Boiling point of water increases with pressure. If so, can a planet with the right atmospheric pressure contain oceans (made of water) irrespective of its temperature?
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2 Answers
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A very important point to make is that water has a critical point around $220$ atm and $370 $ Celsius, so at pressures and temperatures higher than that, gas and liquid water can't exist, only a supercritical fluid. We do not know much about the air pressure of exoplanets, but seeing that Venus has about $100 $ atm atmospheric pressure, we can probably make a guess that an order of magnitude larger might be possible.
This all means that liquid water, in the forms of oceans, can't exist for super hot planets - but supercritical water is a real question.
Whether supercritical water can exist on a planet or not mainly depends on the mass. The reason there's no He in the atmosphere of the Earth is that the gravitation field is not strong enough to hold the He atoms down, so they just fly off to space. We do have, however, enough mass to keep water molecules. If your hypothetical planet has a large enough mass to keep water vapor trapped, then you will always have the possibility of liquid microcondensing out of the air. If you want to be more technical, here's a somewhat recent paper exploring the possibility of having water on hot planets as a function of their mass and rotation (which I neglected for simplicity)
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1$\begingroup$ But the temperature must be below temperature of water critical point (374 °C, 218 atm). Otherwise, there would be no ocean with liquid/gas boundary but supercritical fluid with progressively and continuously changing properties. $\endgroup$– PoutnikCommented Jul 4, 2022 at 14:14
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$\begingroup$ Yes, I skipped over the discussion regarding the maximum temperature and jumped straight to atmospheric water - added this part. $\endgroup$– SzgogerCommented Jul 4, 2022 at 14:34
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$\begingroup$ "If your hypothetical planet has a large enough mass to keep water vapor trapped, then you will always have the possibility of liquid microcondensing out of the air." Certainly not if there's no place in the atmosphere with a temperature below the critical temperature. And even if there is such a place, the pressure must be high enough at that place to allow water to condense. $\endgroup$ Commented Jul 4, 2022 at 16:20
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Pure water's critical temperature is 647K. Dissolved salts can raise that a bit. Above the critical temperature, regardless of pressure, the gas and liquid phases are indistinguishable, so you can't have an ocean.
What if you dissolve a lot of salt? You'll have to decide whether you consider the resulting liquid to be water or molten salt.
