The Solar System now has 12 planets (and counting!)

[reds4ever]

http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html

New (proposed) definition of a planet:

The part of "IAU Resolution 5 for GA-XXVI" that describes the planet definition, states "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." Member of the Planet Definition Committee, Richard Binzel says: "Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet."

I have to admit I'm surprised I thought this decision was going to go the other way and strip Pluto of its planetary status. That definition means, in addition to Pluto and the other distant candidates, Charon (Plutos moon) and Ceres (an asteroid orbitting inbetween Mars and Jupiter) are now planets.

[Blaupanzer]

How did Charon qualify? It seems to violate the "b" part of the definition. It's not the biggest moon in the solar system, is it?

[reds4ever]

Ooops, missed that!!

[Lorizael]

This, I think, is actually a useful definition for a planet. I doubt this silly debate is over, but I think this is a step in the right direction.

[reds4ever]

I was all for Pluto being stripped, leaving us with 8 planets but thats a good definition.

"Plutons" seems a bit artificial and spurious though.

[Japher]

you're just racist!

[reds4ever]

Originally posted by Blaupanzer
How did Charon qualify? It seems to violate the "b" part of the definition. It's not the biggest moon in the solar system, is it?

Just checked, it maybe because the barycenter of the 2 bodies has its center of gravity outside the sphere of the primary member. This makes it a double planet and both members are named as planets. If the COG was within the sphere of the primary member then the secondary member would be defined as a satellite instead. Although by this rule our own moon will be a planet in about 40 million years

[Lorizael]

Damn straight. Pluto has to sit at the back of the bus.

[Colonâ„¢]

Jupiter is like the US of the solar system.

[Proteus_MST]

Interesting,
so (according to the drawings) Ceres,
the largest member of the asteroid belt
will get planetary status, too.

[reds4ever]

Yep, its massive enough to form a (flat) sphere and doesn't orbit another planet.

[Dinner]

Bah! There are only 8 real planets.

[Az]

Personally, I think that saying that the satellites of planets are not planets just because they're circling other planets is silly. Or why does it have to orbit a star? why is a star defined outside of a context of a solar system, while planets aren't?

[Odin]

Originally posted by Oerdin
Bah! There are only 8 real planets.

[Colonâ„¢]

I think we should just call anything that's round a planet.