Crapweed, Stenchblossom, Rose....

"Just as a Chihuahua is still a dog, these ice dwarfs are still planetary bodies," said Alan Stern of the Southwest Research Institute in Boulder, Colo., the [New Horizons Pluto] mission's principal investigator. "The misfit becomes the average. The Pluto-like objects are more typical in our solar system than the nearby planets we first knew." (Trip to Pluto to Take at Least 9 Years)

Is Pluto a planet? That's a point of contention. But this cannot be disputed: Pluto does not 'fit' with the physical distribution of the eight undisputed planets: the other four rocky planets comprise the inner solar system, and the four gaseous planets the outer solar system. Whatever the processes of solar-system formation, it does seem that rocky-planet formation favored the inner solar system. Not that rockiness was prevented elsewhere, just that rocky planetness seems to have been. (Pluto's presence was hypothesized from orbital irregularities in the other planets. As it happened, Pluto's postulated mass as discovered was insufficient to explain all of the irregularity. The subsequent discovery of Pluto's satellite Charon means that Pluto's mass is even less than was then thought. Ergo, there's something[s] else out there [probably Kuiper-Belt Objects {KBOs}].) Pluto's orbital inclination (17°) is more than twice that of the next-most-inclined planet's (Mercury, 7°). It also has the largest orbital eccentricity (0.25, with Mercury again having the next-most eccentric at 0.21. All other planets' eccentricities are >0.1).

There are all sorts of planetary attributes at which one might look, but these seem to be the most salient in terms of formation, and that seems like it really should be the principal consideration with respect to planethood. I considered axial tilt, but that seems to be subject to too many other influences (Mars' axial tilt is thought to vary substantially, for example); Venus and Uranus both have unusual axial tilts as well as Pluto, and it seems unlikely that the same event caused them all.

So Pluto is different — from the rest of the planets. And this is where Stern's comment is significant, because planets are strongly atypical solar system objects. It is their difference from common solar-sytem bodies that makes them special. The planets are sufficiently different that the best operational definition of the category "planet" in terms of solar-system objects seems to be "large Other". So if Pluto is not atypical, then it is not a planet.

Pluto is not a small statistically-outlying planet. It's a big statistically-outlying KBO. (And I realize that Mercury's formation-related attributes are well in-between Pluto's and the rest's. To my mind, Mercury illustrates the bottom range of planethood.)

FWIW, I have a fondness for a 9-planet system. Pluto the Planet is familiar. But Stern's comment flipped me.