How large could a planet made entirely of water be...

…and remain liquid all the way down to the centre?

In other words how large could a spheroid composed entirely of liquid water get before gravitational forces at the core begin to compress the water into a solid state.

Does it matter if its freshwater or seawater? How about if its rotating at the same rate as Earth or not rotating at all?

I think we’ll call this planet Bob…I mean Water.

*I initially typed ‘worlds’, calling Dr Freud…

Well, it depends on the temperature of the planet. If it’s far from its sun and the temperature is low, it will be solid ice even on the surface at zero pressure. Once you pick a temperature, the maximum pressure that water will remain liquid can be determined from water’s phase diagram.

Yes, good point, I was thinking at roughly Earth temperature.

Small water planets would have less dense atmospheres and lower gravity resulting in atmospheric escape, which would also be problematic without an iron core and magnosphere to protect against the solar wind.

As Ice VII starts to form above 3 GPa (435,113 psi), and you would need about 10 GPa (1,450,377 psi) and a radius around 8500 km to avoid evaporation for a few billion years you best be looking for moons with ice surfaces but even Europa which just has a radius of 1500 km most likely has a Ice VII seafloor.

If you are willing to move away from full water, to other liquids or allowing for a rocky core your size ranges will be much greater. Most ocean worlds probably have a rocky core and most pure “water worlds” are probably ice like.

(note this is all really quick simple estimations based on overly simplified math, a.k.a back of the envelope quality)

I finally found a pop-sci post for this paper that wasn’t as hyperbolic as most, but note that Ice VII is in the edge of our current understanding.

The implications of a water ball only several 100 km across spontaneously freezing at 1000 mph is quite interesting though.

I wonder how large a ball of water you’d need before people would start calling it a planet.

According to the IAU definition, it has to be large enough for self-gravity to achieve hydrostatic (heh heh) equilibrium (that is, it’s a sphere) and it has to have cleared other objects out of its orbit. The first part is trivial for a ball of water; it’s always spherical no matter how big it is. So as long as there are no other objects near its orbit, any ball of water would be a planet. That’s probably not what the IAU people had intended, but balls of liquid water are probably exceedingly rare or nonexistent in the real universe.

Post 2006 IAU definition it is probably not possible for a planet to be entirely water due to the requirement that it has cleared other objects out of its orbit and how we think solar systems form. However, it is probable for a planet to be entirely covered by liquid over a solid core which is the typical meaning of ocean planets. This really relates to what else exists when you have Oxygen and nucleosynthesis.

Note that if the Earth was the size of a basketball, all the water on the surface wouldn’t even fill half a ping pong ball. Yet anyone who has been to sea will have a very different perception of the amount of water on Earth.

Here is a great NASA visualization of water on Earth vs Europa.

The answer is such a planet couldn’t exist. Even ignoring the IAU definition of “planet” (which is the best thing to do with that definition).

If it’s over a certain size, the center is going to turn to some form of ice (ice VII as rat avatar points out). If it’s not that large but still at Earth-type temperatures, it’s going to evaporate away into space. It won’t have enough gravity to hold on to the water vapor atmosphere. The only way a small body of mostly water can keep itself together is to freeze its surface.

I think you’d have to call any inhabitant “Bob”.