How do I calculate the maximum possible atmospheric density/ surface pressure a planet can have?

Question

I'm not entirely sure if this is the right stack but here goes.

I'm trying to design a fictional planet that has less gravity than Earth but I would like to give it an atmosphere as or even more dense than Earth.

But I'd like to know if it's possible for a planet with less gravity to have a denser atmosphere than Earth, that is also stable over a long time frame, or would I have to settle for lower atmospheric pressure.

What's the maximum stable air density I can give my world and how could I calculate it?

These are the details of the planet I've designed so far;

Surface Gravity: 0.67g

Escape Velocity: 8.11 km/s

Average temperature: 30°C/ 303K

Radius: 4993 km

Atmospheric Composition: 62% Nitrogen, 37% Oxygen, 1% argon, 1% Carbon dioxide

• PM 2Ring: Consider Venus en.wikipedia.org/wiki/Venus
• my2cts: This is an answer.
• BlackCat42: @PM2Ring I'm aware, but that's because the vast majority of it's atmosphere is carbon dioxide which is denser than normal air. I'd like the planet I'm designing to have a breathable atmosphere and habitable temperature range. Which means I can't just give it a Venus type atmosphere.
• Michael Seifert: This might be a better question for Earth Science; consider asking that question over there if you don't get an answer here. Worldbuilding might be a good place to ask as well.
• PM 2Ring: The scale height equation doesn't care about the total mass of the atmosphere. With a scale height of 8.5 km, ~98.17% of the atmosphere is within 34 km altitude, no matter what the total air mass is. If you doubled the Earth air mass (without changing the temperature profile), the rate of air loss would also be (approximately) doubled, so the relative rate loss would be (almost) unchanged.

Answer 1

This question is more suited for some planetary science and or astrochemistry discussion in a separate forum, but I will attempt to answer it anyways. Firstly, I believe it is necessary to state that it is nearly impossible to calculate the maximum possible atmospheric density as so many factors go into such a calculation that it is computationally intractable. However, because of that exact reason, there is a large number of factors you can use to design this hypothetical planet to give it the traits you want:

• Stellar Configuration: The planet could orbit a cooler star at a closer distance to maintain temperature along with a powerful greenhouse effect.
• Magnetic Field: Maybe the planet could have a large amount of moving, electrically conductive metals internally to produce a strong magnetic field, similar to Earth. This would help prevent gases being stripped away from it due to charged particles from the star.
• Volcanic Activity: More volcanoes erupting, more gases.
• Atmospheric Composition: While you wish for a breathable atmosphere, likely because you want to place hypothetical organisms on the planet, you can get creative with this part beyond just oxygen and carbon dioxide. If you can think of a hypothetical organism with a biochemistry designed to use other sources of energy, you can fulfill both of your wishes while increasing project creativity.

You can get extremely creative here with how you would synthesize a planet like this. Hopefully this answer give you some ideas of where you can start.