The Solar System is the gravitationally bound system of the Sun and the objects that orbit it. The largest eight objects, which form a planetary system, are, in order of distance from the Sun: four terrestrial planets, Mercury, Venus, Earth and Mars; and four giant planets which include two gas giants, Jupiter and Saturn, and two ice giants, Uranus and Neptune. The terrestrial planets, which all have a definite surface, are mostly made of rock and metal. The gas giants are mostly made of hydrogen and helium, while the ice giants are mostly made of volatile substances such as water, ammonia, and methane. The terrestrial planets are also called the inner Solar System and the giant planets the outer Solar System.