Nothing short of caring deeply can cause astronomers to publish such a burst of papers on an as-yet theoretical planet, too distant to have already been captured by our numerous telescope facilities. A recent paper counts twenty two new publications from planetary scientists in 2016, assessing the specific claims of a paper that came out almost exactly a year ago, on the 20th of January, 2016. Over twenty papers in a year is a pretty substantial achievement for any individual object - in fact, off the top of my head, the only other single object that reached those dizzy heights in 2016 is Boyajian’s Star (also known by its technical name KIC 8462852), which you may remember as the alien megastructures star (it was not alien megastructures).
So what did those twenty-two papers turn up? Well, as you say, they’ve narrowed down the mass and orbital parameters of the planet proposed by Batygin & Brown in 2016, which elaborated on an original suggestion by Trujillo & Sheppard in 2014. The 2016 suggestion was covered quite widely - it’s not every day you hear that there might be a mystery planet floating around the very edges of our solar system - but it was, fundamentally, only a suggestion. Batygin & Brown looked at the sample of data that they had, and concluded that one explanation would be a giant large planet, nudging tiny little icy worlds into very specific orbits.
What followed was a flurry of papers, as other planetary scientists who specialize in the outer solar system descended on the idea of this specific incarnation of a distant 9th planet. One team went hunting through a bunch of existing survey data, and found nothing, which meant if such a planet really is out there, it had to be fainter (either more distant or cooler) than the faintest objects captured in the survey. That result doesn't rule out the planet entirely, but it puts boundaries on it.
The papers have kept coming, and with each one, the range of options for this planet grow smaller and smaller. Simulations have been run to test how unlikely our observations of the solar system are. If a planet were there, would we expect to see this setup in our solar system? That particular simulation leans towards no - which should make you suspicious of whether we understand the data as well as we would like. More data is being taken to see if the planet’s signature in our view of the solar system is a bias in the way we hunt for these objects.
Scientists, and astronomers are no exception, are cautious creatures. What we are seeing at the moment is a live, highly publicized, discussion on the various merits and demerits of the idea of a ninth, very distant, planet. But each piece of the puzzle, as data is taken and analyzed, takes time to process, time to analyze, and time to figure out how it fits in with everyone else's work. It's a lengthy process, and everyone wants to get their part right. We won't have a final answer on this version of Planet Nine until it is directly discovered or its existence can be entirely ruled out - in either case it will be a few years of work.
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