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Calculations for a theoretical planet + a little biology

  1. Jun 13, 2018 #1
    So I'm attempting to write a scientifically accurate story and i need some help with the planets characteristics. I have a few things set up already and need some educated guesses and ideas for how the rest will work. So to start with what i have:
    Planet name: Remvale
    Radius:~8370 miles
    Gravity: 8.73 meters/sec
    Atmosphere pressure: 3.7 atmospheres
    Atmosphere composition: haven't decided except for around 34% oxygen
    Surface: 62% water 38% land

    So the questions i have are as follows:
    -Density of planet? It has roughly twice the radius of earth but a lower gravity so it will be much lighter, is the density a realistic mass?
    -Is the atmosphere pressure survivable for an entire human lifespan? If so what is the max pressure for normal human life and what adaptions would be needed?
    -Could humans adapt to live a normal lifespan in such an oxygen rich environment?(sorry this isn't astrophysics but i heard that increased oxygen levels would shorten lifespan)
    -What planetary events could cause a rapid increase in oxygen level? (roughly 20% in almost 1000 years?)
    -Would the increased atmosphere pressure cause any optical effects? (ex. venus atmosphere being impossible to see through)
    -What gas mixtures would be needed for life and temperatures similar to earth's to exist? (just basics like nitrogen oxygen and carbon dioxide)
    -What basic animal types would survive best on this planet? ( out of reptiles, birds, insects, mammals, amphibians, fish, ect.?)

    Sorry, i know this is a lot of questions from a lot of fields. I'm not expecting any one person to try to answer them all, feel free to do as many or as few as you want. Thanks!
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  3. Jun 13, 2018 #2


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  4. Jun 13, 2018 #3
    If I did the math correctly, the partial pressure of oxygen there would be around six times of on Earth. And yes, Oxygen is toxic.
    I would say it would be quite bad within hours. (Six bar partial pressure is not even on this graph below.)
    (Graph is from the linked Wiki page)
  5. Jun 13, 2018 #4


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    Those numbers require the average density of 2.3 g/cm3*. Which begs the question: what happened to all the metals during the formation of the planet?

    *the relationship is simple: starting with Earth - X times the radius means X times the gravity. Y times the density means Y times the gravity.
    Here you have (roughly) twice the radius and the same gravity, so the density must be 1/2.
  6. Jun 13, 2018 #5


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  7. Jun 14, 2018 #6


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    This video, and others on the channel, should help you come up with a realistic planet:

    At 3mins 38secs he goes over something that I think is quite relevant: how the relationship between radius and mass determines what kind of planet you get. If the planet is large and low mass it will have formed as a mini-gas planet. If it's too high mass you get a world covered in incredibly deep water/ice. There is a handy chart drawn from a book (which I think is referenced in one of the videos) that shows the range of conditions necessary for a rocky world.
  8. Jun 14, 2018 #7
    Not really, as other posters explained.
    Pressure probably is, but...
    No. As other posters explained. 1,2 bar partial pressure of oxygen is fatal in a few days.
    Cannot think of any that are this fast.
    Increased Rayleigh scattering and refraction. And the larger radius of planet would contribute to both.
    Other safe components are Ne and He
    All 6 are classes in Earth Cenozoic, and all save one are vertebrates.
    High air density would favour birds.
  9. Dec 29, 2018 #8
    How would a planet with lower gravity than ear have higher pressure atmosphere? They're related.

    A massive die-off of aerobic life could cause plant life to increase oxygen levels greatly.
  10. Dec 30, 2018 #9
    Easily. Venus does. So does Titan.
  11. Dec 30, 2018 #10
    Of course. By having atmospheres of heavy, poisonous gasses. But the OP is talking about a breathable atmosphere, not one full of methane, CO2, sulphuric acid.

    Which is why I asked how the OPs planet manages to have such high pressure. Is it full of argon?
  12. Dec 31, 2018 #11
    Not by this.
    Methane is not poisonous, nor heavy. Actually, methane is lighter than neon. And Titan´s atmosphere is just 4,9 % methane - the rest, 1,4 bar, is nitrogen.
    Venus is mostly heavy and poisonous CO2 - but 3,5 % nitrogen in the 93 bar of Venus´ atmosphere is still 3,2 bar.
  13. Dec 31, 2018 #12
    Thank you for the correction. But my basic point is that Venus and Titan are nowhere close to liveable places because of the conditions that make their atmosphere's dense. In the case of Titan the scant sunlight makes less atmosphere bleed into space while keeping temps low enough to keep much of the methane and other compounds in a liquid state so they can act as a recharging reservoir for that gaseous state. Raise Titan's temperature via increased sunlight 300°F and methane is no longer condensing into rain but breaking down in the ultraviolet. But Titan is actually only 40°F above the nitrogen condensing into an ocean.

    There may be a way to get a planet in a liveable temperature range with super high atmospheric pressure, but there would need to be a pretty hefty mechanism that either replenishes the atmosphere due to outgassing or have the planet in a state of rapid change were it is in the process of breaking down an enormous quantity of frozen atmospheric components as it warms through the liveable range.

    The main point I was trying to make to the OP is that atmospheres are a balance between content, heat and gravity. There would need to be a very good reason a lighter planet has managed to retain more atmosphere than is stable while not being too hot or cold to live in. Atmospheric pressure comes mainly from mass. What keeps that mass in place or replaces it?
  14. Dec 31, 2018 #13

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    Huh? Titan's atmosphere is mostly nitrogen, not methane.
  15. Dec 31, 2018 #14
    I didn't say it isn't.

    So why does Titan have a heavier atmosphere than Earth's? You seem to be implying that methane and other compounds that move easily between liquid and gas at those temps aren't part of the equation.
  16. Dec 31, 2018 #15

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    No, I seem to be implying that discussing the behavior of methane has little to do with an atmosphere that is nearly pure nitrogen.
  17. Dec 31, 2018 #16
    Does it have little to do with the atmosphere?
  18. Dec 31, 2018 #17

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    Tiran, what the heck are you getting at? You started prattling on about methane being liquid as being an important factor for an atmosphere that is just over 1% methane. I pointed this factr (composition) out. That's all, no more, no less.
  19. Dec 31, 2018 #18
    And the CO2 level on earth is only .041%. Why are you suggesting that methane isn't an important factor in why a small planet has an atmosphere that is larger than its gravity suggests it should?

    The articles I've read about Titan all make mention of how methane is a factor in Titan's mysteriously dense atmosphere, and seem to indicate the liquid/vapor cycle of methane and compounds with low vapor points are a factor. What is it you are arguing? If you are discounting the effect of everything by N2, what mechanism are you suggesting is at work?
  20. Jan 1, 2019 #19
    I´m suggesting that no mechanism is needed. Atmospheric escape, for atmospheres of reasonable size, is small compared to the size of atmosphere, and the effects of smaller or higher gravity are unable to overcome chance effects of initial composition and size. So, within reasonable size, atmospheric size and composition are free parametres not dictated by gravity.
  21. Jan 1, 2019 #20
    So you're saying earth could have 3 times its current atmosphere and it could also maintain liveable temperatures?
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