS WE HAVE - CAN REPLICATE IT - NO NEED TO DESTROY
Then, if they are interested in Earth for food, or find some of the creatures here interesting - it will be the easiest of things for them to replicate them - would surely have atom scale replication by then.
We already have 3D printers. They would have them also, but on the atomic scale, able to build almost anything atom by atom.
So whatever they find interesting enough to want it for themselves - why destroy it? If inanimate, replicate it and put it in giant space habitats constructed swiftly using self replicating machines.
If living, then - they may be able to replicate it also, I don't know - but if not - well grow it in giant space habitats constructed using 3D printers and self replicating machines.
OUR FOOD WOULD BE INEDIBLE AND MICROBES QUITE POSSIBLY HAZARDOUS
As for being able to eat our food - that's not too likely - but is one scenario where it could be possible - if life evolved first somewhere else - and Earth was seeded by life from other star systems - either deliberately - or through a star passing through our cloud as it formed. If so - they might be DNA based.
If anything else though - then they'd be at risk from our DNA, or we would be at risk from their XNA - that one or other might take over our world, or their habitats or both.
So, ETs probably would need to have some kind of protection suit before they can land on our Earth to prevent their microbes or other small reproducing lifeforms from taking over the Earth or vice versa.
So that's another omission from the movies. They almost never show the ETs as concerned about Earth life harming them - or vice versa, that their XNA could be a threat to our DNA or vice versa.
HOW THEY COULD TAKE OVER IF THEY WANTED TO - WITH SUB MICROBE SCALE NANO MACHINES
If they wanted to take over the Earth they could surely do it easily. Easiest method - either a life form - or more controlled - a nanoscale replicating machine - programmed to do whatever they want it to do, including building larger machines.
That could literally reform our world, to whatever they want it to be - right up to the capability to take it apart piece by piece and fire the pieces into space to make a Dyson Sphere or more habitats.
With present day technology not much we could do about it. We probably wouldn't even detect it - there are many microbes we can't detect, the "Microbial dark matter" problem.
Microbial dark matter
Their infiltrating spaceship wouldn't just be insect sized. It would be so small you can't see it with an optical microscope, beyond the optical resolution limit so you can only see it with an electron microscope - and it would surely camouflage itself to look like either a microbe, or a nanobe.
We see these nanobes in electron microscope images. Nobody knows for sure what they are- life - or inorganic processes. They are too small for modern life machinery to fit within them. If ETs had nanobe scale tiny machines, camouflaged to resemble nanobes - then even in an electron microscope, we wouldn't notice them as anything unusual.
IF A FEW CENTURIES AHEAD OF US - FINAL PHASE WOULD BE OVER IN MINUTES - AND NOTHING FOR US TO SEE BEFORE THEN EVEN WITH MICROSCOPES
So, surely our world could be infiltrated by nano-scale Alien machines and we wouldn't know, perhaps programmed to reproduce until they are present everywhere in small population - and then in a final phase that lasts a few minutes, grow rapidly and ovewhelm all the Earth life.
So - as the other answers have said, if they are just a few centuries ahead of us - the whole thing would be over before we even know it has started chances are. Might just take minutes in its final phase.
But that would be a boring movie :).
BUT - NOT GOING TO BE JUST A FEW CENTURIES AHEAD SURELY
And - if the reasoning here is right - it's not going to happen that way because they won't be just a few centuries ahead of us. They will be millions of years ahead of us.
If that is so - they had the capability to do this long before humans evolved and didn't. With their technology they could survey our galaxy long ago - an easy thing to do with self replicating probes - and know exactly where every habitable planet is in the galaxy - and know about Earth -but decided to leave it as it is.
So they must be interested in us if they come here - and won't be coming to invade Earth for its resources or habitats - but for some other reason which is in some way to do with us. They would be coming here to find us.
For these reasons I find Alien invasion type movies, and also things like Star Trek utterly implausible in our universe.
(I know Star Trek does have a back story of early proto humans who created a genetic seed that lead to a whole lot of human like creatures evolving to our level of technology throughout the galaxy, almost simultaneously, several billion years later - but - that's just a sci. fi. idea and I find it utterly implausible in reality - though okay for a fun sci fi. movie).
But I do enjoy them all the same, are great fun to watch, just don't take them seriously as things that could happen in our universe.
WE NEED TO TAKE CARE WE DON'T BECOME THE MONSTERS OURSELVES
If we go into space and start colonizing the galaxy - and take over other "habitable" worlds - then we will become the ET monsters for the rest of the galaxy. And our descendants could come back and invade the Earth - either them - or the biological creatures or self replicating machines or cyborgs created by them.
So far nobody has invaded Earth - and they have had millions, probably billions of years to try if they exist. So - chances are we are safe. And in remote chance that anyone did want to do anything to Earth it would be over before we knew it had started.
We can't do anything about that either way and the chance of it happening is so remote - given that we've gone 4.5 billion years without an invasion - perhaps 0.000001% of such a thing happening in any given century - that we are better off thinking about other things, and taking care we don't become ET monsters ourselves for the rest of the galaxy.
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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