If Pluto is a planet, so would be Eris, Sedna, Haumea, Makemake, Chorion, and Pallas.
So how many planets are in our solar system?
8 (or 9 if you include the one that almost certainly exists in the Kepler belt that is about the size of Neptune)
Or 15 (16 once again if you include the highly probable 5th Gas Giant)
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Pluto (and other dwarf planets) does not pass the third requirement to be classed as a planet; It must have cleared its orbit. Pluto is constrained to Neptune's pull and orbits within the Kuiper Belt. It's still a "planet" just a "dwarf" planet. [spoiler]Pluto will always be the 9th planet in my heart.[/spoiler]