Comets are among the most-spectacular objects in the sky, with their bright glowing comae and their long dust tails and ion tails. Comets can appear at random from any direction and provide a fabulous and ever-changing display for many months as they move in highly eccentric orbits around the Sun.Comets are important to scientists because they are primitive bodies left over from the formation of the solar system. They were among the first solid bodies to form in the solar nebula, the collapsing interstellar cloud of dust and gas out of which the Sun and planets formed. Comets formed in the outer regions of the solar nebula where it was cold enough for volatile ices to condense. This is generally taken to be beyond 5 astronomical units (AU; 748 million km, or 465 million miles), or beyond the orbit of Jupiter. Because comets have been stored in distant orbits beyond the planets, they have undergone few of the modifying processes that have melted or changed the larger bodies in the solar system. Thus, they retain a physical and chemical record of the primordial solar nebula and of the processes involved in the formation of planetary systems.