Astronomers in California believe they have found evidence of a vast, mysterious planet 10 times larger than Earth orbiting our sun way beyond Pluto - making it the ninth planet in the solar system. Although no direct sighting has yet been observed, astronomers made the deduction by studying the erratic orbit of asteroids in the Kuiper Belt.

"Planet Nine" orbits at an average distance 20 times further from the sun than the current most outerly planet, Neptune. The vast distances involved mean the sun would appear as a pin-prick of light and barely light the surface, let alone heat the planet. Despite its size, even the most powerful telescopes could only spot Planet Nine if they could calculate exactly where it is on its elliptical orbit round the sun, which takes an estimated 10,000 to 20,000 years.

The reason astronomers at the California Institute of Technology (Caltech) are so convinced of the existence of this mysterious world is the behaviour of smaller rocks in the Kuiper Belt, which move erratically and occasionally in a coordinated way that is otherwise almost impossible to explain. Planet Nine is thought to be a gaseous giant, like Neptune, with ten times the mass of the Earth. The hypothesis is published in the Astronomical Journal.

Dr Mike Brown from Caltech says: "The most distant objects all swing out in one direction in a very strange way that shouldn't happen, and we realised the only way we could get them to swing in one direction is if there is a massive planet, also very distant in the Solar System, keeping them in place while they all go around the Sun. I went from trying very hard to be sceptical that what we were talking about was true, to suddenly thinking, 'this might actually be true'."

It is perhaps ironic that Brown, possibly the discoverer of the ninth planet, was the astronomer responsible for demoting Pluto from that hallowed status in 2006 when he found asteroids that were just as big - and says even his own daughter hasn't quite forgiven him for being the "Pluto Killer" - which is also his Twitter handle.

"All those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found," says Brown. "Now we can go and find this planet and make the solar system have nine planets once again."

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