Beyond the Kuiper Belt (seen right here) lies the theorised Oort cloud, the place an additional planet could possibly be hiding
NASA
Our photo voltaic system could possibly be hiding an additional planet in its outermost reaches. It is feasible {that a} large planet lurks within the Oort cloud, which begins tons of of billions of kilometres past Pluto and has usually been thought-about the area of comets.
Most multi-planet programs, particularly these with large worlds like our personal photo voltaic system, bear what is named a dynamical instability in some unspecified time in the future of their lifetime, whereby the planets …
Source: www.newscientist.com