And then there were 12

[mdlbear]

The committee of the IAU that was studying Pluto's status has finally announced its new definition of "planet" (it will be voted on next week). It's simple, straightforward, sensible, hard to argue with -- and it would mean that the Solar System will have 12 known planets.

According to the IAU's press release (thanks to this article in Scientific American's newsblog for the link) the definition reads:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

There is also a new category of "plutons" -- small planets that orbit with periods of more than 200 years. There are three of them so far, with more expected.

So here's the list you've all been waiting for:

1. Mercury
2. Venus
3. Earth -- the moon is small enough to be considered a satellite
4. Mars
5. Ceres
6. Jupiter
7. Saturn
8. Uranus
9. Neptune
10. Pluto and Charon (tied for 10th place)
11.  
12. 2003 UB313 (Xena)

Comments

[vixyish.livejournal.com]

My Very Excellent Master of Ceremonies Just Served Us Nine Potato Chips. eXtraordinary!

Okay, I got nothin.

I'd never heard of the mnemonic until well into adulthood anyway, though...

[mdlbear]

Nor had I. I don't remember a time when I didn't know the planets; I may have learned the list before I could read.

And that's just for now

[capplor.livejournal.com]

The article I read mentioned 43 other known "objects" that may qualify as planets when they get more data.

Sounds like the old joke about memorizing the names of the US presidents -- "Yea, Dad, but when YOU were a kid there were a lot fewer".

[lapislaz.livejournal.com]

Are they considering that Pluto and Charon are orbiting each other, rather than Charon orbiting Pluto? Because that list doesn't make sense otherwise...

[patoadam]

from Wikipedia:

Charon has been a part of the controversy over Pluto's status as a planet. Under the latest proposal, which will be decided on August 24, 2006, the International Astronomical Union may classify Charon as a pluton, officially making Charon a planet. Under this proposal, Charon would be considered a binary planet with Pluto since the two orbit each other around a center of mass that is outside either body.

[mdlbear]

Exactly: if the barycenter is inside one of the bodies, it's the planet and the other one is the satellite.