The presence of a new planet on the outskirts of the Solar System was proposed by Caltech astronomers Konstantin Batygin and Mike Brown in 2016 to explain the intricate orbital structure of the Kuiper Belt, a field of icy bodies orbiting the Sun beyond Neptune. Since then, astronomers have been busy gathering evidence of its existence. Now Batygin, Brown and two University of Michigan astronomers, Juliette Becker and Fred Adams, have reviewed this...