Five years ago yesterday, the International Astronomical Union demoted Pluto to its “dwarf planet” classification. Here at the Wetherbee Planetarium, that is the single most-heard question we are asked: why? Was something discovered about Pluto that made astronomers second-guess it? Did something happen to it? Not at all. It is the other things around Pluto that made the IAU change their minds about what they classify as a planet, though.
As of August 24, 2006, the definition of a planet by the IAU is: “A body that circles the sun without being some other object’s satellite, is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star) and has “cleared its neighborhood” of most other orbiting bodies.” On the first point, Pluto does orbit the sun. On the second point, as far as we can tell it is relatively round in shape. When NASA’s New Horizons probe cruises by in 2015 we will have close up images of the cold planet for the first time in history and we will know for sure what it is like. On the last point, however, Pluto is not exactly a space hog. It rubs elbows –figuratively speaking—with several other objects in our solar system in a region designated as the Kuiper (pronounced ky-per) Belt, some of which are about the same size as Pluto.
Pluto is only about 1,455 miles in diameter and only 0.2% as massive as Earth. It is tiny, that much is certain. But, it does have at least four known moons: Charon, Nix, Hydra, and the recently discovered P4 (a dismally boring name, we know—NASA presumes will probably be renamed Cerberus in the near future). In orbit around the sun with Pluto are several other objects about the same size as or smaller. Eris is one such object, and was the turning point discovery that led the IAU to re-think their definition of a planet. Another is Haumea, the rapidly spinning dwarf planet that astronomers guess may have radioactive elements in its core. Another is Makemake (pronounced mah-kay mah-kay), a dwarf planet about ¾ the size of Pluto.
Dwarf planets are not just limited to the Kuiper Belt, though. Ceres, another such object, is actually orbiting in our asteroid belt, between the planets Mars and Jupiter.
Each of these objects misses the criteria for a planet in some way, usually in clearing its neighborhood of other objects. They are massive enough to form into relatively spherical shapes and they do orbit the sun, they just usually are not really big enough to push their way through the crowd and clear their own path.