Most comets originate from the deep dark depths of the solar system. From a spherical cloud of debris called the Oort Cloud, that forms the outermost layer of our solar system, like a shell. Most of this debris in this shell is undoubtedly left over from the formation of the solar system, five billion years ago, but some may have come from interstellar space, captured by the Sun's gravitational pull. Since they are so far away from the heat of the Sun, comets are composed largely of frozen gases, mixed with dust and dirt. When their eccentric, elliptical orbits bring them close to the Sun, the frozen gases begin to vapourize, giving rise to the comet's distinctive tail.

Comets actually have two tails. The lighter gases form a long tail that always points directly away from the Sun. This is called the gas tail, or ion tail, and is usually visible only in time exposure photographs. The heavier gases and dust particles usually flow more in the comet's direction of travel, although the added influence of the solar wind may cause this tail to curve slightly. This tail is usually much shorter, and called the dust tail. This is the tail normally visible to the naked eye.

Some comets, presumably under the gravitational influence of the Sun or the gas giants Jupiter and Saturn have migrated a little closer, to a region just outside the orbit of Neptune called the Kuiper Belt. Unlike the Oort Cloud, Kuiper belt objects occupy roughly the same dimensional plane as the planets.

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