https://www.sciencealert.com/a-giant-planet-seems-to-be-lurking-unseen-in-our-solar-system
Could there be a ninth planet?
There's a good reason astronomers spend many hundreds of hours trying to locate a ninth planet, aka ” Planet Nine” or “Planet X”. And that's because the Solar System as we know it doesn't really make sense without it.
Every object in our Solar System orbits around the Sun. Some move fast and some slow, but all move abiding by the laws of gravity. Everything with mass has gravity, including you and me. The heavier something is, the more gravity it has.
A planet's gravity is so large it impacts how things move around it. That's what we call its “gravitational pull”. Earth's gravitational pull is what keeps everything on the ground.
Also, our Sun has the largest gravitational pull of any object in the Solar System, and this is basically why the planets orbit around it.
It's through our understanding of gravitational pull that we get our biggest clue for a possible Planet Nine.