Yeah, these are fun things to look for, and very difficult to find. We had a thread for it on the old forum along with some good information on the limits of survival. I'll copy some of that below.
The trick to seeing if the air is breathable or not is to check the second column in the info > atmosphere tab, which shows the partial pressure
of that gas. Partial pressure means how much of the total surface pressure is caused by that particular gas. For example, if the total pressure is 2 atm, and the composition is 90% oxygen and 10% CO2, then the partial pressure of oxygen is 1.8 atm, and the partial pressure of the CO2 is 0.2 atm.
We can deal with a partial pressure of O2 between 0.1 and 0.6 atm.
For CO2, we need less than 0.05 atm.
SO2 (sulfur dioxide) must be below 10-6
. (Very small!)
and the limit for H2S (hydrogen sulfide) is about 10-4
These all have fuzzy limits where you could survive for at least a little while:
Unfortunately, this means the air on your planet is not breathable. It's actually too much
oxygen, and too much CO2 and SO2. (The SO2 is almost always too high with the current atmosphere generation -- it is exceptionally difficult to find planets where it is below 10-6
There has thus far been found exactly one planet with breathable air... though it is very
And several people have found planets that are very close, or where the air is breathable except for the SO2. We can imagine them being habitable with just a respirator to deal with the toxic component.