[X] My Assistant

[Holder of the Tw...]

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.

[ElkGroveDan]

Anything round by gravity, roughly above 850 km diameter.

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.

[mchan]

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

http://www.spacedaily.com/reports/The_IAU_...lutons_999.html

[remcook]

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

http://www.spacedaily.com/reports/The_IAU_...lutons_999.html

...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"

[paxdan]

...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"

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.

[dilo]

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.

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.