The Solar system - The IAU giveth, the IAU taketh away

**_Butler_** · August 18th, 2006, 02:11 AM

I'm surprised this hasn't been mentioned already, but then again, my search-fu is weak so it might have been.

Over the past week a commitee of the International Astronomical Union (IAC) have been meeting in Prague to come up with an official definition of a planet. The idea was to finally resolve whether of not Pluto was a planet or just a sizable lump of rock.

On Wednesday(-ish) they released their proposal. It's based on the gravity of the object in question and basically says a planet is round and orbits a star (and not another planet, 'cause that would make it a satellite).

This leads to their recommendation that the Solar system be increased to 12 planets.
The new ones being Ceres, largest rock in the asteroid belt; Charon, Pluto's 'moon'; and UB313 (aka 'Xena'), which is beyond Pluto. The interesting thing is that Pluto and Charon form a 'double planet' where the centre of rotation is between the two, so neither is the 'main' planet.

There are those opposed to the idea, including the discoverer of Xena, as it opens up the possiblility of tens, if not hundreds, of planets eventually being classified in our system, including several others in the asteroid belt.

http://www.space.com/scienceastronom...efinition.html

NEWSFLASH:
It now seems as though in the next general assembly of the IAU that Pluto will be/has been dropped as a planet and be reclassified, along with Ceres and Xena, as a 'dwarf planet'. But then, the way these astronomers are acting, it could possibly change again.

**Interceptor** · August 18th, 2006, 02:52 AM

I was just watching a bit about this on the Colbert Report.

Interesting stuff. There's going to have to be alot of new textbooks ordered for the kiddies

**Jack the R** · August 18th, 2006, 05:45 AM

Intrigueing. I didin't realize Ceres had formerly been classified as a planet. Or that it might have more fresh water (as ice) than Earth! Or that Ceres alone makes up 1/3 the mass of the asteroid belt.

Astronomy is brilliant.

**Prometheus|ANJ** · August 18th, 2006, 01:49 PM

I disagree with Pluto and Charon being a double planet. Charon is just 1/8th of Pluto's mass, so the shared gravity center must be pretty close to Pluto, but probably not inside its radius. I have no clue what arbitrary threshold they use to detemine double planet status.

**magicgoo** · August 18th, 2006, 01:58 PM

This is really cool! I'm usually at space.com all the time just 'cause I dig astronomy so much. Can't believe I missed this.

Also, I was really hoping they'd name the 10th planet something with X for 10 in Roman numerals. Xena will be an awesome name if it goes through!

**Prometheus|ANJ** · August 18th, 2006, 02:17 PM

Mmm... Xena and Gabrielle (her moon) would be neat. THis site states: "There is no chance whatsoever that these will become the permanent names of these objects!"

**Elwell** · August 18th, 2006, 02:33 PM

Originally Posted by Prometheus|ANJ

I disagree with Pluto and Charon being a double planet. Charon is just 1/8th of Pluto's mass, so the shared gravity center must be pretty close to Pluto, but probably not inside its radius. I have no clue what arbitrary threshold they use to detemine double planet status.

Probably exactly that: whether the orbital point is inside the radius of one of the bodies or not.

**Elwell** · August 18th, 2006, 02:39 PM

My very excellent mother just sent us nine... oh hell.

**0kelvin** · August 18th, 2006, 02:41 PM

Originally Posted by Elwell

Probably exactly that: whether the orbital point is inside the radius of one of the bodies or not.

Which means in a couple billion years the Earth and the moon will become a double planet. Not that we, or at least our silly definitions, are likely to be around then.

But this may explain why my horoscope is always wrong. Those astrologers haven't been taking these other planets into account!

0kelvin

**brokk** · August 18th, 2006, 03:41 PM

Originally Posted by 0kelvin

But this may explain why my horoscope is always wrong. Those astrologers haven't been taking these other planets into account!

Priceless

**Prometheus|ANJ** · August 19th, 2006, 06:04 PM

Some pics:
http://www.iau2006.org/mirror/www.ia...1_release.html

Faq:
http://www.iau2006.org/mirror/www.ia...NEWS.55.0.html

Q&A:
http://www.iau2006.org/mirror/www.ia...u0601_Q_A.html

**Helium Macaroni** · August 19th, 2006, 06:37 PM

I don't know if anyone has ever seen or used this application.. but!

http://www.shatters.net/celestia/

Its an interactive, space simulation that lets you navigate and view not only the earth, near earth objects and the local solar system, but also other stars, and the entire galaxy or beyond if you choose to install add-on packs. The zoom feature lets you 'travel' light years with just a mouse scroll, and you can type in familiar names, objects, or anything that has been discovered in astronomy and close in on them to view them in 3D. Actually this little app gave me quite a sense of vertigo and.. well whatever the reverse of claustrophobia is.. while I was tooling arond the orion arm of our galaxy.

It's really useful for developing interstellar type stories too, hehe. Also useful for painting space scenes using familiar planets.

**Prometheus|ANJ** · August 19th, 2006, 07:04 PM

Astronomy programs are fun, I wrote my own solar system formation thing a while ago... well, it's very simplified 2D. It's simulates bodies coming together to form larger bodies, while being affected by gravity.

I sent IAU these questions, but I'm not sure if their email form was working.

If a body is orbiting the barycentre of a double planet, it is orbiting a point in space. How will you then determine planethood for that body? Add the double planet masses together and work out the radius?
Will IAU also set thresholds for what constitutes a moon? Are Phobos, Deimos or Prometheus moons for example?
Is Hal Clement's "Mesklin" a planet? It's spinning so fast it's oval and not round.
Since the sizes of solar system bodies are analog, isn't it an arbitrary choice to put a digital threshold for planethood? Personally I would favor a culturally defined threshold, such as using the radius of Mercury. Then I would use the term planetoid for objects down to Your suggested 'roundness' threshold, and go for asteroids, rocks, pebbles and dust after that.

**_Butler_** · August 21st, 2006, 12:17 AM

Thanks for the link Helium. That thing's rather snazzy. There goes the rest of the afternoon.

**Elwell** · August 24th, 2006, 04:13 PM

Probably for the best anyway:
Astronomers strip Pluto of its planet status

My very excellent mother just sent us... nothing.

**gaijin** · August 24th, 2006, 05:46 PM

It's kind of wierd but I'd rather it be eight than twelve+. Sorry Pluto... welcome to the dwarf planet league.

**0kelvin** · August 24th, 2006, 07:55 PM

Originally Posted by gaijin

Sorry Pluto... welcome to the dwarf planet league.

I believe the politically correct term is little people planet.

0kelvin

**Rascar Capac** · August 24th, 2006, 10:07 PM

did you say...the dwarf planet league?

**jfwalls** · August 24th, 2006, 11:13 PM

I refuse to accept that Pluto is not a planet. In my mind there will still be 9 planets. The IAU can go f**k themselves.

**magicgoo** · August 24th, 2006, 11:25 PM

Eh, Pluto was a lump of freakish oblong rock, anyway. And its orbit totally deviated from all of the cooler, bigger, realer planets. It's just not like the other cool kids.