Giant planets exist orbiting many stars. Some of them are free floating in space (see Rough planet). It is possible also that a gas giant can be a moon (like a Neptune-size orbiting a Jupiter-size planet). They are made by a gasses, surrounding an inner solid core, made of metallic hydrogen, ices or rock. Studying these bodies is important, because their moons might be suitable for terraforming.
Depending on average temperature, gas giants can be divided into several categories (made after a list first available at [www.extrasolar.net here]:
Helium gas giants Edit
Assuming a temperature of less then 30 K and almost no inner thermal output, such bodies might have only helium in their atmosphere. Hydrogen should be frozen. In the outer reaches of Oort cloud or in interstellar space, they might exist. Because of their low thermal radiation, detection is impossible for the moment.
Hydrogen cloud giants Edit
They have not been observed. Temperature is very low, so that hydrogen cools and forms clouds (below 45 K). Such cold bodies could be free-floating around the galaxy. They can exist at high distances orbiting stars (like the Oort cloud in solar System). In visible light, they should appear red.
Methane cloud giants Edit
A theoretical gas giant orbiting in Kuiper belt would have temperatures below 60 K, cold enough for methane to form clouds in upper atmosphere. They might exist around many stars, but because their faint luminosity, are very hard to detect.
Methane gas giants Edit
The best example is Neptune. When temperature is around 100 K, methane is in gas phase. temperature is too low for ammonia to evaporate. The planet appears blue, with no clouds.
Ammonia cloud giants Edit
Think about Jupiter and Saturn. Temperature is 150 K, hot enough for ammonia to evaporate and form clouds in the upper atmosphere. If the amount of clouds is small, planet will be green. However, if the amount is higher, we will have a slightly red planet.
Carbon dioxide cloud giants Edit
This is what we will get if we take Jupiter closer to our sun, somewhere in the asteroid zone. As temperature passes over 200 K, conditions are made that carbon dioxide will sublimate and form clouds. Ammonia might still be present, but almost always under the form of gas. clouds will be white, with some pink from ammonia.
Water cloud giants Edit
As temperature rises above 270 K, it is hot enough for water to form clouds in the upper atmosphere. A planet might look like a combination of blue and white, much more Earth-like then any other gas giant.
Sulfuric gas giants Edit
When all water is in gaseous phase (400 K), conditions will look similar with Venus. So, we might expect sulfuric acid to appear in the upper atmosphere, creating opaque clouds and giving the planet a green tint.
Clarified gas giants Edit
If temperature is even higher (600 K), all volatiles should be in gaseous phase. Since there will be no clouds, the expected color of the planet is blue. At such conditions, the planet will look homogenous.
Dark hot giants Edit
Above 900 K, some solid minerals will vaporize and create clouds. This includes sodium, potassium or magnesium. As gasses or clouds, they reflect far less light, giving the planet a dark color, heating it further up.
Overheated giants Edit
What happens if temperature exceeds 1300 K? Silicon evaporates into the atmosphere and rains sand. At higher temperatures, even iron can form clouds and can rain. With so hot temperatures and exposed to strong solar winds, the planet is losing matter.
Terraforming Gas Giants Edit
Terraforming the gas giant is very hard, if not impossible. However, some of their moons can be terraformed, so it is important to know what settlers will see on the sky when they look after the parent planet.
Still, there are ways a gas giant can become useful or even terraformed. See Terraforming Gas Giants for more details.