The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

The "dwarf planet" Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects.

Update: Matthew Yglesias puts on his old fuddy-duddy costume and complains:

I think this is silly. If astronomy has been proceeding since Copernicus without a rigorous definition of "planet" -- which is certainly my understanding -- then obviously the world doesn't need a rigorous definition. It's just a folk-cultural term and it denotes nine entities, one of which is Pluto. There's no need for a bunch of busybody astronomers to make trouble for everyone else.

To which I respond:

Your conclusion doesn't follow from your premise. Science is not in need of a definition of what a planet is or isn't, and the new definition, for all its being framed in scientific terms and voted on my scientists. doesn't flow from any scientific theory that I'm aware of.

However, it was science and scientists (or natural philosophers) who came up with the previous definition of a planet, the one that's got the inertia of cultural baggage you refer to, and Pluto was accepted as one because of that. Sure, everyone eventually became comfortable with nine planets instead of eight, but I'm sure some dunderheads complained at the time that there was no need to upset the well-known system with the brash newcomer.

So science made the old definition, and science deteremined that Pluto fit that definition and added it to the roles when it was discovered, so how does the years of cultural acceptance of that circumstance add up to it not being up to scientists to revise the definition once a better understanding of Pluto called for a change?

It doesn't. A planet is indeed a cultural thing, but it got that way because science defined it that way, and science can undefine it if it suits their purposes -- not every scientific term is supported by strict theoretical framework, sometimes it's all just ad hoc until someone ties it all together.

This ranks right up there with your complaint about the names of foreign cities (here -- and my response here) -- such curmudgeonly behavior in one so young is not terribly becoming.

Update (8/29): Just for the sake of completeness, and defintiely not worthy of a new post: the demotion of Pluto causes Yglesias to become completely unhinged.