Your Own Personal Solar System

While I feel you with the Historical Precedent Choice, I'm open to 10. Eris might make a fine planet as well.

Although Dave Matthews might have to change the lyrics to Typical Situation.

I'm sorry, but "The Nostalgia Factor" (as well as the "Historical Precedent") is based on pure emotion. That is simply not science. People are trying way too hard to keep Pluto as a planet simply because that's what they learned in school, therefore it should not change. Readers of Science Blogs should know better.

I think the problem is pretending that people have a vote here. The definition of a planet should be one that professionals in the field find useful. If that means we have 8 planets, than that's what we have. I can see good reasons for keeping Pluto out of the group and little but nostalgia for keeping it in.

Nostalgia is an awful reason to include Pluto. If you must include Pluto, at the very least go with historical precedent. That said, why is the current answer so bad? I'm OK with the idea of Pluto not being a full planet.

Since when did 78 years become an interesting historical precedent? If you really want history on your side, you would reject anything past Saturn, for those other new-comers aren't visible to the naked eye. And even after the invention of the telescope, it took another nearly 200 years to find Uranus.

Shall we resurrect other bits of scientific flotsam and jetsam that enjoy "historical precedent:" classifications of humans into groups of better and worse (classifications that had the best science behind them at the time); phrenology; Aristotelian physics; Ptolemaic astronomy (where's the "let's revive the sun as a planet" movement?).

On 1 January 1930 Pluto was not a planet. On 12 January 2008 Pluto is not a planet. Get over it.

I would go with the "nostalgia" approach to a point, but instead call the nine planets the "classic" planets, thereby including Pluto.

For the rest, I would opt for a not-a-moon, hydrostatic equilibrium (spherical by own mass/gravity) approach, without the "clearing of the neigborhood" (takes too long for far-away-from-the-sun planets, even small gas giants), but with a minimum density-limit for non-gas-giant planetary bodies (i think no one would dispute the plantary status of those, eh?). The density should be at least a bit above the density of water ice (to exclude cometary bodies), maybe set at 1.5 g/cm³ or higher.

Frankly, as far as I'm concerned, a dwarf planet is still a planet. I mean, a dwarf star is still a star, right? And what about all of those moons? Technically, they should cound as dwarf planets too ("planet" and "moon" don't need to be mutually exclusive). If either Titan or Ganymede were in a heliocentric orbit they'd be accepted as planets without question. IMHO, the definition should be based on mass and not orbital characteristics -- the same way stars are defined.

So I suppose I'd go with the "No planet left behind" option, in a sense...

There's one problem though. Haumea. It isn't spherical. Supposedly, even the IAU's definitions require objects to be spherical under their own gravitational force -- which makes Haumea something of an oddball (if you'll pardon the pun).

Highly recommend the short story by John Scalzi, Pluto Tells All, at
http://subterraneanpress.com/index.php/magazine/spring2007/fiction-plut…

If you really want history on your side, you would reject anything past Saturn,
Hear hear! If the ancient greeks-and-romans knew about it, it's a planet. Otherwise, it's a solar system object thingy.