Let's say there is an exoplanet orbiting its host star well outside its habitable zone. Suppose further that it has a lot of water, being perhaps comparable to earth when it comes to the volume ratio of water compared to the remainder of the planet.

Now water at its densest is 4° Celsius warm. My question is: Could the exoplanet's weight force the water into a liquid state by squeezing it to the temperature of lowest density?

My intuition would say no, because after all, temperature is nothing but molecular movement, and every kind of accelerated movement is supposed to lower the total energy of a system, which seems to be absurd.

Also, this seems to be an explanation of the so-called "faint young sun paradox", which is well-known in geology.

If the answer differs from that, I would be pleased to read an explanation of how my reasoning was flawed.

  • $\begingroup$ This might also fit well on Physics SE. $\endgroup$ May 29, 2021 at 14:55
  • $\begingroup$ I think this can at best be interpreted here or at Physics SE or Chemistry SE. Personally I fail to understand what is being asked and the various lines of reasoning. This said, the answer below is certainly OK as for the answer to whatever is being asked must be the phase diagram of water. $\endgroup$
    – Alchimista
    May 30, 2021 at 11:20
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    $\begingroup$ @Alchimista: Perhaps you could clarify what you don't understand. $\endgroup$ May 30, 2021 at 12:57
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    $\begingroup$ The close vote is unnecessary, as the OP has provided enough details to answer the question. $\endgroup$ May 30, 2021 at 20:51
  • $\begingroup$ I did not vote for closing (I cannot and it wasn't my aim). Just asked for clarification, if possible. $\endgroup$
    – Alchimista
    May 31, 2021 at 9:13

1 Answer 1


Hardly. Maybe. Probably. It depends.

Liquid water at atmospheric pressure is densest at 4°C - but that is a function of pressure and temperature.

The phase of a material (solid, liquid, gas, critical) depends on both, pressure and temperature - and so does the density which depends on both, linearly within one phase, discontinuous at phase boundaries. The phase diagram of water which describes the phase you find for any combination of temperature and pressure is surprisingly complex for water and thus one of the least well-established ones.

Maybe one can answer your question already with a clear 'yes', if you consider a sub-surface ocean an acceptable answer. Enceladus' interior is somewhat heated by tidal interaction with Saturn. From Cassini observations we know that under about a 30km thick ice sheet on Saturn's moon Enceladus we have an ocean of liquid saline water - a source for the geyser activity on that moon.

  • $\begingroup$ Thanks for the clarification. Of course, sub-surface oceans are acceptable to me, but as you wrote yourself, the interior of Enceladus may be liquid because of tidal interaction and not pressure. Does pressure play even the slightest role here? I don't know. $\endgroup$ May 30, 2021 at 12:56
  • $\begingroup$ Also, the phase diagram seems to forbid liquid water at less than -30° Celsius even at very high pressures. So the exoplanet could only be a bit outside the habitable zone. The question now becomes: How much mass is necessary to create a pressure of 10^8 Pascal? $\endgroup$ May 30, 2021 at 12:59
  • $\begingroup$ Apparently, according to arxiv.org/ftp/arxiv/papers/1005/1005.2440.pdf there may be such a pressure at the bottom of a 10km deep ocean. Hence I consider the question answered. Thank you very much indeed! $\endgroup$ May 30, 2021 at 14:05
  • $\begingroup$ No, hang on, can't the temperature itself depend on the pressure? $\endgroup$ May 31, 2021 at 22:59
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    $\begingroup$ Note: They find this high-pressure liquid by heating isobaric, thus at constant pressure they heat their high-pressure ice. IMHO this confirms that there's still a lot of discoveries to be made with water and its phase diagramme and its properties. $\endgroup$ Jun 3, 2021 at 12:26

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