A theoretical view of a distant world so far from our Sun that our Solar System appears barely brighter than the remaining stars in the sky. ( Caltech/R. Hurt (IPAC))
Pluto is definitely not the ninth planet, but a large world beyond Neptune might actually be a reality.
What lies out there, in the outer Solar System, beyond the orbit of the last known planet? Up until 1992, you would have said Pluto and its moon (maybe “moons” if you were willing to speculate), but even the existence of the Kuiper belt was doubted by many. Of course, all of that changed with the discovery of many different objects, including the more-massive-than-Pluto world discovered in 2003: Eris. We quickly realized that Pluto was not unique, but one member of a distinct class of objects thoroughly different than the planets. In 2006, we created the “dwarf planet” classification for non-planetary objects that still were Pluto-like.
But more recently, a compelling but controversial idea has emerged: the idea of a Planet Nine that is more massive than even Earth, but lies hundreds of times farther away that we are from the Sun. Both of these achievements, the theorizing of Planet Nine and the Pluto-killing discovery of Eris, come courtesy of the same planetary astronomer: Mike Brown.