It would take the planet between 10,000 and 20,000 years to make just one full orbit around the Sun, researchers said.

The researchers, Konstantin Batygin and Mike Brown, from California Institute of Technology in US discovered the planet's existence through mathematical modelling and computer simulations but have not yet observed the object directly.

"There have only been two true planets discovered since ancient times, and this would be a third," said Brown.

Brown said that the putative ninth planet - at 5,000 times the mass of Pluto - is sufficiently large that there should be no debate about whether it is a true planet.

Unlike the class of smaller objects now known as dwarf planets, Planet Nine gravitationally dominates its neighbourhood of the solar system.

According to the researchers, Planet Nine helps explain a number of mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt.

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