Jupiter and Saturn represent a class of Gas Giant sometimes called a "traditional Gas Giant" or a "Jovian Planet". Such a planet is composed mostly of Hydrogen and Helium. At the very center it may have a rocky core - such a core is probably required in order for a planet to form in the first place. However, the core (and all other elements heavier than hydrogen and helium) is probably about 3% to 13% of the mass of the planet. The rest is gas. The core of such a planet may exceed 20,000oC. The atmospheric pressure near the core of a gas giant gets so intense, it turns normal hydrogen into Metallic Hydrogen - that is hydrogen that conducts electricity like a metal.
Game and Story Use
- You cannot "land" on a gas giant. If you go far enough down to hit something solid, you'll be destroyed by the gravity and atmospheric pressure. However, there's always the Cloud Planet option, and some aerospace vehicles may be able to fly through the hydrogen and helium clouds without too much trouble.