As for planets with breathable atmospheres do you think you could list out the parameters I should look for in planets with breathable atmospheres? Like what percentages and PPM I should find, and also avoid? I know SO2 is dangerous so it should be as little as possible. 

Check here on the first page. I haven't updated it since then, but should still be useful.

I have done some searching for about an hour or so. Came up with these three here. What do you think?

As for planets with breathable atmospheres do you think you could list out the parameters I should look for in planets with breathable atmospheres? Like what percentages and PPM I should find, and also avoid? I know SO2 is dangerous so it should be as little as possible. 

Check here on the first page. I haven't updated it since then, but should still be useful.

Excellent thread!  As an aside, I miss the vibrant planetary colors of the older version!

Cantra, I'm afraid none of the three planets have sufficient oxygen for breathability. They have less than 0.04 atm partial pressure of O₂, which is equivalent to being above 40,000ft altitude on Earth, and results in loss of consciousness in less than 10 seconds. No amount of acclimatization can help to survive under those conditions.

None of the other gases present are at high enough concentration to cause serious problems. The SO₂ on all three worlds would give a faint smell of struck matches before passing out from the low oxygen.

If you can find a world that has similar numbers for all the other gases but with between about 0.1 to 0.6 atm of oxygen (see the right column in the images you showed), then it could be breathable.

Cantra, I'm afraid none of the three planets have sufficient oxygen for breathability. They have less than 0.04 atm partial pressure of O₂, which is equivalent to being above 40,000ft altitude on Earth, and results in loss of consciousness in less than 10 seconds. No amount of acclimatization can help to survive under those conditions.

None of the other gases present are at high enough concentration to cause serious problems. The SO₂ on all three worlds would give a faint smell of struck matches before passing out from the low oxygen.

If you can find a world that has similar numbers for all the other gases but with between about 0.1 to 0.6 atm of oxygen (see the right column in the images you showed), then it could be breathable.

I went ahead and searched for around two hours straight. These worlds are far from an Earth type though...which may undermine their discoveries, but still I would like to see your thoughts on these worlds. Tell me what you think of these,  and which one is the best out of all of them. I know this may be a lot in one post, but I attempted to find better ones.

Based on what you say, I'll refine my searches next time further. 

I went ahead and searched for around two hours straight. These worlds are far from an Earth type though...which may undermine their discoveries, but still I would like to see your thoughts on these worlds. Tell me what you think of these,  and which one is the best out of all of them. I know this may be a lot in one post, but I attempted to find better ones.

1: Looks good!  Even the carbon monoxide (CO) is tolerable -- I didn't have it on the list, but 1x10^-6 atm of CO is fine. It doesn't get very dangerous until about 10^-5 atm. The most negative thing about this atmosphere is the CO2: at 0.03 atm it would cause some shortness of breath and long term issues, but it's survivable for a while and the effects of exposure are reversible.

2: Too much oxygen
3: Too much oxygen
4: Uncomfortable to dangerously high SO2
5: Too much oxygen
6: Too much oxygen
7: Too little oxygen, comparable to the summit of Everest

Wats what are the side effects of too much oxygen?  Weren't there periods in Earth's history when it had much more oxygen than it does now (and that's why animals were much larger then- like giant sized insects, reptiles, etc.?)

I went ahead and searched for around two hours straight. These worlds are far from an Earth type though...which may undermine their discoveries, but still I would like to see your thoughts on these worlds. Tell me what you think of these,  and which one is the best out of all of them. I know this may be a lot in one post, but I attempted to find better ones.

1: Looks good!  Even the carbon monoxide (CO) is tolerable -- I didn't have it on the list, but 1x10^-6 atm of CO is fine. It doesn't get very dangerous until about 10^-5 atm. The most negative thing about this atmosphere is the CO2: at 0.03 atm it would cause some shortness of breath and long term issues, but it's survivable for a while and the effects of exposure are reversible.

2: Too much oxygen
3: Too much oxygen
4: Uncomfortable to dangerously high SO2
5: Too much oxygen
6: Too much oxygen
7: Too little oxygen, comparable to the summit of Everest

Number one, hmm, that's pretty interesting. I have some other worlds for you to examine here, but that will be it for a few days I think. I've been doing this all day haha. Too much oxygen, attempted to take that into account.

Wats what are the side effects of too much oxygen? 

See Hyperoxia and Oxygen Toxicity for details. Oxygen is a highly reactive gas and at higher concentrations (or partial pressures) it becomes toxic. Above about 0.6 atm, our bodies are unable to manage the oxidative damage it causes.

Weren't there periods in Earth's history when it had much more oxygen than it does now (and that's why animals were much larger then- like giant sized insects, reptiles, etc.?)

Yes, but it is less straightforward than some people think. For example, the following statement might sound reasonable, but is not correct: 

"Oxygen is good for respiration, and therefore animals became larger when there was more oxygen." 

This statement would be incorrect for two reasons. Not all animals became larger, but mostly reptiles and insects. And the reason they became larger is more complicated. For example, being larger could help them avoid the problems of oxygen toxicity and thermal stress. (See: Can Oxygen Set Thermal Limits in an Insect and Drive Gigantism?)

Number one, hmm, that's pretty interesting. I have some other worlds for you to examine here, but that will be it for a few days I think. I've been doing this all day haha. Too much oxygen, attempted to take that into account.

All of these have too much oxygen. Check the number in the right-most column in your screenshots: for oxygen it should be between 0.1 atm and 0.6 atm. Your closest in this set was the first planet at 0.617 atm, which would be tolerable for probably a few days, but is not survivable indefinitely.

Wats what are the side effects of too much oxygen? 

See Hyperoxia and Oxygen Toxicity for details. Oxygen is a highly reactive gas and at higher concentrations (or partial pressures) it becomes toxic. Above about 0.6 atm, our bodies are unable to manage the oxidative damage it causes.

Weren't there periods in Earth's history when it had much more oxygen than it does now (and that's why animals were much larger then- like giant sized insects, reptiles, etc.?)

Yes, but it is less straightforward than some people think. For example, the following statement might sound reasonable, but is not correct: 

"Oxygen is good for respiration, and therefore animals became larger when there was more oxygen." 

This statement would be incorrect for two reasons. Not all animals became larger, but mostly reptiles and insects. And the reason they became larger is more complicated. For example, being larger could help them avoid the problems of oxygen toxicity and thermal stress. (See: Can Oxygen Set Thermal Limits in an Insect and Drive Gigantism?)

Number one, hmm, that's pretty interesting. I have some other worlds for you to examine here, but that will be it for a few days I think. I've been doing this all day haha. Too much oxygen, attempted to take that into account.

All of these have too much oxygen. Check the number in the right-most column in your screenshots: for oxygen it should be between 0.1 atm and 0.6 atm. Your closest in this set was the first planet at 0.617 atm, which would be tolerable for probably a few days, but is not survivable indefinitely.

That's extremely interesting about why reptiles and insects specifically could deal with higher amounts of oxygen better.  It makes me wonder about the cause of the gene turning off that caused dinosaurs to become so large (and thus their evolution towards birds.)  Do you think this gene being turned off was triggered by the environment (the reduction in oxygen in the atmosphere) or was it strictly a chance occurrence that just happened to be beneficial and thus passed down?  This results in a cascade of questions about what caused the level of oxygen to vary so much when (one would assume) the planet was already in a pretty stable "middle aged" phase by the Mesozoic Era....and what caused oxygen levels to go down after that?

Wats what are the side effects of too much oxygen? 

See Hyperoxia and Oxygen Toxicity for details. Oxygen is a highly reactive gas and at higher concentrations (or partial pressures) it becomes toxic. Above about 0.6 atm, our bodies are unable to manage the oxidative damage it causes.

Weren't there periods in Earth's history when it had much more oxygen than it does now (and that's why animals were much larger then- like giant sized insects, reptiles, etc.?)

Yes, but it is less straightforward than some people think. For example, the following statement might sound reasonable, but is not correct: 

"Oxygen is good for respiration, and therefore animals became larger when there was more oxygen." 

This statement would be incorrect for two reasons. Not all animals became larger, but mostly reptiles and insects. And the reason they became larger is more complicated. For example, being larger could help them avoid the problems of oxygen toxicity and thermal stress. (See: Can Oxygen Set Thermal Limits in an Insect and Drive Gigantism?)

Number one, hmm, that's pretty interesting. I have some other worlds for you to examine here, but that will be it for a few days I think. I've been doing this all day haha. Too much oxygen, attempted to take that into account.

All of these have too much oxygen. Check the number in the right-most column in your screenshots: for oxygen it should be between 0.1 atm and 0.6 atm. Your closest in this set was the first planet at 0.617 atm, which would be tolerable for probably a few days, but is not survivable indefinitely.

Wats would all these planets be open to geo-engineering though? We have a machine going to Mars (or already on Mars) that converts CO2 to O2 so I presume we could also create a machine to take O2 out of the atmosphere.....especially with how reactive Oxygen is, for the borderline planets that are just over 0.6 atm especially this should be possible?

We have a machine going to Mars (or already on Mars) that converts CO2 to O2 so I presume we could also create a machine to take O2 out of the atmosphere.....especially with how reactive Oxygen is, for the borderline planets that are just over 0.6 atm especially this should be possible?

Possible, but better question may be how much effort it would take, and if it would be worth it. That would be highly situational, so there is not a simple answer.

We have a machine going to Mars (or already on Mars) that converts CO2 to O2 so I presume we could also create a machine to take O2 out of the atmosphere.....especially with how reactive Oxygen is, for the borderline planets that are just over 0.6 atm especially this should be possible?

Possible, but better question may be how much effort it would take, and if it would be worth it. That would be highly situational, so there is not a simple answer.

Wats, what was the highest the percentage of oxygen was in Earth's lower atmosphere during Earth's habitable stage?  I figure it was sometime during the Mesozoic?

Wats, what was the highest the percentage of oxygen was in Earth's lower atmosphere during Earth's habitable stage?  I figure it was sometime during the Mesozoic?

We don't know.

That might sound ridiculous, like, surely we must be able to know that with some confidence, but no. Figuring out the oxygen evolution is surprisingly hard.

If you were to do a quick search on google or wikipedia, you would be likely to find claims and even figures suggesting oxygen levels peaked around 30% during the Permian and Carboniferous (about 250 to 350 million years ago). If you look more carefully, you might even find error bars for that figure, suggesting the peak was actually anywhere between 25% and 35%.

However, if we do a more complete review of literature, we find models and measurements over that time period go all over the place, and often conflict with one another:

 
Source: Atmospheric oxygen of the Paleozoic (Brand et al, 2021)

What to conclude from this tangle of spaghetti? ¯\_(ツ)_/¯ 

If we look to actual measurements, they don't do much to help settle the question, either. For instance, the above study examined fluid inclusions within certain minerals and rocks, and the results show a fairly constant concentration during that period of time, at odds with both models and prior data from other sources:



Okay, maybe this one particular study is just an outlier, and there is a stronger consensus elsewhere. Except there isn't, really. That's what most surprises me. This is an area of geology of much importance, yet requiring much more work to reach a strong understanding. It's surprisingly more complicated and less certain than the geologic history of CO2. The sources and sinks are different, and the data used as correlations to their concentrations are different. What we are able to say with confidence is that oxygen levels were extremely low until The Great Oxidation Event a little over 2 billion years ago, and then increased further in more recent geologic history. However, we don't know exactly how high it ever got, or even if it was ever higher than today.

One of the strongest constraints on the oxygen concentrations in the last roughly 400 million years is set by the behavior of wildfires and the evidence of soot and charcoal left behind in the geologic record. These constraints tell us the oxygen concentration since 400 Mya could not have been much less than about 15%, or much greater than 35%.

Great analysis Wats, and I'm surprised we don't have more confidence in the numbers, so basically the margin of error of the peak oxygen level is well within our current oxygen level.  The higher occurrence of wild fires is yet another reason why oxygen shouldn't be much more than it is now to maintain a high level of habitability.

As for planets with breathable atmospheres do you think you could list out the parameters I should look for in planets with breathable atmospheres? Like what percentages and PPM I should find, and also avoid? I know SO2 is dangerous so it should be as little as possible. 

I know I'm late to respond, but... I use a cheat-sheet when looking for planets. You might find it useful to save something like it into a .txt file.

O2 0.16atm - 0.5atm (>0.1 can be acclimated to)
SO2 <0.0005atm or 500ppm (<0.00001 or 10ppm ideal)
CO2 <0.05atm or 50,000ppm (<0.005 or 5000ppm ideal)
CO <0.0001atm or 100ppm (<0.000035 or 35ppm ideal)

* Multiply percent content by current ambient pressure for different elevations (Check your decimals! 1% is .01) *
* 0.000n% for ppm *

NARCOSIS occurs at
N2 >1.6 - 3.2atm
H2 >50atm

TOTAL PRESSURE tolerance <70atm (assuming everything is within safe limits)
TEMP tolerance <50c (assuming almost no humidity)

Keep in mind that some of those limits are extremely generous. You can find the reasons for these numbers online and on the first page of this thread (which you've probably read by this point).

SO2 is the big killer of otherwise breathable atmospheres. That should be the second thing you check after O2.