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Planet Info:

  • Star: 1.25x solar mass / F6V Type / ~6300K / 1.97x Luminosity
  • Planet is 1.84x Earth Mass, same density as Earth
  • Orbiting at 1.42 AU
  • atmospheric pressure is 3.68atm

Inputting these values into a Planet Temperature calculator, assuming an identical atmosphere as Earth, gives a temperature of around 12 C. Quite Earth-like.

However, I don't want the planet to be earth-like, and would like the temperature to be around 50 C. Using This Site, this planet would need a Greenhouse Effect about 3 Times stronger than Earth in order to maintain ~ 50C.

My question is: What atmosphere composition would it need to have 3x Greenhouse Effect and temperature around 50C?

For the answer, please note the following:

  • Keep the Planet Info stuff the same
  • Atmosphere should contain similar gasses as Earth (Nitrogen, Oxygen, Argon, CO2, etc), can tweak the percentages as needed
  • Can also add some percent of some other gas (like methane or something else) if it helps achieve the desired heating effect
  • Does NOT HAVE TO be breathable by humans or Earth-life, but should allow for native complex, multicellular, carbon-based life
  • The lack of ice should apparently affect the albedo somehow, but calculating albedo is ridiculously complicated, so just assume the albedo is somewhere similar to Earth
  • Lastly, I want the Oxygen to be around 18-20%, not sure if this would affect the Greenhouse, but try to keep it around this level.

If you need any more information to give an answer, I can (probably) provide it

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    $\begingroup$ Use a spreadsheet i.e Google Sheets then apply the formula given in the website you mentioned, or if you're technical savvy add ChatCPT API to automate, there are YouTube tutorials on that... Caution: ChatGPT likes to hallucinate a lot😱 $\endgroup$
    – user6760
    Jan 5 at 6:37
  • $\begingroup$ BTW for anyone who might be reading this in the future, I decided to add 3% Laughing Gas (N2O) to the atmosphere because its Greenhouse Potency is 273x greater than CO2. It's not exactly stable over geological time, but its extremely easily producable by lifeforms. I could imagine some bacteria over-producing the gas for whatever reason, similar to earth's Great Oxygenation Event. $\endgroup$
    – Foosic17
    Jan 7 at 2:13
  • $\begingroup$ ^^^ this would bring the temperature of the planet up to around 30C. 50C would require 5% Laughing Gas, but i feel comfortable keeping the temperature around this amount. Plus the fact that its literally laughing gas could lead to some interesting worldbuilding ramifications $\endgroup$
    – Foosic17
    Jan 7 at 2:15

2 Answers 2

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It takes a lot of calculations to determine the gases necessary to achieve a particular greenhouse effect, and there are choices involved that a WB SE answer cannot make for you, such as the particular greenhouse gases involved, since different greenhouse gases have a greater or lesser effect than others.

On the principle of teaching someone how to fish rather than giving them a fish, have a look at: https://terraforming.fandom.com/wiki/Greenhouse_Calculator

In case of link rot, the calculation for an earth-like world from the link above is:

Teff = Tv * 0.7062 + Tv * 0.7062 * 21 * dNO * 0.099 + TV * 0.7062 * dHe * 0.218 + (TV * 0.7062/262) * mCO2 + (TV * 0.7062/262) * mGH*C

where:

  • Teff is the planet temperature;
  • dNO is the amount of nitrogen and oxygen (kg per square cm, for Earth it is 1);
  • dHe is the amount of helium (kg per square cm, if the atmosphere has helium as an inert gas);
  • mCO2 is the mass of carbon dioxide (kg per square meter);
  • mGH is the mass of added greenhouse gas (kg per square meter);
  • C is the greenhouse effect (how many times this gas is more potent then carbon dioxide)
  • Tv is the void temperature:

Tv = LOG10(1+(Ks * 1.98) * (1e+100))/((4.6e+22) * 0.58674^(LOG10(1+(Ks * 1.98) * (1e+100))))

where Ks is the Solar Constant:

Ks =(2.512^(-Mag)) * 2.011291223E-11

Where Mag is the star's absolute magnitude.

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    $\begingroup$ Worth voting up just for the link. $\endgroup$ Jan 5 at 6:35
  • $\begingroup$ Make it 2 for not formatting the link properly like me😅 $\endgroup$
    – user6760
    Jan 5 at 6:40
  • $\begingroup$ Thanks for this link, super helpful! Quick question, is the formula meant to be saying "kg/ cubic meter" rather than square? If its measuring mass, I would assume its cubic. Also, in the dNO section, is the value of dNO supposed to be the amount relative to Earth? As in: 50% more N & O would be written as dNO = 1.5? the example states that Earth = 1, so it seems that it would be relative $\endgroup$
    – Foosic17
    Jan 5 at 13:30
  • $\begingroup$ @Foosic17 I would have expected that it would be cubic metres myself. $\endgroup$
    – Monty Wild
    Jan 5 at 13:41
  • $\begingroup$ @MontyWild i realize that dNO is probably referring to atm-pressure. On Earth the atmosphere is indeed 1 kg/cm^2 as the formula states. So I would just enter 3.68, since the atm-pressure is 3.68x more? But then idk what dHe would be, a world like this shouldn't really be able to hold onto that much Helium (Earth = 0.0005% He), do you think it'd be safe just rounding down to 0? $\endgroup$
    – Foosic17
    Jan 5 at 13:49
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This is trickier than it sounds. It isn't a "simple" matter of picking a CO2 level somewhere in crazyland like 3 or 4 kPa of CO2 or something because CO2 is removed by the carbon-silicate cycle unless your planet is either geologically dead (unlikely given it's size and star, the star will die first) or a water world with less than 1% land.

The mass and density of your be planet implies a surface gravity about 22.5% higher than that of Earth, which consequently also means that your atmosphere is squeezed down that much. What this means is you really only have 3.00 times as much atmosphere per square meter as Earth even if it's at a higher pressure than that. Greenhouse effect is based on the atmosphere per square meter while rock weathering is based on atmospheric pressure and temperature, and volcanism releases CO2 insensitive to either. What I'm trying to say though is that you're somewhat squeezing your be planet's atmosphere against CO2-eating rocks, so don't expect there to be much CO2 if the planet is that hot. I doubt this effect is going to make more than a few degrees of difference though, especially given greater volcanism of a larger planet could plausibly come to the rescue. Water runs into a similar issue though. It cannot amplify the effect of CO2 as much when there is just less atmosphere scale height to humidify.

What you almost need is a whole bunch of rarer greenhouse gases or something, but I can't think of a naturalistic mechanism that would emit a bunch of exotic greenhouse gases without resorting to intelligent life. Is there anything there naturally producing a diverse portfolio of exotic CFC-lile refrigerants with their huge global warming amplification potential?

You could add some CH4, NO2, NO, etc, the problem is that most of these are pretty whatever greenhouse gases, and having enough methane to turn the place into that kind of extreme hothouse in an atmosphere that also contains oxygen is unlikely because the two react.

I also heavily recommend AGAINST an 18-20% oxygen atmosphere. 18-20 kPa is fine, but an 18-20% oxygen atmosphere at 368 kPa total pressure is gonna leave you with 70+ kPa of oxygen, which pretty much means any life there is going to be extremely flammable.

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  • $\begingroup$ If worse comes to worse I might need to scale down the temperature slightly (still want it to be a hothouse tho) About the oxygen levels, I think life could likely evolve protection against the severe flammability, assuming you're talking about forest-fires and such. Animals could possibly have exoskeletons that (in theory) don't light on fire as easily as skin, hair, feathers. Unless you mean the animals themselves will sponaneously combust (would they?) $\endgroup$
    – Foosic17
    Jan 5 at 14:11
  • $\begingroup$ Also they could hide underground or in water, I was thinking the planet was around 85-90% ocean, so most places would be pretty close to water anyways. You stated that a planet thats 99% ocean would be able to hold onto a high amount of CO2, I don't think 90% would be enough to do that, so my search for greenhouse effect is still going $\endgroup$
    – Foosic17
    Jan 5 at 14:14

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