What gases could be present in a viable atmosphere for humans?

I'm working on a story involving humans looking for another habitable planet, and I have been scouring the net for days, finding lots of info on varying levels of oxygen and nitrogen, but none that really answer this question:

Earth's atmosphere is a mix of oxygen, nitrogen, argon, carbon dioxide and water vapor; are there other viable possibilities?

For instance, argon -- I know it is inert, which is important, but could it be replaced with another inert gas?

Thank you so much in advance for any info you have on the topic or anywhere you can point me where I might find the answer.

EDIT: Okay, target atmospheric pressure would be, I suppose, in the same realm as what we have on earth? Sorry, for the lame answer, but I am a writer trying to learn science here. I don't need examples of possible atmospheric blends; I'm assuming that is nearly infinite. I'm looking for whether, on a planet on which humans could live outside of pressure suits, etc., the atmosphere might look slightly different to what we have on earth, or should we reasonably expect it to look virtually identical? Thank you!

EDIT 2: Let me boil this down to my real question, I think. If we found an exoplanet that had the exact same gases in its atmosphere (N, O2, Ar, etc.), albeit at slightly different ratios, would we be surprised by that, or would we expect to see that? If you were reading a novel and in it a planet was discovered that fit the above parameters, would you think, no way, what are the odds? Or would you think, yeah, probably?

Thank you for bearing with me on this. I genuinely appreciate the expertise.

posted over 5 years ago

thelefthorse‭

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