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Warning: I am wordy and I overexplain things. I'm also Autistic, so I may come off as odd or unusual (and that also feeds into why I overexplain LOL). I'll do my best to be clear and concise, but you are probably in for a wall of text (sorry)!

I'm hoping to engage in a big (but casual) speculative biology project. I want things to be very much grounded in science, though I will add a fantasy spin to it so it doesn't have to be strictly HARD science. I just want it to feel plausible and like something that could happen based on humans' current knowledge.

I'm working on the planet itself, and I've come up with a few configurations of its orbit around its sun. Still, the one I'm most fond of is also the least plausible - using Universe Sandbox (2) and Artifexian's Worldsmith, this planet would be outside of the habitable zone by way of being too close to the star. I THINK a dense atmosphere and strong magnetosphere could counteract that issue by protecting it, but I am unsure - both programs say that an atmosphere that dense couldn't support life. Obviously, an Earth human would struggle to exist on a planet with such a dense atmosphere, but there are extremophile bacteria and deep sea fish that deal with a lot more pressure than higher up in the water, let alone at the surface.

If a planet with a dense atmosphere and strong magnetosphere to offset harsh solar rays of its star developed closer than the normal, numbers-based habitable zone, would that allow that planet to be still habitable? What sort of makeup of gases would that highly-dense atmosphere have to be able to support carbon-based, water-dependent, oxygen-using (breathing, and plants producing) life, based on what we currently know? Are there other atypical setups that could do the same?

(For my particular planet - it is 0.7 Earth masses with an atmospheric pressure of 8 atm at the surface. I do not know the exact makeup of the atmosphere, other than some vague ideas of having a lot of nitrogen and needing some oxygen and carbon dioxide. It has an eccentricity of 0.05 with an SMA of 1.3 - so a pericenter of 1.24 and an apocenter of 1.37 - around a sun that is 1.4 solar masses. Numbers are approximate and can be varied if needed.)

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    $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Jan 12 at 4:23
  • $\begingroup$ I believe the OP is asking about the best atmospheric composition for a planet that would make life the easiest to build. It’s possible I’m misreading. $\endgroup$ Jan 12 at 16:42
  • $\begingroup$ I'm asking more generally about making planets life-supporting when they may not typically support life. For specifics, controlgroup has the general concept: I'm asking for what the makeup of a dense atmosphere (8x denser than Earth's) might still allow life to prosper. I'm not sure how else to clarify but there's clearly some confusion. $\endgroup$
    – Kunabee
    Jan 13 at 1:30

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This is an interesting concept! The planet in particular reminds me of KSP’s “Eve”, which was a bit smaller than the home planet but closer to the sun with a dense atmosphere. Kerbals can survive on Eve; I don’t see why other life couldn’t survive on your planet.

I concede that evolution would be difficult to start given the atmospheric pressure. Life as we humans know it has issues surviving pressures like that, so I think the safest bet would be having an oxygenated atmosphere so that oxygen can dissolve into the water for aquatic creatures to breathe.

Then again, too much oxygen is problematic: a lot of chemicals that are important for life (and even some that aren’t, like diamond) ignite at room temperature when exposed to pure oxygen, and given the proximity to the star you’re talking about, we might be a bit above room temperature here. So, I suggest a dense atmosphere with some oxygen (around 2.6%) and otherwise filled with closer-to-inert gases like argon and nitrogen. Pick the noble gases carefully and you can make your sky pretty colors, too!

The choice of the percentage is found by taking Earth’s ~20% oxygen and accounting for the vastly-increased atmospheric pressure on your planet. As for the inert gases, anything would work - although as far as cosmology goes, lighter elements like argon and nitrogen are more common.

With this setup, oxygenation of the water bodies on your planet will happen more often, which might allow aquatic life to emerge. The water would also act as a coolant layer so that the heat from the star and the radiation that gets past the strong magnetosphere don’t hurt life that much.

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