Yukmay

insane_alien,

You're right. After a little more prying I was able to find an academic site with a high pressure phase diagram for water. I guess this does mean that a planet made entirely from water would have a solid core.

insane_alien said:

yes, it would definitely have a high temperature from energy left over from its formation. but the extraordinarily high pressures would be sufficient to keep it solid. it would likely be a couple thousand kelvin, perhaps in the tens of thousands, depending on how old the planet is.

Since we're talking about water ice here, wouldn't it be past the critical point at that temperature? In other words, no matter how much pressure is exerted on it the phase will always be a mixture of gas and liquid.

http://en.wikipedia.org/wiki/Image:Water_phase_diagram.svg

On another note, I think it's very unlikely to find a planet that is entirely water. Doesn't the current model for the accretion of the planets deal with cometary/asteroid impacts? In order to have a fully water planet with no rocky core not even a comet (dirty iceball) could have ever impacted it.

Yuk.