Currently, Pluto is the planet we think of being on the edge of our planetary system. But the new body would be 30,000 times more distant from the Sun than the Earth - putting it a significant distance on the way to the nearest star.

What is more, it seems that the new planet cannot be a true member of our Sun's family of planets. It may be a planet that was born elsewhere, and roamed throughout the galaxy only to be captured on the outskirts of our own Solar System.

The controversial suggestion that there is another planet in deep space comes from Dr John Murray, of the UK's Open University.

For several years, he has been studying the peculiar motions of so-called long-period comets.
 

Comets deflected

Comets - flying mountains of rock and ice - are thought to come from the cold and dark outer reaches of the Solar System, far beyond the planets in a region called the Oort cloud.

They spend millions of years in the Oort cloud, until they are deflected into an orbit that takes them into the inner Solar System where we can see them.

By analyzing the orbits of 13 of these comets, Dr Murray has detected the tell-tale signs of a single massive object that deflected all of them into their current orbits.

"Although I have only analyzed 13 comets in detail," he told BBC News Online, "the effect is pretty conclusive. I have calculated that there is only about a one in 1,700 chance that it is due to chance."

In a research paper to be published next week in the Monthly Notices of the Royal Astronomical Society, he suggests that the so-far unseen planet is several times bigger than the largest known planet in our Solar System, Jupiter.

Being so far from the Sun - three thousand billion miles - it would take almost six million years to orbit it.

"This would explain why it has not been found," explained Dr Murray to BBC News Online. "It would be faint and moving very slowly."

Opposite direction

He has calculated that it lies in the constellation of Delphinus (the Dolphin).

But the planet orbits our Sun in the "wrong" direction, counter to the direction taken by all the other known planets. It is this which has led to the remarkable suggestion that it did not form in this region of space along with the Sun's other planets, and could be a planet that "escaped" from another star.

But, if it is discovered, will Dr Murray get a chance to name it? "Probably not," he says. "That will be up to an international committee. But it would be nice to make a few suggestions." Further evidence to support Dr Murray's claims will be presented at a conference in Italy next week.

Professor John Matese, of the University of Louisiana at Lafayette, has carried out a similar study and reached broadly similar conclusions.

His research is to be published in Icarus, the journal of Solar System studies.