I was only thinking about Pluto/Charon style double planets, but you guys astutely pointed out some quirky outlyers. I was only thinking of 2 objects orbiting a common point which in turn orbits a star. Something that travels concurrently in a lagrangian point or in one of those strange gravitationally bound yet not elliptical orbits was off my radar. Either way I would think that their mass has to be small to remain stable in such orbits which would make them clearly not a double planet. Am I missing something or were you guys just ponting out that such objects need to be specifically excluded?

I left out the definition of a brown dwarf as there seems to be some sort of debate on this that I don't get. It wasn't all that relevant either because no one is proposing that we wouldn't count high mass planets as planets because there would be too many of them. Deuterium burning seems like an obvious and major transition point. Fusion is characteristic of stars not planets, but it's not the same type of fusion that predominates in normal main sequence stars. Is the question at what mass this actually happens, or is the line actually fuzzier depending on characteristics other than mass?

I would guess that if you plotted minimum diameter to form a round object versus density you would have a fairly flat line starting at a density of one until some point past the density equivalent to 50:50 ice/rock by volume (a density of about 2.0) which equates to 25% water by mass. Up to that point I would think ice would easily fill all void space and basically form the weak link in structural strength so that there would not be much increase in size needed to form a round object. Either at that point or somewhere past it I would expect to see a sharp change in slope of minimum radius for a round object to density to allow a transition to the value for pure rock. It is that inflection point that I would use as a boundary marker between icy dwarf and rocky dwarf. Anything near that transition point would have to be visited before we would know for sure which way to classify it.

Thinking about Vesta more what should we call it if it turns out to be the shattered remnant of something that was previously big enough to be a terrestrial planet yet it is no longer round? This seems to me to be the easiest explanation for data indicating geochemical alteration of meteorite fragments from Vesta yet a current size that would seem to preclude that. Seems like a special case that could be handled with a term like "planetary fragment". Don't think we'd need a special term for a shattered dwarf planet as we couldn't really tell that happened if no active geology ever took place.

Regards,
Mark