It's a planet! Wait, no it's not, but it is. Hold on, there are 12?

Remember the mnemonic you learned as a kid so you could remember the names of the nine planets? It was probably one of these:

"My Very Eager Mother Just Served Us Nine Pickles."

"Mother Very Easily Made Jane Stop Using Nail Polish."

"My Very Enormous Monster Just Sucked Up Nine Planets."

Well, it's time to unlearn them, because the nine-planet solar system everyone thought was set in stone after the discovery of Pluto in the 1930s just got tossed. Welcome to the age of the 12-planet solar system, thanks to the new definition of "planet" proposed at a meeting of the International Astronomical Union. They also propose to create a new class of mini-planet called a "pluton."

With the new definition comes the following changes:

1. Pluto gets downgraded from planet to pluton
2. Charon, Pluto's moon (more on that in a second), gets bumped up to pluton designation
3. Ceres, a really big asteroid in the belt between Mars and Jupiter, bypasses pluton-ness and goes straight to planet
4. The large body discovered in 2003 beyond Pluto, which has the melodic name 2003UB313 (unofficially dubbed Xena by its discoverer), becomes the third member of the plutons.

Pluto and Charon are seeing the biggest change in the org chart of Solar System, Inc. Charon gets promoted to independent status - a moon no more - because it is so heavy that the center of gravity between it and Pluto is smack in between them (meaning they revolve around a point in space). Contrast this with the Earth and our moon, where the center of gravity of the two-body system is in the Earth itself (so that the moon actually revolves around us). Thus, Pluto and Charon become the solar system's first double planet.

There may be more plutons on the way; if the definition is approved, then a host of other bodies in the solar system, including other asteroids and maybe even other moons, will earn the title. Meaning we could potentially end up with a solar system with hundreds of such mini-planets. And some really long mnemonics to remember them.

Oh, and the main criterion for joining the planetary club? Roundness.

[Read on at New Scientist to learn about the debate over the definition of planet. And many thanks to my father-in-law for the mnemonics.]

UPDATE: Turns out that if the IAU accepts the new planetary definition, things could get really weird in the solar system, with our moon even eventually qualifying for planethood. Tune back in in about, oh, 5 billion years.

ANOTHER UPDATE: Scratch pluton, make that "dwarf planet." And it's 8, not 12.