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A comet is a block of ice and dust with perhaps a rock in the middle, that orbits around the Sun. Most comets come from far beyond the furtherst planet and fly through the inner part of the Solar System in a short time. When comets come close enough to the Sun, then they usually develop a coma and one or more tails.

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There are many differences between planets and comets. The most important one is that a planet is very much larger than a comet. A comet is at most a few dozen kilometers in diameter, while a planet is more than a thousand kilometers in diameter. The Earth, for example, has a diameter of about thirteen thousand kilometers.

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Planets tend to have almost circular orbits (with low eccentricity) that lie in nearly the same plane (i.e., that have a low inclination). Comets tend to have very elongated orbits (with high eccentricity) that are randomly oriented compared to the orbits of the planets (i.e., they can have high inclinations).

Planets and the Sun show up in the sky only near the ecliptic (the zodiac), but comets can show up anywhere.

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Last updated: 2016-02-07