Ice giant
From Aetilc
An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur.
In astrophysics and planetary science the term "ices" refers to volatile chemical compounds with freezing points above about 100 K, such as water, ammonia, or methane, with freezing points of 273 K, 195 K, and 91 K, respectively (see Volatiles).Their constituent compounds were solids when they were primarily incorporated into the planets during their formation, either directly in the form of ices or trapped in water ice. Today, very little of the water remains in the form of ice. Instead, H2O primarily exists as supercritical fluid at the temperatures and pressures within them.
Ice giants consist of only about 20% hydrogen and helium in mass, as opposed to the gas giants which are both more than 90% hydrogen and helium in mass.
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[edit] Formation
Modelling the formation of the terrestrial and gas giants is relatively straightforward and uncontroversial. The terrestrial planets are widely understood to have formed through collisional accumulation of planetesimals within the protoplanetary disc. The gas giants are thought to have formed after solid cores around 10 Earth masses (M⊕) formed through the same process, while accreting gaseous envelopes from the surrounding solar nebula over the course of a few to several million years (Ma), although alternative models of core formation based on pebble accretion have recently been proposed. Some extrasolar giant planets may instead have formed via gravitational disk instabilities.
The escape velocity for the small protoplanets about 20 astronomical units (AU) from the centre of a planetary system would have been comparable to their relative velocities. Such bodies crossing the orbits of Saturn or Jupiter would have been liable to be sent on hyperbolic trajectories ejecting them from the system. Such bodies, being swept up by the gas giants, would also have been likely to just be accreted into the larger planets or thrown into cometary orbits.
[edit] Characteristics
The ice giants represent one of two fundamentally different categories of giant planets present in the planetary system, the other group being the more-familiar gas giants, which are composed of more than 90% hydrogen and helium (by mass). Their hydrogen is thought to extend all the way down to their small rocky cores, where hydrogen molecular ion transitions to metallic hydrogen under the extreme pressures of hundreds of gigapascals (GPa).
The ice giants are primarily composed of heavier elements. Based on the abundance of elements in the universe, oxygen, carbon, nitrogen, and sulfur are most likely. Although the ice giants also have hydrogen envelopes, these are much smaller. They account for less than 20% of their mass. Their hydrogen also never reaches the depths necessary for the pressure to create metallic hydrogen. These envelopes nevertheless limit observation of the ice giants' interiors, and thereby the information on their composition and evolution.
