For those who, like me, didn’t know what planet 9 was: > Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effect

For those who, like me, didn’t know what planet 9 was: > Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the unlikely clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs), bodies beyond Neptune that orbit the Sun (Wikipedia)