The definition of planets has changed over the years, and the scientific community in 2005 voted to demote Pluto's classification to that of a "dwarf planet". They weren't wrong or right, per se, they simply adjusted the way they viewed the world, mainly because the word "planet" is a culturally defined word that has changed its meaning over the ages, according to Harvard astronomers.
Though, since they have reclassified Pluto, this means that the number of planets in our solar system could go as high as 25 very soon, possibly more; how would you like to memorize all those names in school?