Tim Dowling learns about the Planet 9 theory from one of its originators Professor Konstantin Batygin. The theory is that way beyond Neptune and Pluto, on the outer reaches of the Solar System a 9th planet, 5 times the mass of Earth is orbiting the Sun once every 10,000 years. We also hear from Professor Samantha Lawler, who is not as convinced by the evidence for the existence of Planet 9.