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Media playback is unsupported on your device Media caption Why do the Caltech astronomers think there may be a ninth planet? American astronomers say they have strong evidence that there is a ninth planet in our Solar System orbiting far beyond even the dwarf world Pluto. The team, from the California Institute of Technology (Caltech), has no direct observations to confirm its presence just yet. Rather, the scientists make the claim based on the way other far-flung objects are seen to move. But if proven, the putative planet would have 10 times the mass of Earth. The Caltech astronomers have a vague idea where it ought to be on the sky, and their work is sure to fire a campaign to try to track it down. Full story

14 March