Today the International Astronomical Union (IAU) decided that Pluto will not be keeping its "planet" title. Instead, Pluto, as well as asteroid Ceres and the larger-than-Pluto 2003 UB313 are now classified as "dwarf planets." According to the IAU, a "planet" is rounded by self gravity and has cleared its orbital zone, a "dwarf planet" is rounded by self gravity but has NOT cleared its orbital zone, and "small solar system bodies" are not rounded by self gravity. Because Pluto is part of the Kuiper belt and Ceres is part of the asteroid belt, neither has "cleared" its neighborhood, so they don't qualify as planets.
From the Planetary Society blog: It's Official: Pluto is NOT a Planet.
As it turns out, the proposed resolution that made so much notice when it was presented to the public last week -- and which would have expanded the solar system to 12 planets -- passed, but it was a different draft of the document in that it added the third criteria of a planet having to clear its neighborhood around its orbit.
Also from the Planetary Society: Pluto Gets the Boot - Solar System Shrinks to 8 Planets.
They also seem to have updated their excellent article on 'the new solar system' to include a section at the bottom entitled, Eight is Enough:
Even though it deprives him of the honor of being the discoverer of a planet, Michael Brown [one of the discoverers of 2003 UB313] likes the IAU resolution. If aliens came upon the solar system, he suggested on his website, "they would quickly come to the conclusion that there are 8 major bodies orbiting the Sun." Isn't it time, he argues, that we did the same?
My gut feeling is that creating a new category of 'dwarf planets' is a very sensible way to divide things up. Of course, that's not to say that the given definition of a dwarf planet is especially solid. Bad Astronomer Phil Plait has some criticisms of it here.
Finally, in an article entitled What’s in a Name? Explore ALL Worlds, says The Planetary Society we read that:
In response, Louis Friedman, Executive Director of The Planetary Society commented:
"The classification doesn’t matter. Pluto -- and all solar system objects -- are mysterious and exciting new worlds that need to be explored and better understood. Anytime we visit a new world -- planet, moon, asteroid, comet, whatever -- we make exciting and surprising new discoveries about the evolution of our solar system and about our own planet."
Yes, my idol, and one of the founders of the Planetary Society, Carl Sagan described our solar system as having hundreds of worlds, and I like that definition best of all.