For seventy-six years, Pluto was a planet. Then, in 2005, astronomer Dr. Mike Brown discovered Eris - a distant icy world roughly the same size as Pluto - forcing astronomers to confront a question they had quietly avoided for decades: what actually counts as a planet? The answer led to the 2006 vote that redefined the solar system and stripped Pluto of its status, turning Brown into the unlikely public face of a controversial decision.  But this story isn’t just about Pluto. It’s about how scientific definitions evolve. It’s about the vast, largely unexplored region beyond Neptune known as the Kuiper Belt. And it’s about Brown’s current hunt for something even bigger: a massive, unseen ninth planet whose gravity may already be shaping the outer solar system. In this episode, we go to the edge of our cosmic neighborhood with the astronomer who changed it - and may be on the verge of changing it again.