Set up

I'm currently building a planet that I want to have both alien life forms and humans. It has the following traits:

  • Humans migrated from Earth a long time ago, but only landed on the planet 24 years ago.

  • The planet has Earth-like life and humans on it together.

  • The planet is about twice as big as the Earth in radius, and is 4 times as massive (I think that's the term in astronomy for having more mass...?), and has 1G gravity (To Earth).

  • The planet has 2 stars:

  • An F-type star of 1.2 solar masses with about 2.3 billion years left as of current times

  • A K-type star of 0.4 solar masses

  • The planet orbits the two stars at about 1.6 AU on average.

  • The planet has 2 moons:

  • 1 around 80% the size of the Earth, orbiting at 238,900 miles away on average.

  • 1 28% the size of Earth, orbiting at about 72,527 miles away on average.

I want:

  • Humans to be able to live on the surface naturally.

Difficulties in Living

My planet has:

  • A global temperature is around 23 Celsius (78 Fahrenheit), with the poles having a typical average of about 10 Celsius (50 Fahrenheit), and the hottest regions (Mostly sun-scorched deserts) having an average of about 54 Celsius (129 Fahrenheit).

  • An atmospheric pressure of 144% Earth's at sea level

  • Atmosphere:

    • 32% oxygen (a bit over 50% more than modern Earth, but a bit more than 21% less than the Carboniferous period)
    • 67% nitrogen
    • 1% other gases, with atmospheric carbon dioxide level at about 1,600 ppm (In the range of the Cretaceous (1,000-2,000 ppm)). T


The temperature and atmospheric composition came to their current values about 12 million years ago, so life on the planet has adapted.

Desired Outcome

I don't care whether humans can breathe on the planet without an oxygen mask. They came well after life evolved, so their ability to breathe isn't my problem, and they have the technology to make their own in my world anyway.

What I want to know is whether humans could live out in the open on this world with nothing more than an oxygen mask. A major concern of mine is the atmosphere, which is, at sea level, about 130% thicker than Earth's, and the temperature. This temperature is around the normal average temperature of Earth during the Paleocene-Eocene Thermal Maximum (PETM), so Earth-like life could survive, and I don't care about ice caps. However, I wanted to know whether humans could survive with only an oxygen mask.

Things To Consider

The atmosphere is thicker, but according to this question's answer, humans could theoretically survive up to 4 times Earth's atmospheric pressure.

The oxygen levels are likely too high (By how much oxygen %, I don't know).

Maybe the temperatures make the equator too hot for humans to survive in the open all year round, so humans might have to settle more at the poles.

The biochemistry is similar to Earth's, meaning that humans should be able to consume food safely (For those of you who don't know, Earth biochemistry is almost all left-handed amino acids, and they cannot be swapped for right-handed ones).

Could humans survive on this planet in to open with only an oxygen mask?

  • 3
    $\begingroup$ Yes, a ~0.5 bar oxygen partial pressure is pretty toxic for humans. Also, terrible firestorms. $\endgroup$
    – biziclop
    Apr 5 at 20:16
  • 2
    $\begingroup$ @biziclop: At 0.5 atm partial pressure oxygen is maybe very very mildly toxic; adverse reactions have only been observed with partial pressures of oxygen greater then 0.5 atm, but then nobody knows for sure what happens over an exposure longer than a few days. For short term exposure even 1 atm partial pressure of oxygen is quite safe. $\endgroup$
    – AlexP
    Apr 5 at 21:35
  • 1
    $\begingroup$ @AlexP Yes, I meant to live in it all your life. Short exposures to that level are mostly fine. Anyway, I was merely trying to warn OP who mentioned that in the Carboniferous the oxygen percentage in our atmosphere was higher than in the planned atmosphere, that that's only half the story. Percentage AND air pressure multiplied together are what determines whether you've got too much or too little oxygen. $\endgroup$
    – biziclop
    Apr 5 at 22:30
  • 1
    $\begingroup$ When you say "global temperature is 23C", what is the range? Global temperature average on Earth was 13.9C in 2022, but at the moment Accuweather shows Mumbai at 33C and Berlin at 9C (and there are probably hotter places and definitely colder places). A bit of air-conditioning and some annual migration should make it easily survivable. $\endgroup$ Apr 6 at 10:36
  • 1
    $\begingroup$ As a former diver, I can say high pressure + high oxygen partial pressure is a no-no. You can live for a few days at high oxygen partial pressures in a low-pressure environment (like the Apollo astronauts did) but people on your planet can only manage to survive by using masks to increase the partial pressure of inert gas (and I bet they need to protect their eyes too). Also with that level of oxygen expect insects to grow large, very large, and b very cautious on making fire. $\endgroup$
    – jean
    Apr 6 at 12:53

3 Answers 3


What are you worried about?

Is it pressure?

I wouldn't worry about it too much. As someone who has experience diving let me calm your concerns.

It is true that humans are limited to diving at a depth of 10 meters (2 bars, 50% higher) for less than 130 minutes without requiring decompression. It is not the pressure that limits your time under water but rather the fact that you need to return to a low pressure environment again, and how fast your body absorbs nitrogen (it is also different for different people, 130 minutes is the safe no decompression for most people). As you spend time under high pressure your body dissolves nitrogen into your blood (because the higher pressure of nitrogen allows for more dissolved nitrogen). When you go to a lower pressure the dissolved nitrogen slowly leaves your body. If you have too much dissolved nitrogen, it will form nitrogen bubbles throughout your body causing lots of cellular damage... It is extremely painful, but can be prevented/your life saved by a decompression chamber that will slowly bring your body back to a low pressure environment.

At a 1.44 bar atmosphere you could spend years without much harm (except an increased risk for a heart attack). Humans have spent 73 days at twice that pressure (~3 bar) with no reported negative side effects. Team 31 spent 31 days and were able to safely depressurize in 16 hours.

Is it the oxygen?

You probably don't need an oxygen mask. Thoough you might want a mask that reduces how much oxygen breathe in, replacing it with

With 144% atmosphere at 33% oxygen == 1.44 bars * .33 = .4752 bars of oxygen, you are below the point where oxygen toxicity starts (0.5 bars partial pressure). Though some people are more sensitive, and others less. I don't know if there is a long term oxygen toxicity exposure.


Serious mental impairment from Nitrogen narcosis does not set in until a partial pressure of 3.2 bars (there is a lot of variation), though some people have a lower tolerance, and some have a higher. hallucinations occur at 8 bars. At a 1.44 bar atmosphere at 67% you only have .96 bars of nitrogen, which is pretty safe. Very long term exposure might make everyone a little tipsy, I don't know.

  • 1
    $\begingroup$ "I don't know if there is a long term oxygen toxicity exposure" - I sort of think there should be, but can't find any supporting evidence either. Maybe where it's used (COPD etc.) its benefits so outweigh any potential detriments that no-one's bothered with it. $\endgroup$ Apr 5 at 21:26
  • 1
    $\begingroup$ I Know that for saturation diving, there was one company that had a decompression procedure that used a pp of 0.6 to 0.8, because it allowed for a shorter nitrogen off-gassing cycle... So the 0.5 bar cut-off is really taking it safely. (They have since shifted to a more conservative decompression regime). $\endgroup$
    – Questor
    Apr 5 at 22:35
  • 3
    $\begingroup$ @AngryMuppet, my gut says there might be an increased rate of cancer, as low oxygen environments are associated with a significantly lower rate of cancer (naked mole rat research). $\endgroup$
    – Questor
    Apr 5 at 22:37
  • 2
    $\begingroup$ Diving at 10m the pressure is 100% higher, not 50% higher. Also, when I was diving in the UK in the 70's the dive time was unlimited provided you didn't go below 10m. $\endgroup$ Apr 6 at 10:30
  • 1
    $\begingroup$ @MartinBonnersupportsMonica 2 bars is ~50% higher than a 1.4 bar atmosphere $\endgroup$
    – Questor
    Apr 6 at 16:32

Wouldn't a mountain counteract the effects of higher air pressure and more oxygen?

The highest human settlement is about 5 000 metres above sea level. That is more than enough to counteract the differences.

For reference at Mount Everest, the air pressure is around 1/3 of it at sea 🌊 level at 8 848 m

The humans could start settling the mountains. then they might venture down.

  • 1
    $\begingroup$ This sounds like a viable way of thinking about it. It's easy to forget that humans are an exception on Earth, having spread to almost all terrestrial habitats. The norm is very much for organisms to occupy a smaller niche and a small, technologically advanced population of humans could absolutely select the most habitable niche for them. Like older mountains with gentle slopes. $\endgroup$
    – biziclop
    Apr 6 at 10:56

The planet has 2 moons in an orbit range of around 1.6 AU on average:

1 around 80% the size of the Earth

1 28% the size of Earth.

Moons can not orbit habitable planets - that orbit in the habitable zones of their stars - at distances of 1.6 AU. Such a distance would be many times the Hill radius of the habitable planet. The gravity of the stars would pull the moon out of orbit around the planet.

You didn't specify the The mass of the planet or its distance from the star. But there is a relatively narrow range of mass for habitable planets compared to all planets. There is also a relatively narrow range of mass and luminosity for stars that can have habitable planets compared to all stars. Thus there is a relatively narrow range of orbital distances within the habitable zones of stars for habitable planets.

And no habitable planet can have a Hill radius of 1.6 AU.

The word "size" in relation to the moons is very vague. Do you mean radius, surface area, volume, or mass?

Going with radius, a moon with 0.80 the radius of Earth would have 0.512 the mass of Earth (if it had the same average density as Earth) and so it would potentially be habitable for humans if other factors were favorable for habitability. It would be large enough compared to any planet habitable for humans for the two worlds to be considered to be a double planet.

A world with a radius of 28 percent Earth radius would have a mass of 0.021952 Earth if it had the same average density as Earth. But it would probably be a lot less dense and have a lot less mass.

If the smaller moon had 0.28 the mass of Earth it would have about 0.65 the radius of Earth if it had the same average density. A world with 0.28 the mass of Earth would potentially be habitable for humans.

So one possible reason for humans to settle on the planet would be to live there for decades, centuries, or millennia while terraforming one or both of the moons to make them habitable for humans and/or native intelligent and/or for non intelligent lifeforms on the planet.

Added April 6, 2023.

The star is said to be an F class star with a mass of 1.2 the mass of the Sun. According to the table here:


An F7V star should have a mass 1.21 times the mass of he Sun while an F8V star would have a mass 1.18 that of the Sun.

With a luminosity of 2.45 the luminosity of the Sun, the Earth Equivalent distance (EED), where a planet would receive as much radiation as Earth gests from the Sun, of a F7V star would be about 1.565 Astronomical Units (AU).

With a luminosity of 1.95 the luminosity of the Sun, the EED, where a planet would receive as much radiation as Earth gests from the Sun, of a F8V star would be about 1.396 AU.

A global temperature is around 23 Celsius (78 Fahrenheit), with the poles having a typical average of about 10 Celsius (50 Fahrenheit), and the hottest regions (Mostly sun-scorched deserts) having an average of about 54 Celsius (129 Fahrenheit).

You forgot to convert the temperatures to absolute Kelvin temperatures.

0 degrees Celsius is 273.15 degrees Kelvin.

So the average global temperature would be about 296.15 K, the average polar temperature would be about 283.15 K, and the average tropical temperature would be about 327.15 degrees K. By contrast the average temperature on Earth is about 15 degrees Celsius, or 288.15 K, or 59 Fahrenheit.

So measured in absolute degrees Kelvin, your planet would be about 1.0277 times as hot as Earth. So it would have to orbit the star closer than the EED distance to receive more radiation from its star.

I don't know the formula for calculating how much more radiation the planet would have to receive to be 1.0227 times as hot as the Earth. I think I remember that temperature varies with the fourth power of the energy, so a world with 1.0277 times the temperature would have to recieve about 1.1154892 times as much energy as the Earth.

How close to a star can a planet get before it is too hot for liquid water using life? To find the inner edges of a star's circumstellar habitable zone, or Goldilocks zone, you find the ratio of the star's luminosity divided by the luminosity of the Sun. Then you find the square root of that ratio and multiply it by the inner edge of the Sun's circumstellar Habitable zone. Very simple - except for one problem.

According to the list at:


There have been over a dozen different estimates of the inner or outer Edges of the Sun's habitable zone, or both edges, in the last 60 years. Some are very different from others. So which estimate do you choose?

A star can only have habitable planets if it stays on the main sequence phase of its development long enough for a planet to become habitable.

Habitable for human beings and for other oxygen breathers means the planet has to have an oxygen rich atmosphere. Earth developed a breathable oxygen rich atmosphere about 600 million years ago, when it was about four billion years old. That oxygen rich atmosphere was produced by photosynthetic plants after non oxygen using lifeforms had existed on Earth for about two or three billion years.

So it should take about four billion years, give or take a billion or two, for a planet with non oxygen using life to produce a breathable oxygen rich atmosphere which humans and multicellular land animals can breathe.

So how long would a spectral class F7V or F8V star with 1.20 the mass of the Sun stay on the main sequence?

This online calculator gives a main sequence lifetime of about 6.33 billion Earth years (6,330,000,000 years).

You wrote:

An F-type star of 1.2 solar masses with about 2.3 billion years left as of current times.

So the age of the star should be about 6.33 billion minus about 2.3 billion or about 4 billion years old. So if the planet's development went at exactly the same rate as Earth's it should be beginning to have an oxygen rich atmosphere as you say. Maybe it developed a little bit faster than Earth due to various chance events to have such a lot of oxygen so early.


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