The Discovery of Charon

Artist’s concept of Charon seen from the surface of Pluto

On June 22, 1978, US astronomer John Christy discovered Charon, the largest moon of Pluto. Although there was a discussion after the reclassification of Pluto as a dwarf, Charon is not in the list of dwarf planets currently recognized by the IAU.

On June 22, John Christy had examined the magnified images of the former planet Pluto, taken with the 61-inch Flagstaff telescope two months prior. He noticed a periodically appearing elongation, which was confirmed shortly after. During subsequent studies, it was determined that the bulge was due to a smaller accompanying body. Also, it was noticed that the periodicity of the bulge corresponded to Pluto’s rotation period. Between 1985 and 1990, Pluto and Charon entered a five-year period of mutual eclipses and transits and all doubts about Charon could be abolished. A few weeks after the United States Naval Observatory astronomer James Christy had discovered Pluto’s largest moon, Charon, his achievement was officially announced by the International Astronomical Union.

Charon is half a big as Pluto and it is assumed that its surface consists mostly of water ice. Also, scientists believe that Charon has no atmosphere. However, there are several theories on Charon’s internal structure. It is believed that Charon was created by a giant impact into Pluto’s icy mantle and that it therefore consists of an icy body, containing less rock by proportion than its partner Pluto. One theory suggests, that Charon has a rocky core and an icy mantle, others believe Charon to be of uniform composition throughout.

Pluto and Charon are considered gravitationally locked, which means that each object keeps the same face towards the other. Due to the fact that neither object orbits the other, many scientists argued that Charon should be considered as a dwarf planet itself, however, the International Astronomical Union stated that Charon is considered to be just a satellite of Pluto.

A side view of the PlutoCharon system
Image: Stephanie Hoover


At yovisto, you may be interested in a video lecture on ‘Pluto, Eris and the Dwarf Planets of the Outer Solar System‘ by Professor Mike Brown.

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