A US astronomer has said he had discovered a 10th planet in the outer reaches of the solar system that could force a redrawing the astronomical map. If confirmed, the discovery yesterday by Mike Brown of the respected California Institute of Technology would be the first of a planet since Pluto was identified in 1930 and shatter the notion that nine planets circle the sun."Get out your pens. Start re-writing textbooks today,' said Brown, a professor of planetary astronomy, announcing what he called "the 10th planet of the solar system,' one that is larger than Pluto. "It's the farthest object ever discovered to orbit around the sun,' Brown said in a conference call of the planet that is covered in methane ice and lies nearly 15 billion kilometers (nine billion miles) from Earth.