The Solar System extends beyond Neptune and there is much yet to be discovered.
Beyond Neptune is the domain of dwarf planet Pluto and probably several other dwarf planets.
Then there is the Kuiper belt and the more distant Oort cloud. These contain comets and possibly interlopers from distant stars.
There is probably another planet, planet X, beyond Pluto. It’s presence is felt by its gravitational pull, but it is yet to be discovered.