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, according to the Daily Mail.
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."
In 2016, Batygin published a study that examined the orbits of six objects in the Kuiper Belt - a distant region of icy bodies stretching from Neptune outward toward interstellar space.
His findings revealed that the objects all had elliptical orbits that point in the same direction and are tilted 30 degrees "downward" compared to the plane in which the eight planets circle the sun.
To investigate this further, the researchers used computer simulations of the solar system with Planet Nine included, and showed that there should be more objects tilted at 90 degrees to the plane of the eight planets.
The team realised that five objects already known to astronomers fit the bill.
Following this study, two more clues emerged about Planet Nine.
A second article by Batygin's team, led by Elizabeth Bailey, showed that Planet Nine could have tilted the planets of our solar system during the last 4.5 billion years.
Batygin said: "Over long periods of time, Planet Nine will make the entire solar-system plane precess or wobble, just like a top on a table."
Finally, the researchers demonstrate how Planet Nine's presence could explain why Kuiper Belt objects orbit in the opposite direction from everything else in the solar system.
Batygin said: "No other model can explain the weirdness of these high-inclination orbits. It turns out that Planet Nine provides a natural avenue for their generation.
"These things have been twisted out of the solar system plane with help from Planet Nine and then scattered inward by Neptune."
The researchers now hope to find Planet Nine itself using the Subaru Telescope at Mauna Kea Observatory in Hawaii, which they describe as the "best tool" for the job.
Batygin added: "I think Planet Nine's detection will tell us something about its origin."