“What is Pluto?” by Sandra May (2015). NASA Knows! (Grades K-4). http://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-pluto-k4.html.
This is a more scientific explanation of why Pluto isn’t a planet anymore. It’s more scientific, and it uses more complicated words, but it’s still made for kids!
These webpages can help adults to understand more about why Pluto is now called a dwarf planet.
“Pluto Demoted: No Longer a Planet in Highly Controversial Definition” by Robert Roy Britt (2006, August 26). http://www.space.com/2791-pluto-demoted-longer-planet-highly-controversial-definition.html
This is a news update from the day scientists decided that Pluto wasn't really a planet. It gives both sides of the argument, and also gives links to much more information about the change.
“Pluto and the Developing Landscape of Our Solar System”. International Astronomical Union. http://www.iau.org/public/themes/pluto
This is an article from the website of the International Astronomical Union (IAU), the group of scientists who decided to officially change Pluto to a dwarf planet. It gives explanations, along with a series of specific questions and answers about Pluto and other related items within the Solar System.
“Dwarf planet” (2016, March 8). In Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/w/index.php?title=Dwarf_planet
Wikipedia offers a detailed overview of the concept of dwarf planets, as well as helpful links to further reading. This article also includes a brief description of the IAU’s decision to reclassify Pluto as a dwarf planet and the controversy surrounding the decision.