Beyond the Kuiper Belt (seen right here) lies the theorised Oort cloud, the place an extra planet could be hiding
NASA
Our solar system could be hiding an extra planet in its outermost reaches. It is feasible {that a} big planet lurks in the Oort cloud, which begins a whole lot of billions of kilometres past Pluto and has sometimes been thought of the area of comets.
Most multi-planet techniques, particularly these with big worlds like our personal solar system, bear what is named a dynamical instability in some unspecified time in the future of their lifetime, whereby the planets …