In 1930, Clyde Tombaugh discovered Pluto: our Solar System's ninth planet. For over 60 years, the Plutonian system was the only one known beyond Neptune, and Pluto retained its planetary status for all that time despite its diminutive size. Yet an explosion of exoplanets and of other Trans-Neptunian Objects within our own Solar System beginning in the 1990s meant that we'd need to reconsider what it means to truly be a "planet". The debate still rages today, but astronomers agree: when it comes to the planetary club, Pluto simply doesn't belong. Here's why!