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Charon Moon: Definition & Facts

Instructor: John Williams
Named after the mythological ferryman of souls, Charon is Pluto's moon that was discovered in 1978 by astronomer James Christy. Explore a definition of this relatively large moon and facts about its composition, orbit, and unique rotation. Updated: 02/03/2022


In 1930, scientists in the United States identified a planetary body orbiting the sun beyond Neptune. They named this planet Pluto, after the Roman God of the underworld. Pluto, at the time, was identified as the ninth planet in our solar system. (Since then, it has been re-classified as a minor planet). In 1978, a U.S. astronomer named James Christy discovered another object in the same vicinity as Pluto. This object was identified originally as a moon, or naturally orbiting object, of Pluto. In keeping with the Roman mythology, this moon was named Charon, after the mythological being that carried souls to the underworld. Let's discuss the characteristics of this unique moon and its place in our solar system.

Composition of Charon

Charon is the largest moon in the solar system when compared to the parent planets. This means that the size of Charon compared to Pluto is larger than any other planet/moon groups. Very little is known about Charon in terms of its terrestrial (land surface) chemistry, but it has been confirmed that water is a primary component of the surface. Also, while it has not been confirmed, scientists believe that Charon does not have an atmosphere, or collection of gases surrounding the body. These characteristics of Charon were determined by comparing physical properties of Charon with the known properties of Pluto.

Orbit and Rotation

Like Pluto, Charon rotates once every 6.38 Earth days. This is much slower compared to Earth's rotation, and it is interesting to note that both Charon and Pluto have the same rotation period even though they are of different sizes. Both Pluto and Charon orbit the sun once every 248 years, and this is due primarily to the distance of both objects from the sun.

Planets often serve as a central body for their moons to orbit. This is due to the fact that large objects, such as Pluto, often exert a gravitational pull, or attraction, on smaller objects. This does not appear to be the case with Pluto and Charon. The center of gravity between these two bodies does not lie within Pluto. Rather, the center of gravity is found between the two. Therefore, scientists have debated whether or not Charon is a moon of Pluto or if Pluto and Charon could be considered 'twin planets.' While many theories exist as to why Charon and Pluto developed in this arrangement, many scientists believe that, at one point in time, Pluto and Charon collided with each other and, as a result, began rotating around each other in a twin planet orbit. This is known as Impact Theory.

Pluto and Charon
Pluto Charon

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