Vulcan Out, Styx And Kerberos Win In Pluto Moons Name Game

Kerberus and Styx join Pluto’s ever-growing family of moons. This discovery image, taken by the  Hubble Space Telescope, shows five moons orbiting the  icy dwarf planet Pluto. The darker stripe in the center is because the picture is constructed from a long exposure  to capture the faint satellites of Nix, Hydra, Kerberos and Styx, and a shorter exposure to capture brighter Pluto and Charon. Credit: NASA/ESA

The International Astronomical Union (IAU) has spoken and christened Pluto’s newest and tiniest moons Kerberos and Styx. Maybe you cast a vote in the informal Pluto moon naming contest in February and hoped that the number one choice Vulcan, home of Mr. Spock in the Star Trek TV series, deserved a chance.

Sorry to disappoint, but in a moment you’ll see why that hot world doesn’t work for a chilly planet on the solar system’s fringe. Hint, hint.

Pluto is one of five dwarf planets in the solar system. Its fellow dwarfs include the asteroids Ceres, Eris, Haumea and Makemake. Three of themare orbited by their own moons. Credit: NASA

The moons were picked up during a survey of the Pluto system with the Hubble Space Telescope in 2011 and 2012. Kerberos’ diameter is somewhere between 8 and 21 miles (13-34 km); Styx is even smaller at 6-15 miles (10-25 km). In early 2013 Mark Showalter, senior space research scientist at the SETI Institutewanted the public to be involved in the naming of the new moons, so he started a contest.

To be consistent with the names of the other Plutonian satellites, they had to be picked from classical mythology and reference the mythological underworld where souls traveled in the afterlife. Pluto’s deep, dark location has always given it underworld credentials.

Detail of Roman sculpture from 180 A.D.showing the three-headed dog Cerberus. From the Archaeological Museum in Crete. Credit: Tom Oates

None other than William Shatner himself, a.k.a. Captain Kirk from Star Trek, suggested Vulcan. The response was overwhelming with almost 500,000 votes and 30,000 write-ins. When the votes were tallied Vulcan handily took 1st place followed by Cerberus and Styx. Cerberus was changed to Kerberos, the Greek spelling of the word, to avoid confusion with the asteroid 1865 Cerberus.

Styx fit perfectly since this goddess ruled over the underworld river of the same name. Likewise Cerberus, the multi-headed dog that guarded the entrance to the underworld. But what about Vulcan? As the ancient Roman god of fire there’s no question about its mythological pedigree, but the IAU had some issues.

First, Vulcan had already been used as the name of a hypothetical planet 19th century astronomers hoped to find between the orbit of Mercury and the sun. Second, the current term ‘vulcanoids’ refers to hypothetical asteroids that orbit inside the orbit of Mercury. Finally, Vulcan simply doesn’t fit the underworld scheme.

You can’t get EVERYTHING you want. Showalter is “grateful to the IAU for giving such careful consideration” to the public’s suggestions. I’m thrilled that we can finally address all members of Pluto’s family by their first name.


14 Responses

  1. Edward M. Boll

    After hearing Clyde Tombaugh in person, I have never liked that Pluto was demoted as a major planet. It was considered one so long and with all those moons. I mean there is no one living anymore that heard the discoverer of Neptune in person.

    1. astrobob

      No doubt about it, Pluto’s a sentimental favorite, and I understand exactly how you feel. I suspect the “planet definition” will change again in the near future. Wish I could have heard Clyde speak. Lucky you!

      1. Edward M. Boll

        Yes, I was in the audience at the University of MN on APRIL 27, 1990 when Clyde Tombaugh told every one there that he was a plutocrat.

      2. Pluto’s planet status is supported by a lot more than sentiment. Scientifically, the geophysical planet definition, in which a planet is defined as any non-self-luminous spheroidal body orbiting a star, is just as legitimate as the dynamical one adopted by four percent of the IAU, the one that requires an object to “clear its orbit” to be considered a planet. Astrobob, I sincerely hope you are right that the planet definition will be changed and broadened to include dwarf planets and exoplanets. I would expect any definition to be changed many times to accommodate new discoveries.

        1. astrobob

          After hearing Alan Stern out on this one, I agree with him that the definition of a planet needs to change to become more inclusive. My favorite thing he mentioned was about Earth. If Earth were at Pluto’s current position, it would not be considered a planet since it wouldn’t be able to effectively “clear” the vast amount of orbital space available to a body at that distance from the sun.

  2. Joel Willman

    I guess when a planet is named after a Disney character, a moon named after a 1980’s Hair band is acceptable. I liked Vulcan better, though.

  3. Troy

    I’m glad they listened to Mr. Spock (as in logic) rather than Capt. Kirk. It didn’t seem all that fair for Shatner to put his thumb on the scale along with the other issues with Vulcan that you mention. It is likely Disney named Pluto the dog after the planet and not the otherway around. One could argue a dwarf planet is still a planet. I think the IAU decision on Pluto was a good one, likely to stand. Keep in mind Ceres, Pallas, Juno, and Vesta were all considered planets until about 40 years later in 1845 when Astraea was discovered. The parallels to Eris are very stark. They have to keep out the riff raff (though of course Pluto is still very cool!)

    1. The problem is, the current IAU definition states that dwarf planets are not planets at all. This makes no sense and was not the intention of Dr. Alan Stern, the scientist who first created the term dwarf planet. The analogy with Ceres, Pallas, Juno, and Vesta is flawed for one reason: 19th century astronomers’ telescopes could not resolve Ceres into a disk and therefore did not know it is spherical and therefore rounded by its own gravity. That makes it a small planet, not an asteroid, a complex world with geology and differentiation. Now that we know Ceres is spherical, we know that demotion was wrong and should be overturned. Vesta and Pallas are intermediate cases because they are close to being in hydrostatic equilibrium but appear to have had part of them knocked off during an impact. The Dawn mission showed Vesta to be a very complex world that some scientists describe as “the smallest terrestrial planet.” Many believe there should be a middle category between asteroid and dwarf planet for Vesta and Pallas, something like protoplanet or sub-dwarf planet. These objects are definitely different from the tiny asteroids, which are simply rubble piles of rock shaped only by their chemical bonds.

      1. astrobob

        Scary to think about adding yet another term but maybe for precision’s sake, assuming ‘planet’ itself is redefined, sub-dwarf planet would work.

      2. Troy

        Ceres, Pallas, Juno, and Vesta were considered planets for 40 years though. Asteroid is probably the worst of terms as it was Herschel’s condescending description of the new planets (it means star like refering to their lack of disc in a scope). Calling them asteroids is right up there with calling Uranus “George” When Astraea was discovered I think it became clear to 19th century astronomers the asteroids were a group that hadn’t cleaned up their neighborhood, which is essentially how IAU defined it. Planetary definitions are a bit like arguing how many angels can dance on the head of a pin. I sometimes think if there were Jovians they’d only have 4 planets, it is rather arbitrary. That said I’d be disinclined to add a category. The big asteroids are special to those in the know. I agree with you, Dawn’s Ceres encounter will be utterly fascinating. I’m sure it will bring new respect to the dwarf planets.

        1. astrobob

          Hi Troy,
          I like your Jovian perspective. Yes, to Jupitereans all the inner planets would look like “asteroids”. Perhaps a broader definition of a planet might include any body differentiated into core-mantle that wasn’t an orbiting moon. That way the number of new planets would be manageable and a little more intuitive.

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