How I Killed Pluto and Why it Has it Coming

The book I am reading for my project is called "How I Killed Pluto and Why it Had it coming" by Mike Brown and in this book there are 231 pages. I think that the topic of this book is about defining a planet and discussing how astronomy has changed throughout history. I think that this topic is important because throughout the book many things change in history and it affected people a lot. I learnt that to be an astronomer is a hard job but is a very important job as astronomers were the ones to discover that they are not the center of the universe but there are other planets and universes out there. One thing that confuses me is that there were/are many other planets before but then were forgotten and I don't understand why. What I really want to learn is what is the current definition of a planet as it seems throughout history the definition changed.