The hunt for the mysterious Planet Nine is on and one of the proposers of the existence of such an object now claims that it would be difficult to explain some features of the Solar System if the unidentified planet is not discovered.

That Planet Nine existed was first proposed by Konstantin Batygin and Mike Brown in January 2016, based on the curious configuration of some distant objects beyond Neptune. This world is expected to have 10 times the mass of our planet and orbits 20 times further out than Neptune. No direct observation has seen it but the case for it being out there is becoming more and more compelling.

"There are now five different lines of observational evidence pointing to the existence of Planet Nine," Batygin said in a statement. "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."

The first three lines of evidence have to do with the objects of the Kuiper Belt, the region of the Solar System beyond the orbit of Neptune, and were part of the first paper Batygin and Brown published about Planet Nine. All detected objects so far have elliptical orbits pointing in the same direction and their orbits are tilted at about 30 degrees with respect to the plane of the Solar System. The existence of such a planet would suggest the presence of objects orbiting at a 90 degrees tilt and five of such objects fit the bill.

The fourth comes from a follow-up study led by Elizabeth Bailey that showed how Planet Nine could explain the tilt of the Solar System. The planets all orbit very close to a single plane, but the plane is tilted at 6 degrees with respect to the Sun’s equator. This mystery can be solved by adding something like Planet Nine to the mix. The final clue is about “contrarians”, objects in the Kuiper Belt that orbit in the opposite direction to rest of the Solar System stuff. Planet Nine could cause their unusual behavior.

"No other model can explain the weirdness of these high-inclination orbits," Batygin added. "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 case for a ninth planet is solid but until we actually get an observation, all of this is speculation. The researchers will continue the search in December.