Wednesday, August 16, 2006

The Twelve Planets

When I was young and foolish (14 or so) I dabbled a little in Astrology. The book I learned Astrology from (whose title I have forgotten) had open spaces for 3 more planets: Vulcan (the hypothetical planet between Mercury and the Sun, not the Star Trek one.), Planet X and Planet Y (which were hypothetically beyond Pluto.) I learned enough to reject Astrology and pretty much lost interest in hypothetical planets (While extra-solar planets were quite another matter.)

Now it does appear that there are 12 planets because the term planet now has a more precise definition.

"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 still no Vulcan inside of Mercury's orbit, or it is quite unlikely but now the asteroid Ceres could be re-promoted to full planet status, as would Charon (Pluto's moon because Pluto and Charon are so close to the same size that they orbit a common point between them rather than one orbit the other so they are technically twin planets -- by this definition.) The twelve planet is Xena, previously known as 2003 UB313.

The article correctly points out that our moon is larger than 3 of these planets (and Jupiter's moon Ganymede is larger than Mercury) and they are round so should they be considered planets too in spite of being satelites?

I have a feeling this isn't going to stand long and they will be redefining again.


Anonymous Anonymous said...

Well I'm not real wise about astrology. I have interest in it though. If a satelite is bigger than a planet then why isn't called that? I'm not sure what that is based on...gases, matter, etc.I mean whats the critera? Seems if it's bigger it should be a planet. I dunno but this was an interesting post.

11:04 PM  
Anonymous Anonymous said...

As you point out yourself, the reason Charon is included is because of the double-planet thing. However, orbit trumps size in the proposed definiton, so even an object larger than Earth would be called a moon if it was in orbit around something the size of Jupiter. I'm not sure why some people are stuck to the idea that size must be more important than the orbit.

9:00 AM  
Blogger Lone Pony said...

Everybody is talking about this and I haven't had time to read much. Using the IAU committee's definition, in our solar system, there are 53 known planets and will be many more. What? Did they call it a big 'ol mess? lol

8:59 PM  
Blogger Patrick Joubert Conlon said...

I like the idea if calling those smaller than Pluto plutinos as has been suggested. They aren't satellites of a planet like our moon.

2:15 PM  

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