Even though the giant planets share many characteristics, there are significant differences among them. Jupiter and Saturn are similar to one another in radius and are composed primarily of hydrogen and helium. Uranus and Neptune are also similar to one another in radius, and both contain larger amounts of water and other ices than either Jupiter or Saturn has. These differences are significant enough that many astronomers divide the giant planets into two classes, referring to Jupiter and Saturn as gas giants (Figure 10.1) and to Uranus and Neptune as ice giants (Figure 10.2). Other differences between the gas giants and the ice giants become evident as scientists more closely examine their physical and chemical characteristics.