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There is a class of planets called ice giants. Let's talk about Neptune. The top layer is atmosphere, consisting of hydrogen and helium. The amount of these gases is about 10% but they produce high pressure onto the second layer. The top of second layer consists of water and methane with temperature about 1700K. It's obvious that it is not ice but liquid! So why ice giants?

I have also heard that small amounts of ice exist on Jupiter. I would also like to know where and what is the real form of them.

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The term "Ice Giant" is given to planets which are thought to have formed from materials in their ice phase, not because they are made of ice. The ices changed to gas and liquid during planet formation.

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  • $\begingroup$ But didn't all planets which formed beyond the frost line for X, which for water is about 3 AU from the Sun, form as an ice giant then? And Neptune's (upper?) atmosphere consists of almost 99% hydrogen and helium which do not form ice. Don't current amount of aerosols play a role for the definition? $\endgroup$ – LocalFluff Feb 29 '16 at 14:06
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    $\begingroup$ Gas giants are made primarily of hydrogen, which freezes at so low a temperature that it was likely in gaseous form then the planet formed. Ice giants may not be the best term, but as it's the opposite of gas from a phase point of view it makes some sense. $\endgroup$ – GdD Feb 29 '16 at 14:25
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We can probe this matter a little more in-depth. This introductory reference describes all the giant or Jovian planets, noting that only the two more massive ones, Jupiter and Saturn, are made primarily of hydrogen and helium. Uranus and Neptune, which did not have as much material to work with and did not become powerful enough to draw large proportions of hydrogen and helium, accreted materials that were originally ices (water, ammonia, methane, etc). That's where the "ice" in "ice giants comes from as GdD's answer states.

The introductory site is not a primary reference. This reference (free version in http://www.boulder.swri.edu/~hal/PDF/un-scat_nature.pdf) provides such a primary reference. Using models, it is predicted that a core of about 15 Earth masses was required for accretion of large amounts of gas, and the "gas giants" reached that critical core mass. The "ice giants" fell short and thus were left with more ice that phase-transformed (as did much of the gas in the gas giants).

One more relevant fact: the whole concept of "ice giants" is of recent vintage, born in 1990. Before then, less was known about the subtle differences between these planets and "gas giants" was used indiscriminately for all the Jovian planet's.

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