Differences in temperature from place to place within the protoplanetary disk have a significant effect on the makeup of the dust grains in the disk (Figure 7.11). In the hottest parts of the disk (closest to the protostar), only the most refractory substances can exist in solid form. In the inner disk, dust grains are composed of refractory materials only. Somewhat farther out in the disk, some hardier volatiles, such as water ice and certain organic substances (organic refers to a large class of chemical compounds containing the element carbon), can survive in solid form. These add to the materials that make up dust grains. In the coldest, outermost parts of the accretion disk, far from the central protostar, highly volatile components such as methane, ammonia, and carbon monoxide ices and other organic molecules survive only in solid form. The different composition of dust grains within the disk determines the composition of the planetesimals formed from the dust. Planets that form closer to the central star tend to be made up mostly of refractory materials such as rock and metals. Planets that form farther from the central star also contain refractory materials, but in addition they contain large quantities of ices and organic materials.