In January, researchers at Caltech in the US suggested a large, additional planet might be lurking in the icy outer reaches of the Solar System. Now, a team at the University of Bern in Switzerland has worked out what they say are upper and lower limits on how big, bright and cold it might be. The study has been accepted by the journal Astronomy and Astrophysics. Prof Mike Brown and Dr Konstantin Batygin made their case for the existence of a ninth planet in our Solar System orbiting far beyond even the dwarf world Pluto. There are no direct observations...