They seek it here, they seek it there...

From PhysOrg:

A study recently submitted to The Astronomical Journal continues to search for the elusive Planet Nine (also called Planet X), which is a hypothetical planet that potentially orbits in the outer reaches of the solar system and well beyond the orbit of the dwarf planet, Pluto.

The goal of this study, which is available on the pre-print server arXiv, was to narrow down the possible locations of Planet Nine and holds the potential to help researchers better understand the makeup of our solar system, along with its formation and evolutionary processes....

Dr. Brown tells Universe Today, "This would be the 5th largest planet of our solar system and the only one with a mass between Earth and Uranus. Such planets are common around other stars, and we would suddenly have a chance to study one in our own solar system."

Scientists began hypothesizing the existence of Planet Nine shortly after the discovery of Neptune in 1846, including an 1880 memoir authored by D. Kirkwood and later a 1946 paper authored by American astronomer, Clyde Tombaugh, who was responsible for discovering Pluto in 1930.

More recent studies include studies from 2016 and 2017 presenting evidence for the existence of Planet Nine, the former of which was co-authored by Dr. Brown.

This most recent study marks the most complete investigation of narrowing down the location of Planet Nine, which Dr. Brown has long-believed exists, telling Universe Today, "There are too many separate signs that Planet Nine is there. The solar system is very difficult to understand without Planet Nine."

He continues by telling Universe Today that "…Planet Nine explains many things about orbits of objects in the outer solar system that would be otherwise unexplainable and would each need some sort of separate explanation."