Discovery of possible '10th planet'

If confirmed, the discovery 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.

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.

The planet was first spotted on October 31, 2003 with the Samuel Oschin Telescope at Palomar Observatory near San Diego, California.

But it was so far away that its motion was not detected until the scientists reanalysed the data earlier this year, Brown said.

The planet has not been noticed previously because its orbit is at a 45 degree angle to the rest of the solar system, he said.