Could a heavy exoplanet squeeze water into being liquid and 4° warm? 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 &quot;faint young sun paradox&quot;, 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.
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, linearily within one phase, discontinuous at phase boundaries. The phase diagramme 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 geysir activity on that moon.