Planet 9, or the planet which supposedly orbits out in the kupler belt, is supposedly a massive planet that we can not observe with our telescopes from home. The literature has been rife with a back and forth characterizing both evidence in support and in opposition to the existence of this purported celestial body.
Planet 9: The Invisible Monster
Way back alllll the way in april of 2017, researchers had been analyzing the movements of asteroids out in the kupler belt, and found no evidence in support of a massive planet being there. So the internet was a blaze with articles stating that "Planet 9 doesn't exist," blah blah blah. Journalists, tch. Getting all riled up and making definitive claims based on one published article, that's not how science works. Science is iterative, its the amalgamation of a plethora of studies, the summation of those works and when using all of the data at once attempting to come up with a unifying explanation that works for all pieces of data at once.
As it turns out, the researchers saying the planet doesn't exist may not have been studying in the right place. As new work has come to light indicating that there is an unusual movement in some of the "trans-neptunian" objects of the kupler belt. While some researchers state that these observations are consistent with bias in their measurements, and are consistent with random movements of the kupler belt objects. Others, performing more careful analytical experiments state that the movements are consistent with a planet being out there.
This second piece of work was published in the journal Monthly Notices of the Royal Astronomical Society: Letters and adds to the growing body of work describing potential gravitational effects by the supposed planet 9.
One might ask themselves after hearing about all of this, how are we supposed to trust any of these findings? The answer to that is we must consider each experiments individual perspective and realize that the experiments not finding effects on the kupler belt objects are just as informative (assuming there is a planet 9) as experiments that show more direct evidence. As knowing what the planet's gravity is not capable of doing also helps us paint a picture (experimentally of course, not a literal picture) of what this planets movements, size, composition etc may be like.
Is there a planet 9? There may or may not be, however the summation of the research is not sufficient to say that there is not. For those of us scientists who like to dream, we can hold out hope that more data is generated that finally allows us to identify this now invisible planet.
Sources
https://en.wikipedia.org/wiki/Planet_Nine
https://en.wikipedia.org/wiki/Kuiper_belt
http://www.agenciasinc.es/en/News/New-evidence-in-support-of-the-Planet-Nine-hypothesis
http://www.inquisitr.com/4166280/planet-9-may-not-exist-after-all-new-study-suggests/
https://academic.oup.com/mnrasl/article-abstract/doi/10.1093/mnrasl/slx106/3892345/Evidence-for-a-possible-bimodal-distribution-of