With all the controversy surrounding Pluto's status as a planet it is easy to lose track of the bigger story—a rich new class of worlds is being uncovered in the outer solar system. These discoveries strongly suggest that there may be an unknown world, far beyond the other eight planets, a world much larger than anything discovered in that region so far. Come join astronomer Mike Brown, "Pluto Killer," on the journey toward the scientific discovery of the century—the discovery of Planet Nine.