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Posted by: Holder of the Two Leashes Aug 16 2006, 03:31 AM

The fifth planet in the solar system is ... CERES!

So says the draft from the IAU working committee that is trying to settle the question of the definition of a planet. And the agreement on the committee was unanimous.

I knew it. I knew it all along. First time I ever saw Ceres I thought "Yep, that's a planet". Although admittedly my trusty astroscan couldn't quite resolve it.

I would have fought for Vesta and Pallas, too. But this is good enough.

Oh, incidently, Pluto is too. So is 2003UB313.

Well now, that should give a little extra insurance to the DAWN mission getting of the ground.

This all has to be voted on by the whole body, but once again, the committee was unanimous, all seven members. Anything round by gravity, roughly above 850 km diameter.

Posted by: volcanopele Aug 16 2006, 03:47 AM

Sweet. Would have been better had Dawn already launched. Dawn: The Mission launched toward an asteroid, but arrived at a planet (kinda like that forgettable Hugh Grant movie).

Posted by: Holder of the Two Leashes Aug 16 2006, 03:57 AM

Good one.

Posted by: ElkGroveDan Aug 16 2006, 05:27 AM

Did they mention that it had to be orbiting the Sun? Lacking that, we'd have a whole lot of satellites that will now become planets; Luna, Io Europa, Gannymede, Callisto, Dione, Titan, Iapetus, Areil, Umbria, Titania, Oberon, Triton and Charon.

Posted by: mchan Aug 16 2006, 05:34 AM

Yes, a planet may not orbit around another planet...

http://www.spacedaily.com/reports/The_IAU_Draft_Definition_Of_Planets_And_Plutons_999.html

Posted by: dilo Aug 16 2006, 06:20 AM

Wow, 12 planets (up to now)!
I will drink a Ceres to celebrate the "new", italian-discovered planet!!!

Posted by: abalone Aug 16 2006, 07:54 AM

A very sensible compromises

Posted by: remcook Aug 16 2006, 08:08 AM

...except charon

"If the proposed Resolution is passed, the 12 planets in our Solar System will be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313"

Posted by: paxdan Aug 16 2006, 09:37 AM

the barycentre of the Pluto-Charon system is above the surface of Pluto, hence the Pluto-Charon system is a double planet, not a planet-moon system.

Posted by: ugordan Aug 16 2006, 10:45 AM

Geez... So, you get two planets for the price of none (for those of us who argued Pluto isn't a planet)!?

Posted by: ngunn Aug 16 2006, 10:51 AM

What rule excludes the other larger KBOs and Sedna? Is it just a question of illumination?

Posted by: abalone Aug 16 2006, 11:21 AM

Is that really the definition of a double planet system? Charon is only 11% of the mass of Pluto.

Also does not the barycentre depend on the distance of separation of the two bodies? This would mean that it is a moon if separated by "x" and a double planet if separated by another larger distance "y", surely not!!!

Charon is a moon??

Posted by: ugordan Aug 16 2006, 11:35 AM

In principle, yes. The ratio of the distances from the barycenter to each body is proportional to their mass ratios (or something like that). So you could have a scenario where a massive-but-close-in moon is not a double planet, but a less-massive-and-far-out moon is. It depends upon the radius of the primary body (and hence its density) and also upon the separation distance. Frankly, that's why I don't think this is a neat clasification.

Posted by: abalone Aug 16 2006, 11:53 AM

This would mean that as our Moon recedes from the Earth over the next several billion years our system will become a double planet system when the moon's orbital radius get to about 550,000 km, and be reclassified as a planet.

Posted by: Rob Pinnegar Aug 16 2006, 01:25 PM

The IAU can cross that bridge when we come to it.

Posted by: dilo Aug 16 2006, 06:39 PM

Moreover, extending this criteria, the Sun is a double star because Jupiter-Sun baricenter falls slightly outside photosphere (and remember that Jupiter is a failed star...)
The baricenter positions seems a neat "objective" criteria, but it isn't. I think a mass/size ratio between the two bodies is better, even if somehow arbitrary...
I would like to know how the IAU commission concluded that Pluto-Charon is a double.

Posted by: volcanopele Aug 16 2006, 07:55 PM

Jupiter is not quite big enough to be considered a "failed star". It would need to be quite a bit bigger.

Anyways, regarding the Pluto-Charon double, see my new post in the Planet definition thread:

http://www.unmannedspaceflight.com/index.php?s=&showtopic=3064&view=findpost&p=64582

Posted by: Jyril Aug 16 2006, 08:02 PM

If 800+ km objects can become planets, it's possible that binaries comprising two such objects are found. Which one would be the planet? Also, there may be extrasolar giant planets that orbit each other (for example, a sub-brown dwarf pair with a few M_Jupiter each in a distant orbit around their star).

Since the discovery of Charon they are called a "double planet". Now, if the draft is adopted, it would be a double planet. Big difference.

Posted by: MarcF Aug 16 2006, 08:56 PM

All these solar system bodies are interesting, wether they are called planets or not.
May be I'm wrong, but I do not think that considering Pluto or Ceres (or others) as planets or not is so important. Pluto is interesting as Pluto (the same is true for all the other solar system bodies).
Pluto and Earth are completely different objects, but aren't Earth and Jupiter even more different ?
I think one solution would have been to focus the debate on a finer classification which is already used and might be updated (Earth-like or rocky planets, gaz giants, ice giants, outer solar system icy planets, ...), and not just focus on the simple term "Planet" which definition might still change several times in the future (for scientifical as well as political reasons).

Marc.

Posted by: alan Aug 16 2006, 11:32 PM

I don't believe Sedna and the others candidates's diameters have been determined precisely enough for the IAU to certain they meet the criteria. For example, if Sedna had the same albedo as Pluto it would be too small to meet the criteria.

Posted by: ngunn Aug 17 2006, 01:39 PM

Thanks, Alan. Presumably we don't yet know how 'round' they are either.

Posted by: punkboi Aug 24 2006, 05:57 AM

Ceres won't be a planet. What a letdown

Posted by: Myran Aug 24 2006, 03:12 PM

....nor Pluto.

Posted by: volcanopele Aug 24 2006, 03:57 PM

Nor Earth IMHO

Posted by: stevesliva Aug 24 2006, 04:00 PM

Of course, Earth is the center of the universe! Sort of like a fulcrum for the heavens to sit upon, rather than a planet.

Posted by: djellison Aug 24 2006, 04:02 PM

With the number of NEO's and Earth crossing ateroids, Earth certainly hasn't cleared it's neigbourhood.

SL9 tells us that Jupiter is still doing the job of clearing it's Neigbourhood.

And the existance of Pluto tells us that Neptune's not finished either. As Levy put it so well, SL9 reminded us that at the edge of the solar system is a large sign marked "Warning - Solar System under construction"...NOWHERE has a 'clear neighbourhood" at this time.

The 'neighbourhood' clause in the current description is the worst thought out description I've ever seen. Utterly shocking. They had a change to finally get this right....and they screwed it up.

Alan's right - "I'm embarassed for astronomy....it's a farce" I agree on both points.

Perhaps what the IAU have done, by using the Neigbourhood clause is actually, eliminate the term planet all together as one could aruge that none of them have finished clearing their back yards. Cunning eh

Doug

Posted by: Myran Aug 24 2006, 04:10 PM

A farce - well yes thats what the cartoonist appear to be saying.
Its Goofy that have gotten the role of a news anchor, which is a hint he means to say that the IAU goofed with the suggestion of adding new planets including Charon the moon of Pluto. And im happy that proposal went out the door. Even volcanopele could be happy, since it obviously is a Jupiter for a 'real' planet in that cartoon.

Posted by: Betelgeuze Aug 24 2006, 08:16 PM

...and again Ceres lost its planethood I was really hoping this lost planet would finally get some recognition. Maybe better next time because there will defenetly be a 'next time'.

Posted by: Jyril Aug 24 2006, 08:19 PM

Again? It is a dwarf planet now, not only an asteroid. I wouldn't call that demotion.