Beyond the Kuiper Belt (seen here) lies the theorised Oort cloud, where an extra planet could be hiding
NASA
Our solar system could be hiding an extra planet in its outermost reaches. It is possible that a giant planet lurks in the Oort cloud, which begins hundreds of billions of kilometres beyond Pluto and has typically been considered the domain of comets.
Most multi-planet systems, especially those with giant worlds like our own solar system, undergo what is called a dynamical instability at some point in their lifetime, wherein the planets …