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Can water planets exist whose atmospheric pressure at sea level is just above the triple point of water, say at about 690 Pa (0.1 psi)? If no, why not? Just curious. How thick or thin can atmospheres of ocean planets be?

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    $\begingroup$ If you look at the phase diagram of water, then just above the triple point there is just a tiny temperature window in which water is in a liquid state. So I'd say it's possible in theory, but you'd need a vary stable temperature: you could switch to either ice or vapour with a few degrees change. upload.wikimedia.org/wikipedia/commons/3/33/… $\endgroup$ – Jean-Marie Prival Nov 19 '20 at 16:39
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    $\begingroup$ @Jean-MariePrival Some of that is covered in answer(s) to Mars' atmosphere and the triple point of water in Space Exploration SE, perhaps you can add an answer here? $\endgroup$ – uhoh Nov 21 '20 at 3:34
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There's a couple reasons why it would be difficult, but not necessarily impossible (and I'll toss in a hypothetical at the end). I don't know how to answer this questions with a lot of "in theoreys" and "this is unlikelys", but I'll give it a shot. Maybe when the James Webb Space Telescope gets up and running we'll get a better look at planet's atmospheres and maybe even identify a planet somewhat like your scenario. For now, exoplanet atmospheres are incredibly hard to measure, so there's mostly just models and educated guesses to work with.

One reason it would be tricky is because the vapor pressure alone approaches the triple point air pressure, so this means that if the planet got too warm too often, say, mid-day noon or during summer, assuming it has seasons, then the vapor pressure, just from the ocean would probably rise above the triple point and that's before you add any additional gases. CO2, CH4, N2, O2 if you want to get creative and get rid of a lot of the CO2 by photosynthesis and lets not forget Argon which forms from decaying potassium 40 in the planet's crust. Earth's atmospheric Argon alone is about twice the triple point pressure.

So you'd need a planet with oceans but virtually no other gases and that might be hard to do. Planets can lose atmosphere to space by solar wind or jeans escape, but water is a light molecule and arguably gets removed from a planet more easily than gasses like CO2, N2, O2 or Argon. That's not to say it's impossible that a planet might be sufficiently water rich that it loses atmosphere but retains water. I just think it would be a rare occurrence.

A second potential problem is temperature variation, which means, if the planet is too warm, too much ocean becomes atmosphere and adds too much vapor pressure and if the planet is too cold, the oceans would be largely covered by ice. Planets are dynamic and a thin atmosphere planet should have a wide temperature range, just as Mars does. Latitude, day/night cycle or day side/night side if tidally locked and eccentricity could all play roles in significant temperature variation, and lets not forget, the planet's internal heat and potential volcanism. There would be a narrow range where the planet could maintain both oceans and a very thin atmosphere.

A final consideration is what you mean by triple point. Oceans are likely briny and astronomers as a rule, are more interested in the "triple point" of briny water not pure water, though a chemist would say that triple point applies only to pure substances not salt water or possibly ammonia water.

If you consider the triple point of salt water oceans, then it goes below freezing where ice sheets become possible, and with an expected natural variation in the planet's temperature, ice sheets become very likely. That doesn't make your scenario impossible, but I think you'd need a cold planet mostly covered by ice with narrow equatorial regions of exposed ocean - just a thought.

The triple point of briny water is discussed here - a fun read.

And a chart with some thing like the triple point of salt water in this research gate question here.

If, in theory, Mars was given oceans, by some terraforming efforts, as a result, initially the water would evaporate but over time, it's combination of water vapor and CO2 would push it's atmospheric pressure further above the triple point, and as it's south pole releases more CO2 as it presumably gets a little warmer, Mars would move away from this scenario if it had enough liquid water. It has too much CO2, though for now, the majority of it's CO2 is frozen.

And Ammonia/Water ocean is especially interesting as the freezing point of the correct ratio can dip below the freezing point of CO2, this drops the triple point significantly, below -80 C° but it may solve the CO2 problem. Frozen CO2 at the poles with Ammonia/Water oceans and lots of water ice. Most of the oceans would have a permanent water-ice cover.

Mars, for example, undergoes a variation in atmospheric pressure as it goes through it's seasons by as much as 25%. (Scroll down to the Mars section). That's mostly eccentricity driven and happens because Mars is further from the sun during it's Southern Hemisphere winter and this allows some atmospheric CO2 to freeze, which thaws again as the planet moves back towards it's perihelion and Southern Hemisphere summer.

Certainly a planet with oceans and triple-point like conditions could be designed if you could create your own planet, but in actuality, the likely mix of compounds in the atmosphere and temperature variation and general dynamics of a planet would, I think, make such planets rare, if they exist at all. That said, observations have shown astronomers things that they didn't expect to see many times before, so there's a chance.

Hypothetical: a Mars like planet has ice at it's poles, but warms up enough to lose it's atmosphere. Ice is stable even in a vacuum, provided it stays below freezing temperature.

After losing it's atmosphere, the planet warms up and it develops a magnetic field. The ice melts and now it has oceans and a very thin atmosphere. Perhaps not impossible. It requires a few things to go just right, so, still unlikely, I think but who knows.

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