Beyond Neptune and to the edge of the solar system is what is collectively known as the trans-Neptunian region, and objects in this region are sometimes referred to as TNOs. They are considered to be pristine remnants of the early solar system forming out of the protoplanetary disk. At the lower temperatures further out form the Sun, fewer TNOs formed because of the lack of building materials—fewer rocky objects to serve as the nuclei and smaller amounts of condensable gases that could add icy layers onto the rocks. Various forms of ice are a principal component of many of the outer planet moons. In addition to the icy moons, it has been estimated that there may be around 100,000 other icy objects with diameters greater than 100 km orbiting the Sun in the area of the outer planets and beyond.