Comets are small icy bodies that orbit the Sun and are often described as leftover building blocks from the formation of the solar system about 4.6 billion years ago. Most of the time they remain frozen and inactive in the distant outer regions of the solar system, but when a comet’s orbit brings it closer to the Sun, heat causes its ices to vaporize. This process releases gas and dust, creating the glowing appearance that makes comets visible from Earth.
Comets form in two main reservoirs far beyond the planets. Short period comets originate in the Kuiper Belt, a disk shaped region beyond Neptune that also contains dwarf planets such as Pluto. Long period comets come from the Oort Cloud, a vast, spherical shell of icy objects that surrounds the solar system at distances thousands of times farther than Earth is from the Sun. These comets likely formed closer to the giant planets early in the solar system’s history and were later scattered outward by gravitational interactions.