The planet I am making is a super Earth, a celestial body slightly larger than Earth, but not big enough to be considered a gas or ice giant.

The atmosphere is the most important so here it is:

  • The atmosphere I am taking inspiration from is from the Carboniferous and Cretaceous period.
  • A warm/hot, wet climate is intended, although deserts and glaciers would be acceptable

Is this possible with the following atmospheric composition?

  • Oxygen: 30%
  • Carbon Dioxide: 10%
  • Nitrogen: 60%
  • Argon: 0.9%
  • 2
    $\begingroup$ What makes you think that you couldn't have a hot wet climate with that particular mix of gasses? No matter where you are on Earth the mix of gasses is the same and depending on where you are it can be hot and wet; cold and wet; hot and dry; or cold and dry. Your atmospheric composition seems to be about what it was like on the Earth during the Cretaceous. Your planet is earthlike. Seems like it's trivially obvious what the climate would be like. I'm unsure what you are looking for from us? $\endgroup$
    – sphennings
    Commented Sep 10, 2022 at 2:43
  • $\begingroup$ Hang on, that's nearly 10% of the atmosphere unaccounted for. Apart from that the question is fully answerable, so I'm voting to leave open in faith that that detail can be added, and won't change anything major. $\endgroup$ Commented Sep 10, 2022 at 4:20
  • 1
    $\begingroup$ 7-10% carbon dioxide will suffocate humans, even 1% is bad news - not sure about the animals of the Carboniferous, since they aren't available to run tests on! So re-think that part of the atmosphere, since at the moment the percentages add up to 100.9% and include that lethal level of CO2. $\endgroup$ Commented Mar 30, 2023 at 5:20
  • 1
    $\begingroup$ Given the increased gravity It is probably also worth thinking about air pressure too, as with 30% oxygen and a 0.5 bar threshold of pulmonary oxygen toxicity, an air pressure beyond ~1.6 bar would mean this mix would be...bad. At least for creatures with a biology similar to ours. $\endgroup$
    – biziclop
    Commented Mar 30, 2023 at 10:42
  • 1
    $\begingroup$ Where does '… not big enough to be… a gas or ice giant' come from, please? If you mean 'any kind of giant' why not explain that? Else, why are 'gas or ice' giants singled out? $\endgroup$ Commented Apr 1, 2023 at 19:56

1 Answer 1


Clearly, the planet has lots of life on it, which includes life that produces oxygen as a waste product. On Earth, that's mostly done by plants, but a different planet might be differently arranged.

You need life to be doing this, and for there to be a lot of it, because free oxygen is too reactive to stay in an atmosphere over geological time unless it is refreshed. On Earth, the initial atmosphere had no free oxygen, but early life began to produce it. After it ran out of things it could easily react with, it began to accumulate and The Great Oxidation Event, which ended about two billion years ago, was when the atmosphere started to contain free oxygen.

The problem with your atmosphere is the 10% of carbon dioxide. That's a lot, and it would produce a huge greenhouse effect. That would likely run away, producing an atmosphere rather like that of Venus. Your translation of 2000ppm in Earth's Carboniferous atmosphere to 10% is wildly wrong: it should be 0.2%. That will produce a warm climate, but it will support life.

With that mistake corrected, your atmosphere seems perfectly plausible, for a planet at the correct distance from its star.

  • $\begingroup$ Wouldn't that level of carbon be fine if it were further away from it's star or had a colder star? $\endgroup$
    – Zautech
    Commented Mar 5 at 18:48
  • $\begingroup$ @Zautech: With that much CO2, you'd likely always be on the edge of a runaway greenhouse effect. A minor increase in the star's output could potentially fry the ecosystem, and over geological time, it would happen eventually. $\endgroup$ Commented Mar 5 at 18:54

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