Pluto's moon Charon was discovered in 1978, and ultimately led to the decision more than 30 years later that Pluto wasn't a planet

Much to the dismay of many professional and amateur astronomers, the 1978 discovery of Pluto’s largest moon Charon ultimately led to Pluto being demoted from the status of a planet to that of a dwarf planet by the International Astronomical Union (IAU) in 2006.

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In June 1978, American astronomer James Christie, working at the US Naval Observatory (USNO) in Washington, DC, USA, was examining photographic plates of Pluto, taken with the 1.55-meter Kaj Strand Reflector Telescope in Arizona, USA, to refine calculations of the planet’s orbit around the sun.

While examining the photographic plates, which were not of particularly good quality, Christie noticed a strangely distinct bulge on one side of Pluto. His background in photographing double stars enabled him to deduce that what he was seeing was something unusual. Although the images of Pluto (and the bulge) on the plates were fuzzy, the background stars appeared as perfectly sharp points of light.

Researching photographs of Pluto in the USNO’s archives, Christie discovered the same planetary bulge. This indicated to him that the apparent bulge was not the result of either an improper telescope alignment or atmospheric turbulence. The archive photos also showed that the bulge systematically moved around Pluto over time.

With his colleague Robert Harrington, Christie tracked the position of the bulge across the archived images of Pluto as far back as 1965, and calculated that the bulge circled Pluto once every 6.4 days, which just happened to match Pluto’s own known rotation period.

This indicated to Christie and Harrington that the bulge was, in fact, an unknown, massive companion moon to Pluto that was locked in a synchronous orbit with Pluto. The newly discovered moon orbiting Pluto at a distance of approximately 19,400 km was provisionally labelled S 1978 P1.

As the discoverer of the new moon, Christie had the right to name it.