ELI5 Pluto - is it a planet or not?

Why did Pluto suddenly became a dwarf planet? Why did they change it in 2006, and who defined what makes a planet and what doesn't? Why did we get so excited about new photos of Pluto a few years ago?   ... We explain like i'm five!   Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: silentwalrus92, calc3, ursus1337, sirkendizzle, lithium, staggerlee024, mason11987, lonewolf220 & bobbycorwin123. To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]