Ceres is a planet! Options

[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.

[volcanopele]

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).

[Holder of the Tw...]

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).

Good one.

[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

[dilo]

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

[abalone]

A very sensible compromises

[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.

[ugordan]

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.

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

[ngunn]

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

[abalone]

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.

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??

[ugordan]

Also does not the barycentre depend on the distance of separation of the two bodies?

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.

[abalone]

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.

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.

[Rob Pinnegar]

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.

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