You see in the nineties Pluto was still considered to be a planet and was the 9th planet. At the end of the nineties astronomers changed their perspective of Pluto because it is smaller than our moon and smaller than objects in space that behave like commets so in the following decade they officially reclassified pluto as a dwarf planet.
Quite a lot of people were not happy but there are so many objects that are a similar size to Pluto that it make the solar system map very messy. Ceres, Sedna and Iris are just a few of the objects along with Pluto that are considered to be Dwarf Planets. If our moon wasn't orbiting earth it would have been called a Dwarf Planet as well.
Well going back to the Nineth planet, they know it exists because it has pulled a bunch of commets into a very oliptical orbit and they are all pointing in the same area of sky. It is believed that this planet is a giant world but so far away that it may take years to find it.
Good luck to them
