How many planets are made up of gases?
All of the planets in the solar system have at least some trace
gases in their atmospheres, but the planets known as gas giants are
composed of mostly gases. They are the four outermost planets,
Jupiter, Saturn, Neptune, and Uranus. In contrast, the four
innermost planets, Mercury, Venus, Earth, and Mars, are called the
terrestrial planets and are mostly composed of solids.