And this is a problem how?

You would have to come up with a new definition for what we currently call planets, since they are clearly different (more massive, more regular orbit, etc). It's more reasonable to call the newly discovered things something different instead of renaming old things.

I dilutes all the meaning out of the word "planet".

Words gain meaning not from what they mean, but from what they don't mean.

The mnemonic would become even more tortured than "My very excellent mother just served us nine pizzas"?

Mother very thoughtfully made a jelly sandwich under no protest.

(Terra and Asteroid Belt)

How long does it take to get to Pluto at a constant 8g, with a skew flip for deceleration? Don't use your slipstick.

According to Heinlein it was apparently 5 days.

Well, yes, that was the subtext for both my comment and (I believe) its parent comment.

FWIW, 2 * sqrt(32.6 AU / (8 * 9.81 m/s/s))) = https://www.google.com/search?q=sqrt%2820+AU+*+2+%2F+%288+*+... = 5.77 days.

Perihelion of 29.657 AU was in 1989, when it would have been 5.5 days. "Have Spacesuit Will Travel" was written in 1958, so the distance to Pluto was about what it is now. Earth orbit variability makes that +/- about 3 hours.

Top speed is sqrt(2 * (8 * 9.81 m/s/s) * (32.6 AU/2) ) / (speed of light) = 6.5% of light speed.

I'm not sure that Kip and Peewee would have survived 5+ days at 8 g. Apparently the only week-long >1 g tests we've done have been 1.5g, says http://space.stackexchange.com/questions/6154/maximum-surviv... , though we've done tests for 1,000 seconds (16 minutes) at 4 g.