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Consider a planet with Earth like gravity but an atmosphere that is 10x what Earth has at sea level (atmospheric composition is equivalent to Earth). If the vegetation is similar to Earth's, would the plants be bigger due to having access to more CO2?

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Depends on the solar constant at the top at the atmosphere. A thicker one will be absorbing a lot more light on the way down, so the available light at sea-level will be less.

On the other side, a larger total atmospheric absorption of energy will:

  1. heat that atmosphere more
  2. impede on radiating the excess of heat at night time

A very likely result will be an atmospheric thermal runaway, in which the atmosphere heats up until the amount of IR radiation is high enough to counterbalance the trapped heat. Very similar to a Venus scenario.

There may be a way to have your planet as you describe, but this involves the planet's star to emit in UV/hard UV maybe with a bit of X-ray spectrum. In such a scenario, the light that reaches the sea-level is not the one emitted by the star, but is produced by fluorescence in the upper side of the atmosphere.

Accepting such a situation, whatever energy reaches the sea-level has a lot less infra-red and more in the visible spectrum (for which the atmosphere is transparent) and the fluorescence will disperse the incoming radiation so that heating is less direct and less intense. In such a case the planet will actually need a thicker atmosphere (and a matching magnetosphere to keep it in place long term). An A-type star (young, steadily burning hydrogen without flares) may do that for you.


Here's the blackbody radiation (energy density) spectrum for Sun's light (5700k)

blackbody Sun

And here's the energy density spectrum for a star twice as hot (11400k)

blackbody - Atype

Note how:

  1. the maximum energy density moves into ultraviolet range
  2. a hotter star will radiate a lot more energy - just look at the energy levels on the vertical axis

Placing your planet further away from a hotter start is self-evident. Feeling of guts, about 2 times further.
With a star and planet mass similar to Sun/Earth, it will mean an orbital period around 2.8 Earth-years.

I might be completely off in my estimations, but here are the reasons for the "feeling of guts - 2AU or thereabouts should do":

  • the radiation flux goes down with the square of the distance, yes. At 50 x more power integrated on the entire spectrum (feeling of guts), then at 2AU the total radiance is around 7 times stronger than having the Sun at 1AU.

  • The pressure is 10atm, the height of the atmosphere is 10x the Earth's (note how pressure gets into the equation as a linear factor);

  • Rayleigh scattering - increases with the power of 4 of frequency (and we have more energy at UV) - acting on a 10x higher atm should scatter back close to 50% of the incoming radiation (remember how you see orange-red sun-set on Earth and very little blue due to the blue sunlight being scattered more?). So we get to 3.5 more energy to account for

  • flourescence - happens on the upper part of the atmosphere. Has more channels:

    • ionization, which results in charges directed to move along the magnetic lines of the magnetosphere (stronger) thus part of the energy is directed to poles (and keep them warmer - the aurorae should be more visible and descend to to lower latitudes - you wouldn't see the starts at night)

    • excitation falling back towards the base level with IR radiation. That will heat the atmosphere, but this heating takes place in the upper side of it so getting rid of the extra heat at night should be easier

    • the rest is just shifting UV into Vis

One on top of the other, I'm going to write of another 0.5 times from the incoming energy and have the rest of 3x to justify/use.

  • then, I have 10x higher atmosphere to 'pierce' with enough light for the plants to grow; and a tad more than 10x the mass of gases to keep warm. I think it may be safe to assume that somewhere in the [1x ... 3x] range of more energy there is a point where there can be an equilibrium that doesn't imply a thermal runaway. That may require some tweaking of the amount of greenhouse gasses and their altitude (e.g I think I can handwave a thin layer of methane higher - CH4 is lighter than air, even if much heavier than H2), but the handwaving is not implausible.
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  • $\begingroup$ Given that the energy emission of the brighter star is more than 30 times as bright, and that the thicker atmosphere retains morte heat, I would assume roughly 6-8 times as distant from the star. $\endgroup$ – Klaus Æ. Mogensen Mar 6 at 9:09
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    $\begingroup$ @KlausÆ.Mogensen the radiation flux goes down with the square of the distance, yes. At 50 x more power integrated on the entire spectrum (feeling of guts), then at 2AU the total radiance is around 7 times stronger than having the Sun at 1AU. The pressure is 10atm, the height of the atmosphere is 10x the Earth's. Rayleigh scattering (increases with the power of 4 of frequency) acting on photons with shorter wavelen on a 10x higher atm should scatter back close to 50%, then flourescence should keep the heat generation upper so easier to get rid at night. But… gut feels, don't know for sure. $\endgroup$ – Adrian Colomitchi Mar 6 at 11:52
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Nope.

Gravity still have a role to growth of everything, in different gravitational conditions plant bodies will have different sizes. A smaller gravitational acceleration could improve height of everything.

Plants need access to sunlight to photosyntesis. A denser atmosphere can filter more, then there will be less light to plants.

Also the pressure will act to minimize any potential to plants grow.

However, biodiversity will be boosted due to more resources.

Still about pressure of atmosphere and Adrian's example in comment about Venus: in the surface level the venusian dioxide carbon is under a high pressure its almost doesn't behave like a gas and, instead, looks like more a supercritical fluid. Its 94 atm, not 10 atm like in your planet, but, still need to acount for different consequences:

surface winds will be slow

Will this huge atmosphere be really stable? I would not care of atmospheric escape to space or how high is the magnetosphere, but how much of gases will react with water and ground and be absorbed? I can imagine oxygen reacting in a deeper layers in all the ground, oxidating everything. On another side the diazotrophs will have tons of nitrogen to fixate, all those reactions will make a superfertile soil, favorable to biodiversity. Then, would the high pressure atmosphere allow gases come back from hydrosphere and litosphere in the air and complete their cycles? Looks like this atmosphere will soon or later be partially eaten by biosphere, sea and soil, and be a bit thiner.

Adrian suggestion to planet be further than Earth is to the Sun will make the atmosphere absorb less heat and make a climate the closest to Earth's. A planet located further would also have more chance of keep light elements like volatives. Although, total insolation will be lesser and plants will evolve to use other ways than photosyntesis.

Perhaps another big kingdom of biological beings would thrive in those conditions better than plants. Forget the trees, fungi would rule, like ruled in the old Earth.

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  • $\begingroup$ Ummm.. about that Gravity still a role to growth of everything. - seems superfluous, adds nothing to the argument. I thought the question made it clear that "a planet with Earth like gravity but an atmosphere that is 10x what Earth has at sea level". And you don't need a 10x gravity to have a 10x atmospheric pressure, Venus has about the same gravity as Earth's but the mass of it's atmosphere is 93 times that of Earth's. $\endgroup$ – Adrian Colomitchi Mar 6 at 0:50
  • $\begingroup$ I meant because gravity in same acceleration than in Earth the growth of plants will remain same, unless other factors surpass it. Not about pressure or mass of atmosphere. Btw, had other answer and I cant coment since I m new in this network, I saw you and other telling about pressure, heat and mass. I went just add in that discussion both was right and pressure will rise if up temperature and/or mass. $\endgroup$ – Rodolfo Penteado Mar 6 at 1:13
  • $\begingroup$ I might be wrong, but you still can edit your this very answer, can you not? I mean, you have the opportunity to be more precise on what you mean and improve the quality of your answer. $\endgroup$ – Adrian Colomitchi Mar 6 at 1:18
  • $\begingroup$ Ah, man, take a bit easy, I m still new with it and my english is also rusty. Still learning tools and reading old posts of persons with previously same questions I have now. To reply an answer of other I need make 50 rep points. $\endgroup$ – Rodolfo Penteado Mar 6 at 1:22
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    $\begingroup$ Hey, man, don't take it the wrong way, I'm not bashing you or something! I just noted a point in your argument that was unclear and which I believe, if you have time/disposition, it may worth explaining or expanding on it. Nothing wrong if you cannot or don't have time for it (I just can't remember if your rep level allows you to edit your own answer). My best wishes to you in adjusting to WB, it's a nice place to spend some time. $\endgroup$ – Adrian Colomitchi Mar 6 at 1:29
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Not appreciably, no.

CO2 is already rarely a limiting nutrient. Growing plants in high CO2 conditions does sometimes result in faster growth, which itself results in slight increases in size, but only of the "shift the peak of the curve over" variety, not "increase the population maximum" variety. And having access to plenty of CO2 quickly becomes irrelevant if:

  1. There's not enough bioavailable nitrogen.
  2. There's not enough water.
  3. There's not enough phosphorus/magnesium/calcium/iron or any of a wide range of other micronutrients that plants need to grow.
  4. Their structure is limited by gravity. High CO2 levels won't, e.g., let a redwood grow any taller--or a sunflower, whose stalk is significantly weaker than a tree trunk.

Consider a planet with Earth like gravity but an atmosphere that is 10x what Earth has at sea level (atmospheric composition is equivalent to Earth). If the vegetation is similar to Earth's, would the plants be bigger due to having access to more CO2?

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