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Let's say we had a mechanism of adding unlimited amounts of water at a certain point in space. How much water could we add until the water molecules would turn into something else due to gravity? What would the composition of this "planet" be if we went past that point? Would it be possible to have an Earth-sized blob of water? Would there be ice at the surface and water inside?

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    $\begingroup$ I think those water molecules will turn into something else (water ice) due to vacuum straight away as soon as they are added to a certain point in space. And then sublimate to vapour. $\endgroup$ – Sergiy Lenzion Dec 6 '19 at 9:28
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    $\begingroup$ This is an interesting question but I don't think that it is on-topic here in Space Exploration SE. However, Worldbuilding SE has many water planet questions, including Could a planet made completely of water exist? and What is the largest I can make my water planet, while remaining within Earth-like parameters? and even A continuous water “planet” ring around a star $\endgroup$ – uhoh Dec 6 '19 at 10:55
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    $\begingroup$ I'm voting to close this question as off-topic because questions about building planets are far too hypothetical for the site. There are some well-received terraforming questions here, but the more hypothetical ones get closed as well. Worldbuilding SE might be a good site for this, but there are several water planet questions there already. Welcome to Space! $\endgroup$ – uhoh Dec 6 '19 at 11:02
  • $\begingroup$ @LeoS it's an interesting thought. If there was a water atmosphere then the water would stay warm; H2O is a powerful greenhouse gas! So it might be bistable; an ice ball may never sublimate fast enough to maintain a water atmosphere, but if you melt it and evaporate an atmosphere of water up to the triple point pressure then its possible it will stay warm and liquid. $\endgroup$ – uhoh Dec 6 '19 at 11:07
  • $\begingroup$ I was going to lament that the worldbuilding answers don't actually come up with any figures, only of course at the pressure limits of liquid water it is actually noticably compressible and trying to work out the radius and mass of a sphere of compressible material being squished under its own gravitation seems like a right hassle. $\endgroup$ – Starfish Prime Dec 6 '19 at 11:34
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How much water could we add until the water molecules would turn into something else due to gravity?

It depends on how close this "waterworld" is to its parent star and thus how hot it is. Water has several phases and/or crystalline structures at temperatures and pressures that aren't typical of Earth.

Water Phases by Pressure/Temperature

What would the composition of this "planet" be if we went past that point?

If by what you mean disassociating the water altogether and/or fusing the hydrogen in the water, you're looking at a very metallic star. The smallest hydrogen-burning stars are roughly 14 times the mass of Jupiter though this figure may be higher due to its metallicity.

What would the composition of this "planet" be if we went past that point?

Atomic hydrogen, fused helium, and atomic oxygen

Would it be possible to have an Earth-sized blob of water?

Gliese 1214 b is a candidate object of such a waterworld, though it has a radius of 2.6x earth and 6.5x earth's mass.

Would there be ice at the surface and water inside?

Again that would depend on the planet's mass and proximity to its parent star. Europa has a thick layer of ice on its surface and liquid water below, though in warmer areas of a hypothetical solar system, it's perfectly possible to have water on the surface as seen on earth. It should be noted that at higher temperatures or lower surface gravity, water vapor will escape the hypothetical planet.

Gas Escape Velocity by Temperature and Gravity

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