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Ice Giant

A classification for a type of giant planet, distinguished from a gas giant by their chemistry. Both types of giant planet are characterised by a small solid core surrounded by outer layers extending for thousands of kilometres, but whereas in gas giants these outer layers consist primarily of lighter elements (especially hydrogen), in an ice giant these outer layers contain heavier elements and more complex compounds.

Ice giants are not literally composed of ice, but rather of substances such as methane, ammonia or water; these are ices in a technical sense of the term, based on their freezing point, but within an ice giant they exist primarily in liquid or gaseous form. Structurally, the solid core of an ice giant is surrounded by a dense mantle of heavier elements, which is itself wrapped in a deep atmosphere of lighter elements such as hydrogen and helium, with methane and other gases.

There are two ice giant planets within the Solar System: Uranus and Neptune (the other giants, Jupiter and Saturn, are both gas giants). This difference is clearly visible in the colours of the planets: while the hydrogen of the gas giants gives them a brown hue, the more complex chemistry of the ice giants' atmospheres makes them appear blue or blue-green. The gases in the outer atmosphere (especially methane and hydrogen sulphide) absorb red light but reflect blue and green, giving the ice giant planets their distinctive colour. There are some differences in the way the atmopsheres are layered between Uranus and Neptune, and those differences produce different colours: Uranus appears as a pale blue-green, while Neptune shows a much deeper blue colour.

Identifying ice giants beyond the Solar System is difficult, but several candidates have been found (one notable possibility, for example, is a potential ice giant in the nearby Proxima Centauri system). Some planets of these general type show variations from the familiar ice giants of the Solar System, as for example those described by the term 'mini-Neptune' (or 'gas dwarf'), planets that seem structurally similar to Uranus or Neptune, but are significantly smaller and less massive. In other cases extrasolar planets appear similar to the Sun's ice giants, but orbit much closer to their star. Planets of this kind (of which the first known example was Gliese 436 b) are commonly described as 'hot Neptunes'.


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