Two years ago the International Astronomical Union (IAU) angered a lot of people when the Solar System’s ninth planet Pluto was demoted and designated a “dwarf planet” along with the asteroid Ceres and the recently discovered trans-neptunian object Eris. Well, the IAU has now come up with an official designation for Pluto and Eris that is almost guaranteed to upset people all over again. While some may think it’s a silly exercise to spend so much energy coming up with classification schemes (years ago the physicist Enrico Fermi complained about the number of sub-atomic particles saying “If I could remember the names of all these particles, I’d be a botanist.”), it is important for scientists to make classifications of the objects they study. The Greeks used the term “planetes astrum” to describe the wandering stars—as opposed to the “fixed” stars—but that definition did not really distinguish the actual planets from the Moon and Sun. But “Plutoid?”
The IAU Committee on Small Body Nomenclature defines a plutoid as “celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit.” For naming purposes, any body that meets these criteria and have an absolute magnitude brighter than H = +1 where H is the magnitude of the planet, asteroid, comet, etc. at one Astronomical Unit from the Sun.
In the above image, Pluto and its three moons are shown on the left while Eris is on the right.
Image Credit: IAU, NASA/ESA Hubble Space Telescope, H. Weaver (JHU/APL), A. Stern (SwRI), the HST Pluto Companion Search Team and M. Brown.
