Grasping Reality on TypePad, by Brad DeLong

At http://delong.typepad.com/sdj/2006/08/a_modest_propos.html#comment-21343339 LowLife writes:

I have no idea why they seem to be bending over backwards to prevent the moon from becoming a planet. As it's bigger than Pluto why would it's sharing an orbit with the Earth disqualify it?

I had always wanted to live in a solar system with a binary planet. Please help make my dream come true!

Of course, Ganymede is a lot bigger than Luna... but IIRC, Ganymede's orbit is at times concave to the sun, while Luna's is always convex to the sun...