How Pluto’s First Moon Got Its Name

The discussion a few years ago about whether or not Pluto should be called a ‘planet’ missed the most interesting bits about Pluto. The mainstream media kind of presented the issue as if Pluto was getting demoted. But just the opposite was true.

Calling Pluto simply a ‘planet’ doesn’t do Pluto justice!

I used to think of Pluto as the ‘plain’ planet of the outer system, the last planet to be discovered, a ball of rock on the edge of nothingness on the fringe of the solar system.

I was wrong on just about all those thoughts.

First of all, Pluto is far from plain. It turns out that Pluto is in fact the solar system’s only known binary planet. Pluto and its largest moon are of such comparable size that neither really revolves around the other, rather they both revolve around a point in space between them. And with two additional small satellites and even, possibly, a ring system, Pluto may be one of the most complicated bodies in the solar system.

And rather than being the ‘last’ planet to be discovered on the edge of nothingness, now we know that Pluto actually was the first of the interesting TNO’s, the trans-Neptunian objects, to be discovered. Now we know there are thousands of TNO’s out there and trying to make sense of them with their odd combinations of some asteroid characteristics and some comet characteristics has become the single most interesting astrophysics issue of the modern era.

Pluto is the star of that show.


Pluto is known to have three moons. The two most recently discovered moons, Nix and Hydra, are very small and were first found with the Hubble space telescope. They have since been imaged from ground telescopes using state-of-the-art equipment and cutting-edge observing techniques.

The first moon of Pluto to be discovered was found by an astronomer named James Christy and he discovered the moon without ever leaving his desk . . .


How Pluto’s First Moon Got Its Name

One Thursday back in the late 70s James Christy was looking for a straightforward job he could start and finish by the end of the week. He decided to review a series of photographs of Pluto an observatory had sent him. The photos had been marked as defective because the observatory noticed that the images of Pluto weren’t as perfectly round as they should have been. Christy thought he could quickly check out the photographs, recover any useful positional information they may contain, and then call it a week.

After looking at the photographs first by eye and then with a microscope, Christy began to wonder if they really were defective. The star images were perfectly formed. Only the image of Pluto looked odd. And he began to notice a pattern to the oddness.

It occurred to Christy that the odd shape of Pluto could be caused if there were a moon orbiting around Pluto and the two images had become blurred together. As the moon changed position, the round image of Pluto would become elongated first in one direction and then another.

Christy studied the photographs and saw exactly that pattern.

He spoke with his boss, explained his moon theory, and the two of them both reviewed the photographs and worked out initial rotational periods independently. When they checked their numbers with each other, their calculations agreed.

It appeared Pluto had a moon.

When Christy reported his find, observatories quickly confirmed it.

Christy had identified a moon of Pluto from his desk, without ever looking through a telescope.

And since Christy had discovered the moon, it was his right to give it a name.

Astronomers Alan Stern and Jacqueline Mitton tell the story of the name:

“...Within a day of his discovery, he offered his wife, Charlene, the honor: ‘I could name it after you—Char-on.’ He was thinking it rhymed with prot-on and neutr-on. Another romantic astronomer.

“To make a very long story short, however, Christy realized within a few days that although it was his privilege to propose a name, the rules of the International Astronomical Union (IAU) nomenclature bylaws within which he operated stated that he would have to choose from Greek or Roman mythology. Someone suggested Persephone, the wife of Pluto. Christy liked that, so he went to a dictionary to get himself out of the trouble this would cause with his wife. But to his amazement, there, in black and white on the page, was the ancient name ‘Charon.’ In Greek mythology, Charon was the repugnant boatman who rowed dead souls over the river Styx into Hades, where the god of the underworld, Pluto, ruled. Christy realized this was the solution to his dilemma with both the IAU and his wife. Although correctly pronounced ‘Khar-on,’ Christy pronounced it ‘Shar-on,’ like Charlene—who promptly got her moon!

“It was done. Pluto’s satellite, which had for billions of years remained nameless in its anonymity, had acquired a name less than a week after its discovery by one very careful Homo sapiens.”