What is a comet?

Comet ISON (C/2012 S1)

Comet ISON was discovered by two amateur astronomers in 2012. Despite high anticipation, it fizzled and broke apart while making its first pass around the Sun.
Adam Block / Mount Lemmon SkyCenter / Univ. of Arizona

Like asteroids, comets are suspected to be remnants of planet formation in the Solar System about 4.6 billion years ago. But while asteroids are generally comprised of rock and metal, comets are more akin to dirty snowballs. They are composed of frozen gases such as carbon dioxide, methane, and ammonia, as well as water ice, in which dust particles and rocky material are embedded.

Where do comets come from? Comets primarily originate from two regions of the solar system: the Kuiper Belt, which is a disk of icy bodies that lies just beyond the orbit of Neptune, and the Oort Cloud, a more distant, spherical collection of objects that marks the very edge of the solar system. Short-period comets, which orbit the sun in 200 years or less, are usually Kuiper Belt objects, while long-period comets that take hundreds or thousands of years to orbit the sun generally come from the Oort Cloud.