Yes, indeed, all that's possible except the last, it's not really possible to settle a planet around a nearby star with current technology if we find one, though people are working on ways to get there.
But in the other cases, we'd have to take especial care, as it surely has its own lifeforms, if it is as close as that, and if it is as habitable. If nothing else, then through shared life through asteroid impacts, in that situation - would be easy to share life.
If it is so close that it is a double planet - then pretty certain to have shared life from the past - but is more isolated than another continent. We already have issues of "invasive species" as plants, animals, fish etc so would have all those issues, but also - invasive microbes also, if they were ones that for some reason or another couldn't make the transition on meteorites.
It could also be a second planet inside the "goldilocks zone" - so like our system but nudge Earth a bit further out, and Venus also, you'd end up with two stable Earth like planets in the same solar system.
Also many stars are binary, or in groupings of three or more stars quite often also - and if they are far apart binaries, then they can have planets in stable Earth like orbits around both stars.
We could also be in a star cluster - but those not too likely for life on present day knowledge. They are either places where stars are born, so too early probably for advanced life - or globular clusters where - probably - there aren't many planets because the stars frequently come close to each other and interact with each other.
However, on very long timescales if a civilization lasts for billions of years, then from time to time, other stars in the galaxy will come close by and sometimes those might have habitable planets. So again if we were very "lucky" or possibly "unlucky" then that might be the situation when a society develops technology for the first time.
I say potentially also "unlucky" because with the enthusiasm to visit another planet a young technological species might not take enough precautions. Back in the days of Apollo we had so little awareness of such things, I think if we'd had another planet with life on it in this solar system, might well have just sent humans there right away. Some like Carl Sagan would surely have warned against it - but would they have prevailed?
And - there are many forms of life we simply didn't know about and couldn't 'detect with 1960s technology. E.g. ultramicrobacteria, or many non cultivable archaea, also Gene Transfer Agents. What's more there could be life on Earth that we still can't detect today, some think the nanobes are alive, or some other "shadow biosphere".
So a young ETI might simply not even know that they have brought life back to their "Earth".
And then - maybe that is okay - but maybe that is disastrous, we don't have any prior experience to know. I think at least a chance that it is disastrous, returning invasive species to Earth, maybe ones that seem just fine but then after a while they suddenly cause problems (like locusts that suddenly swarm and change from harmless innocuous grasshoper like creatures to ones that destroy crops and cause major problems).
It's possible that we might actually be lucky, as young ETIs to not have an Earth like planet nearby. Focuses more on the need to treat the Earth properly. Though in science fiction, or in imagination if it didn't have those issues would be a great thing :).
But in the other cases, we'd have to take especial care, as it surely has its own lifeforms, if it is as close as that, and if it is as habitable. If nothing else, then through shared life through asteroid impacts, in that situation - would be easy to share life.
If it is so close that it is a double planet - then pretty certain to have shared life from the past - but is more isolated than another continent. We already have issues of "invasive species" as plants, animals, fish etc so would have all those issues, but also - invasive microbes also, if they were ones that for some reason or another couldn't make the transition on meteorites.
It could also be a second planet inside the "goldilocks zone" - so like our system but nudge Earth a bit further out, and Venus also, you'd end up with two stable Earth like planets in the same solar system.
Also many stars are binary, or in groupings of three or more stars quite often also - and if they are far apart binaries, then they can have planets in stable Earth like orbits around both stars.
We could also be in a star cluster - but those not too likely for life on present day knowledge. They are either places where stars are born, so too early probably for advanced life - or globular clusters where - probably - there aren't many planets because the stars frequently come close to each other and interact with each other.
However, on very long timescales if a civilization lasts for billions of years, then from time to time, other stars in the galaxy will come close by and sometimes those might have habitable planets. So again if we were very "lucky" or possibly "unlucky" then that might be the situation when a society develops technology for the first time.
I say potentially also "unlucky" because with the enthusiasm to visit another planet a young technological species might not take enough precautions. Back in the days of Apollo we had so little awareness of such things, I think if we'd had another planet with life on it in this solar system, might well have just sent humans there right away. Some like Carl Sagan would surely have warned against it - but would they have prevailed?
And - there are many forms of life we simply didn't know about and couldn't 'detect with 1960s technology. E.g. ultramicrobacteria, or many non cultivable archaea, also Gene Transfer Agents. What's more there could be life on Earth that we still can't detect today, some think the nanobes are alive, or some other "shadow biosphere".
So a young ETI might simply not even know that they have brought life back to their "Earth".
And then - maybe that is okay - but maybe that is disastrous, we don't have any prior experience to know. I think at least a chance that it is disastrous, returning invasive species to Earth, maybe ones that seem just fine but then after a while they suddenly cause problems (like locusts that suddenly swarm and change from harmless innocuous grasshoper like creatures to ones that destroy crops and cause major problems).
It's possible that we might actually be lucky, as young ETIs to not have an Earth like planet nearby. Focuses more on the need to treat the Earth properly. Though in science fiction, or in imagination if it didn't have those issues would be a great thing :).
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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