It's a simple matter of definition. To apply to it a celestial body needs to meet three prerequisites: First: it needs to orbit a fixed star. Check. Second: it must have enough mass to reach the so-called hydrostatic equilibrium (which means it has to have a round shape). Check. And last: it must have cleared its orbit from other celestial bodies such as asteroids et cetera. Not check. That's why Pluto is considered a Trans-Neptunian object or dwarf planet (this terms have not exactly the same meaning but both apply to Pluto) as guest has already said. However, in our hearts Pluto will always be the ninth planet! :)
@wiseguy:
Couldn't have summed it up better.
I will add that a prominent science communicator (can't recall who) said that Pluto's composition is such that, if it were brought in closer into the solar system, it would grow a tail like a comet... and that is not proper behavior for a planet.
Couldn't have summed it up better.
I will add that a prominent science communicator (can't recall who) said that Pluto's composition is such that, if it were brought in closer into the solar system, it would grow a tail like a comet... and that is not proper behavior for a planet.