A gas giant (sometimes also known as a Jovian planet or Jovial planet after the planet Jupiter) is a large planet that is not primarily compose of rock or other solid matter. There are four gas giants in our Solar System; Jupiter, Saturn, Uranus, and Neptune. Uranus and Neptune may be considered a separate subclass of giant planets, 'ice giants', as they are mostly composed of water, ammonia, and methane, unlike the "traditional" gas giants Jupiter or Saturn. While all four planets lack a solid surface, the hydrogen and helium in Uranus and Neptune is mostly in the outermost region. A gas giant may have a rocky or metallic core—in fact, such a core is thought to be required for a gas giant. The majority of the mass of Jupiter and Saturn is hydrogen and helium. (Although familiar to us as gases on Earth, these constituents are assumed to be compressed into liquids or solids deep in a gas giant's interior.) The majority of Uranus and Neptune is mostly icy. See section below for details.