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Is an ice planet made only by ice? What causes the water to conglomerate and form ice in a planet?

Even if the planet is not made entirely by ice but has an high % of ice (e.g. 50% ice), how can it be explained?

I heard about ice planets with surface temperature way above the melting point. Can it be explained with surface pressure? Edit: This is probably wrong but feel free to point it out in a full answer

Thanks.

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    $\begingroup$ Can you provide a link to the 3rd one. Ice planets with higher than melting point surface temperature? $\endgroup$ – userLTK Nov 29 '16 at 12:27
  • $\begingroup$ No link sorry, it was told to me. It could be totally wrong. $\endgroup$ – A.Danzi Nov 29 '16 at 14:45
  • $\begingroup$ Phase diagram for water: www1.lsbu.ac.uk/water/water_phase_diagram.html Liquid There are solid phases up to 1000°K, but that's under multi giga Pascal pressure. Sea level norm pressure on earth is a mere 101325 Pascals. $\endgroup$ – Wayfaring Stranger Nov 29 '16 at 15:02
  • $\begingroup$ Probably wrong then.. will update the OP $\endgroup$ – A.Danzi Nov 29 '16 at 15:07
  • $\begingroup$ Maybe I'll turn this into an answer later, but generally ice planets or ice moons form outside the frost line. en.wikipedia.org/wiki/Frost_line_(astrophysics) where ice behaves like rock, it's frozen. Ice planets aren't 100% ice, on average, maybe about half ice by mass, half other stuff, but that ratio likely varies quite a bit. There's no liquid water in space, just frozen and gaseous water vapor atoms. Vapor and icy debris can coalesce into an ice planet similar to how a rocky planet forms. $\endgroup$ – userLTK Nov 29 '16 at 15:27
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There are several compounds referred to as 'ices' in the solar system, these are mostly hydrogen-based compounds such as water, ammonia and methane (and others) accreted around what is believed to be a rocky core, these compounds are important, as they comprise a great proportion of the two 'Ice Giants' observed in our Solar System (Uranus and Neptune), as seen in the diagram below, compared to the Gas Giants - Jupiter and Saturn:

enter image description here

Image Source: Wikipedia

This answer will just mainly focus on the Ice Giants. as discussion of these answer both your questions regarding the ice planets and what is sometimes described as ices existing at high temperatures.

In terms of planets with proportionally large amounts of ice, as mentioned in the comments, there is a dividing line when the solar system formed known as the Frost Line, illustrated in the diagram below:

enter image description here

Image Source

Inside the Frost Line, the hydrogen compounds remain vapourised, beyond the line, hydrogen compound ices can condense and solidify, as seen for Uranus and Neptune (and an array of dwarf planets, moons and other celestial objects), these 'ices' comprise the mantle of both planets. An important distinction about what these "ices" are is explained in the paper "The Interior Structure, Composition, and Evolution of Giant Planets" (Fortney and Nettelmann, 2009) as being:

The label ice refers to a mixture of $H_2O$, $CH_4$, and $NH_3$ that are supposed to have been in an ice phase during protoplanetary core formation.

The compounds in the mantle in the Ice Giants are under immense pressure (in excess of 10 GPa), far in excess of those existing on Earth. Greater pressures change the properties of the hydrogen-based ices - so it is one of the reasons that it is sometimes referred to as 'hot ice', according to the AstroTech Gallery page 'Ice Giant', describe a model of the Ice Giant planets as being:

Much of this mass (perhaps about 65%) is made up of hot ices, which despite being very hot are probably kept at least partially solid by the enormous pressures within these massive planets.

The effect of pressure on water (as an example) can be seen in the example phase diagram below (of course there would be differences depending on the actual nature of the mixtures and compounds present):

enter image description here

Image Source: Duke University

However, according to the article The gas (and ice) giant Neptune, the present icy (or 'hot ice') mantle of Neptune is described as:

The mantle is equivalent to 10 – 15 Earth masses and is rich in water, ammonia and methane. This mixture is referred to as icy even though it is a hot, dense fluid, and is sometimes called a "water-ammonia ocean".

A similar situation is hypothesised to exist for Uranus.

It is known that ice exist on Mercury, Earth, Moon and Mars, but these comprise a small proportion compared to the planet itself, so not included in this answer.

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