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In my story, people are trying to colonize an Ocean planet that orbit Ultra-cool red dwarf. (Based on this Wikipedia article a planet can retain Water in this type of stars with a mass around 0.08-0.11⊙ and avoid problems to a certain extent).

The planet has a mass of 4 and a radius of 1.7 (Where 1 = Earth) with an average temperature of 11°C, and an atmospheric pressure of 1 atm.

Can this water-planet have a safe atmosphere that won't damage the body and let you walk on it (Floating base, boats, etc) with only an oxygen mask naturally while maintaining an H2O Ocean?

I heard that CO2 will be sucked into the Ocean and change through a carbon cycle, making the atmosphere dominated by other gases such as nitrogen, is that true?

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Having an Atmosphere at all is Unlikely

The Wikipedia article cites this paper. From what I understand this is the problem: Suppose a planet is in the habitable zone of a Dwarf star. Then in the past the star was not a dwarf. It was much hotter and so the planet was in the hot zone. During that time radiation from the star boils off the water from the planet. The mechanism for this is called "H20 Photolysis by FUV (far ultraviolet) photons "

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Don't worry if you don't know what "H20 Photolysis by FUV (far ultraviolet) photons" means. I don't know myself, beyond high energy photons hitting the water vapour in the atmosphere and knocking it off the planet. But it sounds bad enough to knock off any gasses in the atmosphere too. Judging by how Earth's oceans are MUCH heavier than the atmosphere, I'd wager the atmosphere gets knocked off long before the oceans.

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    $\begingroup$ Thanks for the accepted answer. Though it is good practice to wait a few days before accepting an answer, since THE GREEN TICK tends to scare off other, potentially better, answers from appearing. $\endgroup$ – Daron Feb 1 at 21:54
  • $\begingroup$ Thanks for the advice. $\endgroup$ – Khalid Feb 2 at 5:35
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Planets can migrate in position, and changing CO2 content of the atmosphere might mean that its greenhouse effect increases as the star cools.

That said, you asked for other gases ... Xenon would be fun. If one of your characters gets a stroke, just have somebody take off his mask; it may do wonders. I'll admit, I have no idea how you get that much xenon, but if a large planet lost most of its gas who knows if it could work out to spare the heaviest atoms? Barring that, Earth's air has argon in moderate abundance.

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    $\begingroup$ If strokes are due to oxygen not reaching the brain, then breathing pure xenon will only make the stroke worse. $\endgroup$ – Daron Feb 2 at 15:53

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