Just as we were getting used to the idea that Pluto was not a full-fledged planet, but rather a "dwarf" planet, the International Astronomical Union has come up with a new twist.
The IAU, which is the internationally recognized authority for naming and categorizing celestial objects, has created a new family of dwarf planets called "plutoids." And the first two (and so far only known) members of the family are Pluto and Eris.
Never heard of Eris? No wonder. It was discovered just five years ago by a team at the Palomar Observatory, and named for the Greek goddess of strife and discord. It's orbiting the sun three times farther out than Pluto. Astronomers estimate it is about 2,500 kilometers in diameter, and 27 percent more massive than Pluto. That makes it the ninth largest object known to be orbiting the sun. Eris (pictured in an artist's rendering at left) has a satellite of its own, dubbed Dysnomia, the daughter of Eris and demon of lawlessness. Nice.
Anyway, the IAU has decided than any object that's orbiting the sun beyond the orbit of Neptune, and large enough for its own gravity to pull it into a spherical shape, will henceforth be classified as a plutoid. And while there are just two such objects so far - Pluto and Eris - scientists are confident they will find more.
Telescopic searches and the voyage of NASA's New Horizons spacecraft will be looking. New Horizons (at right), built and managed at the Johns Hopkins University's Applied Physics Lab near Laurel, is due to fly by Pluto in 2015. From there it will cruise out into the icy regions beyond, where more plutoids are likely lurking.
The solar system's only other known dwarf planet is the asteroid Ceres. It's big and spherical, too, but because it orbits the sun in the asteroid belt between the orbits of Jupiter and Mars, it's not a plutoid. Astronomers believe Ceres is the only object of its kind, so it likely will not be categorized in any other way.
The term "plutoid" was coined by the IAU's Committee on Small Body Nomenclature and accepted by the IAU Executive Committee meeting in Oslo, Norway.