It has been widely debated among the science community for years, but now Nasa claims that Planet Nine does exist.
The space agency highlights five different lines of evidence pointing to the existence of the mysterious world, and says that imagining that Planet Nine does not exist generates more problems than you solve.
Researchers are now using the Subaru Telescope in Hawaii in the hopes of finding Planet Nine, and hope that its detection will also shed light on its origin.
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WHAT IS PLANET 9?
Astronomers believe that the orbits of a number of bodies in the distant reaches of the solar system have been disrupted by the pull of an as yet unidentified planet.
First proposed by a group at CalTech in the US, this alien world was theorised to explain the distorted paths seen in distant icy bodies.
In order to fit in with the data they have, this alien world – popularly called Planet Nine – would need to be roughly four time the size of Earth and ten times the mass.
Researchers say a body of this size and mass would explain the clustered paths of a number of icy minor planets beyond Neptune.
Its huge orbit would mean it takes between 10,000 and 20,000 years to make a single pass around the sun.
The theoretical Planet Nine is based on the gravitational pull it exerts on these bodies, with astronomers confident it will be found in the coming years.
Those hoping for theoretical Earth-sized planets proposed by astrologers or science fiction writers – which are ‘hiding behind the sun’ and linked with Doomsday scenarios – may have to keep searching.
Planet Nine was first theorised by experts from Caltech in 2014.
And while the planet itself is yet to be found, researchers believe there is strong evidence it exists.
Dr Konstantin Batygin, a planetary astrophysicist at Caltech in Pasadena, whose team is closing in on finding Planet Nine, said: ‘There are now five different lines of observational evidence pointing to the existence of Planet Nine.
‘If you were to remove this explanation and imagine Planet Nine does not exist, then you generate more problems than you solve.
‘All of a sudden, you have five different puzzles, and you must come up with five different theories to explain them.’