Gas giant
From SporeWiki, the knowledge base dedicated to Spore
A Gas Giant is a type of planet that can be seen in the Space stage. They can't be visited by the UFO, according to the game they are "too dangerous". This is probably because Gas Giants don't have a solid surface, and the atmospheric pressure would crush and destroy the UFO. However, some of them have moons that can be visited, mostly ice or lava planets. From lower down on these moons, at certain times of the day, the gas giant covers most of the sky and blocks out the sun, creating an artificial night.
On the list of planets while viewing the star, the moon(s) are listed, but not the Gas Giant that they orbit. Most home systems have a Gas Giant, though it normally doesn't have a moon. The Gas Giants come in many colours and some have rings while others do not. Some Gas Giants appear to be tilted at an angle.
Occasionally, a glitch can occur, and they may be inhabited by an AI empire, and the planet will produce rather strange White spice. Inhabited Gas Giants can be bought, but not captured or entered. As a result it can be irritating to own a Gas Giant because a player cannot protect their colonies.
It is somewhat ironic that the player cannot land on gas giants, even though they can withstand the enormous pressure in a black hole, which would make the pressure in a gas giant seem tiny by comparison. Hypothetically speaking though, any black hole large enough could be traveled through without too much pressure, the Wormhole Key probably just shrinks the ship into a tiny size and be done with it. This probably could not be done for a gas giant. It is possible that the reason you can't go onto Gas Giants is because your engines would ignite the gas. To visit them, you must have some type of mod, otherwise you can't go in a gas giant. Another way is to have Galactic Adventures so convert all the planets into adventure planets, THEN the gas giant get converted to adventure planets and you can visit them even they are still gas giants. If you change the atmosphere lower ,then it will become a normal planet. If higher then it will become a gas giant again.
[edit] In real life
Jupiter, Saturn, Uranus and Neptune of the Sol System are Gas Giants. They have many moons and the "surface" is mainly gas. The huge mass of these planets makes them attract a lot of matter with gravity, this is what makes the moons. Like in Spore, some of these natural satellites of the gas giants are large enough to support life if terraformed. Two examples of these are Europa of Jupiter and Titan of Saturn.
They also have the tendency to "release" small fractions of their matter in massive explosions every few dozen million years. This is what creates the rings, as well as shards of ice from broken up moons or captured comets.
