A hypothesis, not a discovery
Some researchers have proposed that the gravity of a distant planet could help explain features in the orbits of certain trans-Neptunian objects. The planet has not been directly observed and its existence remains a matter of scientific discussion.
Other explanations
Other interpretations are possible, and the selection of observed objects can affect patterns in the data. Further observations and analyses are needed to compare the hypotheses.
Is it Pluto?
No. Pluto is a known dwarf planet. Planet Nine is an informal name for a different, hypothetical planet that has not been identified.