I have developed a fictional planet with the following atmospheric composition;

  • 60.4% Nitrogen
  • 27.6% Oxygen
  • 9.8% Xenon
  • 1.5% Water Vapor
  • 0.64% Argon
  • 0.06% Carbon Dioxide

Is this breathable to humans with an atmospheric pressure similar to Earth's? What about 50%, 200%, and 500% of Earth's atmospheric pressure?

Lastly, what would it look like in terms of color?

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    $\begingroup$ I wonder if lightning strikes would be even more dramatic on this planet than on Earth, due to the abundance of Xenon and its behavior when electrically excited. Definitely not an answer to your question, but the first thing that occurred to me... $\endgroup$
    – amfinley
    Jun 5, 2017 at 19:30
  • $\begingroup$ I think it's breathable. N and O are almost equal in Earth. Xe and Ar are inert gas (not problem), H2O, well, it's quite up, but not much problem, and CO2 it's very low for been dangerous. $\endgroup$
    – Ender Look
    Jun 5, 2017 at 19:38
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    $\begingroup$ Please consider that your big hole isn't so much that people are able to breath whatever but the fact that something like xenon is that abundant anywhere. Check this out: periodictable.com/Properties/A/UniverseAbundance.html . If you plan to write good fantasy/sci-fi, choose your battles wisely. Space faring humans will have highly developed genetics and will be able to adapt. Just because a swamp can exist, it doesn't mean that the scene in the empire strikes back where yoda pulls out the aircraft is realistic. May I ask why you need such specific data? $\endgroup$
    – Raditz_35
    Jun 6, 2017 at 14:39
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    $\begingroup$ Very often, what matters isn't so much the percentage of a gas as its partial pressure. For example, Earth's atmosphere is just a shade over 1000 millibar and 21% oxygen content, for an oxygen partial pressure of about 210 millibar. If you double the pressure and keep the composition the same, that's now 420 millibar partial pressure of oxygen. NASA has done studies on humans and high oxygen partial pressure. Also see eg worldbuilding.stackexchange.com/a/80111/29. $\endgroup$
    – user
    Jun 6, 2017 at 14:49
  • $\begingroup$ @Raditz_35 The exact details of the atmosphere are not super important, aside from the fact that the human colonists need to be able to breathe well enough to live there. I really only added Xenon for aesthetic purposes, but it's not exactly necessary for the story. $\endgroup$ Jun 6, 2017 at 18:48

5 Answers 5


Nitrogen (N₂):

  • 60.4% — 78.08% = −18.48% N₂ (↓22%)
  • Is used in the atmosphere to reduce the percentage of oxygen in the air (if O₂ is too much our atmosphere may burn out).
  • It haven't any very important effect in life. N₂ is a inert gas, it can't burn and don't make any special reaction in the air.
  • However bacteria breath N₂ and make amino acids > then make proteins > and then their are used by plants and animals. A 18% of difference I don't think that could be very problematic.

Oxygen (O₂)

  • 27.6% — 20.95% = +6.65% O₂ (↑31,74%)
  • Is used by all the plants and animals in our world.
  • In a world where there is 24% more oxygen I could guess that metals would oxidize 24% faster.
  • Our bodies could have a bigger metabolic rate (more O₂ to burn). Luckily only O₂ is danger when it's up to 28%, and you has 27.6%.
  • This mean that insect would be much larger. Insect haven't respiratory system, they breath thought their skin, they can't be bigger because their won't have enough surface per gram of insect to breathing. With more O₂ insect could have bigger bodies without suffocate.
  • Fire would have bigger and hotter flames.
  • Organic matter would rot faster.

Xenon (Xe):

  • 9.8% — 0.08 ppmv (0.000008%) = +9.799992% Xe (↑122,499,999%)
  • Is an inert gas, this mean that it can't be burned or used in any reaction in the air.
  • Lightnings and rays would be more blue.
  • Xenon is very lethal to humans in high concentration, luckily you have 9.8% and it's only dangerous over 33% (30% is used by medics); I guess that the minimal effects of 9.8% would be suppressed if your animals have lived in that planet for generations.

Water Vapor (H₂O):

  • 1.5% — 1% = +0.5% H₂O (↑50%)
  • Nothing bad; your planet would have more humidity.
  • Also in winter there would be more drops of water in the surfaces.

Argon (Ar):

  • 0.64% — 0.93% = −0.29% Ar (↓68,81%)
  • Is an inert gas. Actually Argon don't do anything in our planet so it won't do nothing in yours.
  • Also your rays and lightnings would be a little less pink.

Carbon Dioxide (CO₂):

  • 0.06% — 400 ppmv = +0.02% CO₂ (↑300%)
  • Your lakes, oceans and rain would be more acid since that CO₂ would mix with natural water making it acid, so your Coccolithophore, Coral, Foraminifera, Echinoderm, Crustacean and Mollusca life would be quite damage their Calcium Carbonate organics parts (water would be able to dissolve them) and their will have a bite of Hypercapnia.
    • This would make your planet hotter. How? Well, this organic life have shells, exoskeleton or corpses made of Calcium Carbonate, CaCO₃ is white and increase the Albedo of Earth. Less CaCO₃ will reduce the albedo, lower albedo will absorb more of the Sun's heat. How much? I am not god, so I have no idea (maybe despicable).
  • Also your coral would suffer Coral bleaching and they would lose 90% of their energy input and would starve to die.
  • I am not sure but I think that your planet would be a 0.5 – 1 ºC hotter, remember that CO₂ is the second (9%–26%) dangerous gas to the global warming.

Well, also you won't have some gasses like methane or chlorofluorocarbon that increase the greenhouse effect (but very low or null since your CO₂ increase). Remember that some gasses like ozone are very important for global warming and to stop UV radiation from the Sun, your people will have much skin cancer since O₃ is the only gass that reduce the UV rays <290 nm wavelengh.

Atmospheric Pressure:

  • 101.325 kPa (Normal): O₂ Partial Pressure: 27.5 kPa
  • 50.6625 kPa (50%): O₂ Partial Pressure: 13.75 kPa, minimal safe is at 16 kPa but at 13.3 kPa humans suffer hypoxia, your people won't die, but their will be very bad...
  • 202.650 kPa (200%): O₂ Partial Pressure: 55 kPa, maximum safe is at 50 kPa otherwise you would have oxygen toxicity and your people will suffer hiperpoxya. Risk of oxygen toxicity is in less than 10 hours.
  • 506.625 kPa (500%): O₂ Partial Pressure: 137.5 kPa, deadly... you won't survive more than one day. Risk of oxygen toxicity is in one or a couple of hours.
    Also you would have some pulmonary effects:
    • Burning sensation on taking a deep breath.
    • Cough.
    • Pneumonia.
    • Permanent lung damage (and then death).

If you want to know more about necessary oxygen partial pressure to breath you can check the answers of What is necessary for a breathable artificial atmosphere?

With the excel tables I have calculated this:

  • Values based on a temperature of 25 °C (298.5 K).
  • $1 \text{ atm} = 101.325 \text{ kPa}$

$$ \begin{array}{|cc|cc|c|cccc|} \text{Chem}&\text{%}&\text{g/mole}&\text{%}&\text{RMS}&&\text{Pres. (atm)}&&\\ &&&&&1\text{ atm}&0.5\text{ atm}&2\text{ atm}&5\text{ atm}\\ \text{N}_{2}&60.4&14.007&8.460228&729.083468&61.2003&30.60015&122.4006&306.0015\\ \text{O}_{2}&27.6&15.999&4.415724&682.186932&27.9657\text{*}^{1}&13.98285\text{*}^{2}&55.9314\text{*}^{3}&139.8285\text{*}^{4}\\ \text{Xe}&9.8&131.293&12.866714&239.13818&9.92985&4.964925&19.8597&49.64925\\ \text{Ar}&0.64&39.948&0.2556672&431.720126&0.64848&0.32424&1.29696&3.2424\\ \text{CO}_{2}&0.06&44.01&0.026406&411.314605&0.060795&0.0303975&0.12159&0.303975\\ \text{H}_{2}\text{O}&1.5&18.0152833&0.27022925&642.879041&1.519875&0.7599375&3.03975&7.599375&\\ \end{array} $$

  • •1: 27.9657 kPa O₂ = Breathable oxygen.
  • •2: 13.98285 kPa O₂ = Below allowed O₂ pressure (16 kPa) and very close to hypoxia (13.3 kPa). Humans won't survive much time.
  • •3: 55.9314 kPa O₂ = Above allowed O₂ pressure (50 kPa), oxygen is toxic. Humans won't suvive much time.
  • •4: 139.8285 kPa O₂ = Above allowed O₂ pressure (50 kPa), oxygen is highly toxic. Human will die in some minutes (an hour?).

I don't know your planet mass. If it's like Earth, no problem. If not, check that your escape speed is less than some gas RMS (remember that I used 25 °C) (or less than a third using the @Alice advice from comment), if escape velocity is greater than the RMS gases from the planet would take millions (maybe even more) of years to escape

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    $\begingroup$ This is comprehensive and good for analyzing the differences. But I believe some of your specific facts are wrong. ❴…❵ « can't change the atmospheric pressure without changing planet's gravity? » say what? If you change the amount of air, how does the pressure remain constant? $\endgroup$
    – JDługosz
    Jun 6, 2017 at 4:12
  • $\begingroup$ BTW, the degree symbol ( ° ) is different from the masculine ordinal indicator. They look the same in the edit box, but different in the final page. $\endgroup$
    – JDługosz
    Jun 6, 2017 at 4:14
  • $\begingroup$ @JDlugosz Thanks for the edit. Aren't clouds water gas? Wikipedia says that clouds increase global warming (reflect heat from the inside to escape) and also reduces it (reflect heat from the outside to enter in Earth). If you increase the atmospheric pressure, the air would have more strength to escape from the planet so you will need more gravity to pull it down (when I arrive at home I'll check it and explain better). About Calcium Carbonate I mean that all the animal with shells of CaCO3 will have weaker shells (acid corrode them) $\endgroup$
    – Ender Look
    Jun 6, 2017 at 12:42
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    $\begingroup$ What's the pressure on Venus? And the gravity? $\endgroup$
    – JDługosz
    Jun 6, 2017 at 13:28
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    $\begingroup$ @JDlugosz, Venus has an escape velocity of 10.36 km/s (calculated with 4.8675×10<sup>24</sup> kg of mass and 6,051.8 km of radius). The average mol of Venu's air is 44 g/mol, so his MRS is 645.49 m/s, gas can't escape. For more information you can read this post (in spanish) $\endgroup$
    – Ender Look
    Jun 6, 2017 at 21:00

27% Oxygen is absolutely breathable. Other components should not affect breathability. At 50% atmospheric pressure, oxygen has an equivalent concentration of 13.8%, which is a little too low for long-term breathing. However, short trips are totally possible. At 200%, equivalent oxygen would be at 55.2%. This would be harmful with long term exposure, but again perfectly fine for a short time. At 500%, we would be getting into the toxic territory. It is similar to the oxygen effects on deep divers.

For the color, the air would still be colorless, however the effect on color of the sky and color of setting sun should be more pronounced in higher pressure.

  • $\begingroup$ So it's safe to assume that at 1 atm the sky would still be blue during the day? $\endgroup$ Jun 5, 2017 at 23:15
  • $\begingroup$ Oh yes, the blue sky will not go away! $\endgroup$
    – Alexander
    Jun 6, 2017 at 0:38
  • $\begingroup$ Of course the color would change. It would still be blue, but certainly a different blue... $\endgroup$
    – Fl.pf.
    Jun 8, 2017 at 7:17

At 500% you could get anesthetic effects from the nitrogen and possibly the xenon. Xenon is used as an anesthetic but at 1 atmosphere it requires 60-70%. I conclude it must be a more potent anesthetic than nitrogen gas because we already routinely breathe 70% N2 at 1 atm.

Nitrogen narcosis can happen from normal nitrogen-containing air at pressure - I think one can get into trouble between 4 and 5 atmospheres.

Otherwise this looks like normal air with a percentage of oxygen replaced by xenon.

  • $\begingroup$ You mean, I think, a percentage of nitrogen replaced with both oxygen and xenon. Standard air is 21% oxygen. $\endgroup$ Jun 8, 2017 at 14:24

CO2 is a little high.

Breathing does two things: it grabs O2 and it dumps CO2. Of these, the CO2 generally feels more urgent. If you hold your breath for a while and feel your lungs start to burn, you're feeling the effects of excess CO2.

(Hypoxia is also bad for you, but people tend to pass out before they notice it. Pilots get special training in how to spot the symptoms and react to them.)

So you'll have about 50% more CO2 in your atmosphere than Earth. That probably leads to a few rare (and exotic) complications, but also slightly more common acidity in various fluids. Like, tooth decay might be slightly more common on this world. For anyone worried about it though, the easy buffer is calcium cabonate (Tums, but also chalk like for a chalkboard). Natives are likely fine; visitors probably chow down on Tums, and anyone who doesn't drink beer gets kidney stones.

Vistors also likely have their lungs start burning with less exertion than their used to. That probably works about like how exercise in Denver makes you tired more easily, and goes away after a few weeks/months of acclimatization.


Just an addition concerning the colour (as the compositional questions already have been covered):
Rayleigh scattering that turns our atmosphere blue is more efficient the shorter the wavelength. It explains why the sunset is red, because blue is scattered away long before it reaches the observer, and it should make you think why our sky isn't actually purple (which has even shorter wavelength than blue).
The answer to the latter is that our sun emits more blue light than purple, on the steep slope of the black-body function, and there is some purple mixed in in our blue sky, but blue still prevails.

Thus in my understanding your sky colour will change:
At 200-500% pressure, you have essentially the sun-set situation already at noon and during the rest of the day, so the sky will be red-orangeish.
At 50% pressure the blue hasn't been scattered away yet on ground level, so purple might come out better, but you still might want to turn the star a bit hotter so that its purple/blue ratio increases.

This is under the assumptions of pure Rayleigh-scattering, there might be other effects, like high-pressure absorption bands in $N_2$ being activated at high pressure, that could change the colour away from my description.


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