The news that “Vulcan” topped the poll results taken by the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California as a possible name for one of the two tiny moons newly discovered to be orbiting Pluto has gotten quite a bit of press this week. In 2012, Mark Showalter of SETI, working with scientists on the New Horizons mission sending a probe to Pluto, found a tiny fifth moon orbiting the icy world. Showalter was also the lead author of the discovery of a fourth moon in 2011 using observations from the Hubble Space Telescope.
As SETI contemplated what names to propose for these two newly-discovered moons, they opened the question to the public in an on-line poll. Inspired by a tweet from William Shatner, the actor who became famous playing Capt. James T. Kirk in the original Star Trek television program (1966-69), Vulcan, Pluto’s mythological nephew and the name of a fictional world in Star Trek’s imagined universe, became the top vote getter. Leonard Nimoy, who played Spock, the most famous Star Trek Vulcan, reportedly tweeted that naming one of the two moons “Vulcan” would be the “logical choice.”
But did you know that there already was a Vulcan? Or, actually, there wasn’t. But astronomers thought that there was.
Since the 18th century, astronomers worried that the orbit of the innermost planet in the solar system, Mercury, did not behave the way that they expected. By the mid-nineteenth century they knew that perturbations in the orbit of Uranus had just (in 1846) resulted in the discovery of Neptune, the first planet to be predicted mathematically before it was confirmed through observation. Could that also apply to Mercury? Was there another planet orbiting between Mercury and the Sun that could explain Mercury’s orbit? Urbain Jean Joseph Leverrier, whose calculations had been used to discover Neptune, thought so. By 1859, amateur and professional astronomers started searching.
And they found some things. Some scientists reported seeing a bright, star-like object orbiting near the Sun. And others saw circular shapes transiting (or crossing) the face of the Sun. It must be Vulcan, they thought! Some textbooks printed in the 1860s and 1870s even listed Vulcan as a planet. (For, indeed, Pluto is not the first body to have been considered a planet and then reclassified. Ceres, the spherical body in the asteroid belt, was also called a planet when it was discovered in 1801 but then reconsidered when the many other bodies discovered in that same region of space became known as the asteroid belt.) But the observations of Vulcan did not compute. They were not consistent. According to Newton, and to vast experience, planets, above all, were predictable in their orbits. Any deviation was not acceptable. That’s how Kepler decided that planets did not travel in circular orbits. So when scientists looked for Vulcan where it was predicted to be visible and they could not find it, they started to doubt that Vulcan existed.
When something does not move as predicted, astronomers start looking for a perturbing mass. That is in fact how dark matter was detected, and after almost 50 years, finally accepted as a major factor in controlling the motions of things like stars and galaxies in the universe. In the early 20th century, astronomers thought that the existence of a faint disk of material around the sun, called the Zodiacal Light, might be massive enough to make Mercury’s orbit shift in the way it appeared to do. But in the end, Einstein solved the problem (literally) and Vulcan was no more. In 1915, Einstein’s General Theory of Relativity explained the shifts in Mercury’s orbit without the presence of another world orbiting nearer to the Sun. Vulcan, which had never existed, entered the history books. But astronomers still use the name: NASA’s project to detect new planets has been called “Project Vulcan.”
The names of Pluto’s moons have still not been decided. The International Astronomical Union, the worldwide professional organization of astronomers, will make the final choice. They may choose “Vulcan.” Or they may decide that there was already a Vulcan. Except that there wasn’t.
Margaret Weitekamp and David DeVorkin are curators in the Space History Department of the National Air and Space Museum.