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

Question

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.

Answer 1

Firstly it sounds like you want to do bottom-up worldbuilding, meaning you want to get the basic science straight first. I would recommend worldbuilding YouTuber Artifexian, as he has a series where he starts with constructing the solar system and has currently reached climate mapping. He breaks down the science to a very digestible minimum. For your question, I recommend watching his videos on atmospheres, alien atmospheres and sky and plant-color.

Pressure

Pressure may vary significantly on "earthlike" planets. Mars has only about 0.006 atm, Earth has 1 atm, Titan has 1.5 atm and Venus 96 atm. Mars and Venus both experienced cataclysmic events in their pasts, the loss of the magnetosphere and a runaway greenhouse effect respectively. Thus they are out of the interesting zone. Looking at Earth and Titan no one could say you went overboard if you vary the pressure on planets you construct by an order of magnitude in both directions (0.1 atm - 10 atm). The upper and lower end might be unsuitable for unsuited humans.

Composition

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?

Yes, and No. let me break this down in detail and give you some breathability limits for indefinite survival.

• N2 < 3 atm

Is somewhat a given since ammonia is common in protoplanetary nebulas and will end up on most planets. As it gets broken apart the nitrogen reacts to N2 molecules, which are very stable and inert. Nitrogen will usually just accumulate over time in the atmosphere.

• O2 < 0,16 – 0,5 atm (0.3 - 0.35 atm wildfire limit) For the maximum and this and for the miniumum.

Free oxygen is only a surprise if you don't expect a biosphere. O2 is so reactive that it will disappear quickly if it doesn't get replenished constantly. O2 may also occur abioticly on worlds where no land is exposed to the atmospere, as nothing can be found to xidise it away with.

• Ar < 1.6 atm

Nearly all of the argon in the Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most easily produced by stellar nucleosynthesis in supernovas. - Wikipedia

Argon isn't in the atmosphere by chance but as a result of the alpha decay of potassium-40. This means that a planet with no argon in the atmosphere is either very young (not older than maybe 0.5 byr at most), in a low metallicity system (meaning it would most likely be a planet dominated by water), lost its original atmosphere in a geographically speaking recent event or is an artificial world around a gas giant or black hole or is so small (moon-sized) that it had little radioactive material, to begin with. Usually, you will find argon.

• H2O

You'll usually find out the maximum possible water content by calculating the water vapor saturation pressure. Calculate your planets global average temperature for it and multiply the maximum saturation with 0.125. That's the average water content of the atmosphere. That said this rule of thumb only holds for planets with large bodies of water. Desert-planets might have significantly less moisture.

• CO2 < 0.02 atm

Carbon dioxide too is a given and will remain in the atmosphere in moderate concentrations as long there is a functioning carbon cycle, which likely requires plate tectonics. A lot of carbon is stored in the lithosphere as rock. If the planet gets too hot it will be "cooked out", which is the reason for Venuses thick CO2 atmosphere

• CH4 < 0.05 atm (not toxic, but explodes at this point)

Is usually generated by biological processes, so if there is life, some methane is likely. Abiotic methane is an option, too but you usally wont find it in significant quantities on Earth-like worlds.

• SO2 < 0.000005 atm

Is a short-lived gas and a sign for extremely strong vulcanism.

• O3 < 0.0000001 atm i.e. 0.1ppm

A byproduct of having O2 in the atmosphere. Will shield away UV-rays and allow life to colonize the land.

So, no one would be surprised to find very Earth-like atmospheres if the planet in question is very Earth-like. Also finding Mars and Venus-like planets should not surprise anyone, as they show paths along which an Earthlike atmosphere might develope.

Answer 2

Today, we have examples of some radically different breathing environs. The most unique I know of are breathable liquids, made from specific perfluorochemicals; https://en.m.wikipedia.org/wiki/Liquid_breathing Science-wise, there is nothing preventing a planet from being covered in this stuff, although you’re on your own to figure out a natural process that would produce the stuff in quantity.

But as a proof of concept, this suffices to answer your question.

You should also check out nitrox breathing, an alternative mix of gasses used by deep-sea divers. (In chemistry, nitrox refers to any nitrogen-oxygen blend, including regular air, but among divers, it’s a specialty mix.) At extreme depths, regular air becomes toxic... as would occur on a planet with a very deep atmosphere. These alternate blends are not breathable at the surface but work just fine at depth. https://en.m.wikipedia.org/wiki/Nitrox

Answer 3

It's an interesting consideration that you can have halometanes in the atmosphere: they are not very toxic but already make starting fire difficult at some concentrations.

https://en.wikipedia.org/wiki/Halomethane