Explaining the Death of a Planet to Your Child

My eight year old son was upset I whited out Pluto from the solar system coloring sheet that he requested (weird effects of homeschooling- apparently he thinks it super fun to color the planets himself). I need to read aloud "How I Killed Pluto, and Why It Had It Coming" (by Mike Brown) to him. Tried so many ways to explain why Pluto is NOT a planet.

The book is very entertaining to read.  It reads like a story, but has a lot of science in it.  I have only gotten half way through, but I found it fascinating.  Just ordered it again from our library, maybe I will have the time to finish it this time.