This thread is simple, just post planets or moons that have atmospheres that seem breathable.

Here is one I found,

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.)

This was the first planet found with breathable air... though it is very cold:


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.

Added:  A few more breathable atmosphere's have been found, including a temperate terra by Gnargenox!

At least that planet makes some sense as it has life.  I have seen some planets with breathable atmospheres with no life and I can't think of any geologic or physic process outside of life that could keep such high O2 levels.  

Yeah I've had a lot of trouble finding breathable atmospheres in SE thanks to seemingly every danged planet having tons of SO₂ in the air. There's worlds here and there that have enough oxygen, the right pressure, etc. but almost every time that SO₂ has to show up and ruin it >.<
Am I the only one slightly suspicious that SE may reflect an overabundance of SO₂ in terras' atmospheres?

I remember a thread like this on the old forum. I've come pretty close to finding reasonably breathable atmospheres myself, but the S02 is almost always the deal breaker. If the S02 is not high, you can count on the C02 to make up for it instead
@Watsisname Aside from the lethal temperatures on that planet. I would imagine the low mass would also point toward a weak magnetic shield?
If I were to be sent out tomorrow as a colonist to arrive one of these world's, I'd rather land on one of the temperate planets with a poisonous atmosphere. More comfortable to wear a breather than an an entire EVA suite.

I would imagine the low mass would also point toward a weak magnetic shield?

Would need to figure out Ozone content but that would play a decent role in UV protection.  You would probably be fine with limited exposure and considering the temperature you would be wearing layers anyway.  You could always line your clothes a bit to protect from radiation.

I would imagine the low mass would also point toward a weak magnetic shield?

Would need to figure out Ozone content but that would play a decent role in UV protection.  You would probably be fine with limited exposure and considering the temperature you would be wearing layers anyway.  You could always line your clothes a bit to protect from radiation.

Interesting. Thought gives me the chills though. I don't 'fancy' the idea of dodging hazardous radiation. Curious as to what affect lining your clothing would have though?

Curious as to what affect lining your clothing would have though?

If we are talking future technology perhaps you could generate a localized magnetic field or some other fancy shielding technique.  Considering the length of day, temperature, and distance to it's parent star it would probably not be a good idea to spend a lot of time outside.  The big plus on that planet is you won't die from breathing the atmosphere, everything else is a matter of exposure and shielding. 

If we are talking future technology perhaps you could generate a localized magnetic field or some other fancy shielding technique.

That's an idea. If these travelers had the capability of generating a localized magnetic field, then I imagine they would be equipped with a gas filter of some sort too. Wear a fancy belt that screens out S02 and C02 within a localized range, say 5 x 5 metres. In which case, they can work on a more hospitable planet and be more mobile with less need to spend time in an atmospheric controlled hab.

If you have more than about 300,000ppm of CO2, the atmosphere will kill you.

Am I the only one slightly suspicious that SE may reflect an overabundance of SO2 in terras' atmospheres?

No, I think you're right.  Naturally-occurring SO2 is mainly from volcanic outgassing, and on most terrestrial planets this rate should be low enough, and the residence time of the SO2 short enough, that the concentration would remain pretty small.  Some¹ studies² with atmospheric chemistry models show that it doesn't matter very much whether the atmosphere is dominated by hydrogen or nitrogen and CO2 -- either way the SO2 is quickly converted to elemental sulfur and sulfuric acid, which forms an aerosol layer or precipitates out.  This is what happens on Earth pretty quickly after a big eruption.

It could be conceivable that some planets with very high volcanic activity (like on super-Earth's) might have a high enough emission rate such that the chemical cycle would be dominated by sulfur, but even then the SO2 concentration would be small.

At any rate, SE's current atmo generation is still fairly new and primitive.  There's plenty of room for improvement.

@Watsisname Aside from the lethal temperatures on that planet. I would imagine the low mass would also point toward a weak magnetic shield? 

I agree with Doc -- the atmosphere itself provides a good amount of shielding.  And if there is appreciable (i.e. breathable) amounts of oxygen in the atmosphere, the star's UV radiation will break some of it up to form protective ozone.

Added:

If you have more than about 500ppm of C02, the atmosphere will be disorienting to breathe even if you have enough oxygen. Any more than 1000ppm, and you're dead within a few hours.

I would check your source of that info.  First problem is these numbers would depend on what the total pressure is (higher pressure = smaller concentration needed), and secondly if you consider Earth's surface pressure then they are far too small.  At 1atm, you need closer to 10,000ppm to start feeling drowsy, and 50,000ppm to be lethal in hours.

If you have more than about 500ppm of C02, the atmosphere will be disorienting to breathe even if you have enough oxygen. Any more than 1000ppm, and you're dead within a few hours.

I've measured CO2 in my house for a decade, and over the past month it looks like this: The diurnal cycle mostly reflects the presence of people in the house, and by your statement we should be close to dying every night.  In fact, less than 1000 ppm is regarded as good indoor air quality.

Speaking of atmospheres, I think SE generates too few Venus-like terras, that is, planets with thick, opaque atmosphere.  But that, I suppose, is difficult to render.

I've measured CO2 in my house for a decade, and over the past month it looks like this:

What do you use to do these measurements?  I would be interested in doing this myself but I haven't a clue where to start.

DoctorOfSpace:

CO₂measuring, example: Netatmo weather station. https://www.netatmo.com/product/weather/weatherstation

Just came across this. To my knowledge, O2 doesn't occupy enough partial pressure. C02/S02 looks to be pretty borderline?