Mike Brown, the Caltech professor who found a Pluto-sized object past Pluto that led to both being reclassified by the International Astronomical Union as “dwarf planets” (a promotion for Pluto to being the best and biggest of its category, not the demotion you often read about, see The Cosmos, Section 8.1c, pp. 201–202), and Caltech colleague Konstantin Batygin, have deduced the presence of a “superEarth” perhaps 10 times Earth’s size and way far out in the Solar System.
They deduce its presence by its effect on several of the most distant Kuiper-belt objects that Brown and others have found in recent years. So even if Brown’s work led to Pluto no longer being the ninth planet, this could be the real ninth planet. “This may even be real,” they each said to me [The Cosmos author, JMP].
The scientific paper was published in the Astronomical Journal: “Evidence for a Distant Giant Planet in the Solar System,” Konstantin Batygin & Michael E. Brown, 2016 February, Astronomical Journal.