In the new solar system, there are eight planets, at least three dwarf planets and tens of thousands of so-called “smaller solar system bodies,” like comets and asteroids.
For now, the dwarf planets include, besides Pluto, Ceres, the largest asteroid, and an object known as UB 313, nicknamed Xena, that is larger than Pluto and, like it, orbits out beyond Neptune in a zone of icy debris known as the Kuiper Belt. But there are dozens more potential dwarf-planets known in that zone, planetary scientists say, and the number in that category could quickly swell....
According to the new rules a planet meet three criteria: it must orbit the Sun, it must be big enough to gravity to squash it into a round ball, and it must have cleared other things out of the way in its orbital neighborhood. The latter measure knocks out Pluto and Xena, which orbit among the icy wrecks of the Kuiper Belt, and Ceres, which is in the asteroid belt.
Memorization of the solar system for the world's second graders just got 1/9th easier, a concept they can appreciate once they master fractions a year or two down the road.