According to some scientists, Planet Nine might be a "rogue" planet captured by our solar system as it was zooming through space long ago.
Originally described by astronomers Chad Trujillo and Scott S. Sheppard in a 2014 letter to the journal Nature, Planet Nine is now believed to be as much as 10 times as massive as Earth, with a diameter between two and four times that of our home planet and a highly elliptical orbit with an orbital period of approximately 15,000 years.
"Several have been observed in the solar neighbourhood".
For most people, it's an exciting possibility because, well, there's a giant icy planet waiting to be found somewhere out beyond Pluto.
But now there are claims that the planet is a rogue world that has somehow been snared by our solar system - sparking hopes that it could hold alien life.
Dr Dimitri Veras of the Department of Physics said: "The existence of a distant massive planet could fundamentally change the fate of the solar system".
The theory of Planet Nine was born out of the movements of objects in the Kuiper belt, which is a disc-shaped collection of mostly small objects that exists beyond Neptune. It sounds like the setup for a science fiction movie, but rogue planets actually outnumber planets that orbit a particular object.
Space.com reports that after months of research, James Vesper, an undergraduate student at New Mexico State University (NMSU), announced at the 229th gathering of American Astronomical Society (AAS) it's "very plausible" that Planet 9 was a "rogue planet" - a planetary-mass object that wanders around the galaxy unattached to a star.
But Mr Vesper noted that in 60 per cent of simulations, the incoming rogue would be flung out of the solar system. Every once in a while, it took a souvenir: at least one of the solar system's planets.
But 40 percent of the time, the rogue planet stuck around, caught orbiting the solar system.
"We speculate that if rogue planets are abundant as predicted, then, Planet 9 may be a captured rogue". This doesn't eliminate the possibility of Planet Nine, by the way - the planet may be bigger than Earth, but NASAsuggests it's about the size of Neptune.