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What are comets?

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Comet PanSTARRS C/2011 L4 was visible to observers on Earth in early 2013.

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Comets are small celestial bodies that orbit the Sun. They are primarily made of dust and ice. Considered to be remnants of the formation of our Solar System, most comets belong to the Kuiper Belt, a region beyond the orbit of the planet Neptune. Kuiper Belt comets tend to have short orbital periods – usually around 200 years – and are therefore also known as short period comets.

Some comets are thought to come from the Oort Cloud, a region almost a light year from the Sun. Oort Cloud Comets have very long orbital periods – spanning several million years – and are known as long period comets.

Key features

One of the distinguishing features of a comet is that most of them develop a tail or a coma when they come close to the Sun. Away from the Sun, comets are frozen celestial bodies that are hard to detect. However, as a comet comes closer to the Sun, the Sun’s heat and radiation vaporizes the ice and dust of the comet. These vaporized gases collect dust and stream out of the center of the comet like a tail. This tail can be thousands of miles long.

While most comets passing by the Sun are hard to observe from Earth without specialized equipment, some comets are bright enough to be seen by the naked eye. The brightness of the comet is due to sunlight reflecting and refracting off the dust in the tail.

Comets usually have two tails, which point in different directions. The dust in the comet is responsible for one tail. This tail, also called the dust tail, tends to be broad and curved. The gases in the comet make the second tail, called the plasma or the ion tail. This tail is thin and straight and tends to point directly away from the Sun.

Did you know?

A light year (light-year or lightyear) is a unit of distance and is defined by the International Astronomical Union as the distance traveled by light in a vacuum during a Julian year. It is approximately equal to 6 trillion miles (10 trillion kilometers).

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