Eight planets, like, woah.
That's kind of unfair for Pluto.
From now on, generations of school children will say: "Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune--PERIOD." That's... strange. [And just a little bit unfair, since we had to memorize more!]
Although having eight planets nicely divides the solar system into four inner terrestrial planets and four outer gaseous planets.
Aww. Sailor Pluto's home planet isn't a valid planet anymore. Which does kind of fit her background, hmm... [Yes, I just had to say that.]
But Pluto has always been the odd one out, ne? It is more like an asteroid than a planet. Reclassifying it does make the definition less ambiguous.
*trains herself to say "Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune."