On January 20 2016, two American astronomers made an extraordinary claim - they had found evidence for a ninth planet in our solar system, a planet 20 times further out than Neptune which would take up to 20,000 years to orbit the sun. It is a discovery that could completely rewrite our understanding of our solar system and how it formed. As the world's biggest telescopes start scanning the skies searching for Planet 9, the Sky at Night team investigates. If Planet 9 exists, where is it and where did it come from? In California, Chris Lintott meets the astronomers whose study of the distant Kuiper Belt led them to predict the existence of the planet. And while some scientists are still sceptical, Maggie Aderin-Pocock discovers how our models of the formation of the solar system and the discovery of similar exoplanets around other stars all support the existence of Planet 9.