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Originally Posted by humndislocation
We wouldn't necessarily need to find a planet with the same atmosphere as our own. There's the possibility we could change the atmosphere for our own ends.
If I recall correctly, the earth didn't always have an atmosphere that would be breathable to humans. It's my understanding that the oxygen in the atmosphere is actually the result of lifeforms that use photosynthesis, taking in carbon dioxide and outputting oxygen.
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Yes that's true, any organism containing green chlorophyll will photosynthesise using sunlight and produce oxygen, as well as respire using oxygen.
However to introduce plants the planet's soil would need to be conducive with enough of it and containing all the right nutrients derived from millions of years of erosion of the correct kinds of rocks for healthy sustained growth. There would need to be enough water on the planet too, and a weather system like ours to recycle the water, and create a varied landscape and different climates.
The planet would also have to be the correct distance from a star for the right solar energy and gravitational field for plant and human existence, and an ozone layer to protect us from harmful UV rays.
Also we would have to ensure there are no organisms already present that could wipe us out completely as a species.
And of course we will need the technology to travel to it safely with all the necessary supplies, and as a race overcome the complicated politics of who funds the process and goes there.
With all these factors present (and probably others that I haven't thought of) then I agree, we could change the atmosphere by introducing plants and eventually colonise it, which would be awesome
However the chances of all favourable factors being present are very small indeed.