Pluto... smaller than our own moon, orbiting with a bunch of other detritus (Orcus, Ixion), can't even run around with it's own moon (Charon) without the [strike]bathycenter[/strike] barycentre (center of gravity) being outside it's sphere (Pluto and Charon pirouette around a point in empty space)...
...not very planet-like to me. Now, if Triton hadn't been captured by Neptune, there likely would have been a mercury-sized ninth planet by now because it's likely large enough to gather all the plutinos (pluto orbit objects).
Then there's Sedna and 2005UB313, but I digress...
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edited for brain fart, bathycenter means nothing but would be "center of ocean".