The scientific process is, at its core, iterative. There is no such thing as a truly final/definitive answer - there are only solutions that are good enough for now. In a day-to-day sense, what this really means is that when you’ve been chugging away at some problem and you finally arrive at the result, you can be damn sure that you probably haven’t understood the Full Picture (even if in the moment it feels like you have). “Maybe there is a more elegant way to arrive at the solution? Maybe some approximation has introduced a hidden uncertainty? Maybe there’s a better way to look at the data?” Every step in the right direction is inevitably haunted by questions that fall along these lines.
The Planet Nine story is no exception to this rule. Back when Mike and I published our first P9 paper three years ago, we didn’t worry that there might be a lot more work to be done on this problem - we were certain of it. Instead, what we worried about was that there exists a simpler, or perhaps more natural resolution to the anomalies we were seeing in the data, and that the Planet Nine hypothesis will be rendered irrelevant shortly after publication. That didn’t happen.
To our joint relief (and to some extent surprise), thus far, the P9 hypothesis has fared the test of time rather well. Inevitably, questions have come up regarding the role of observational biases in shaping the orbital clustering we see in the distant Kuiper belt, but these concerns have been largely put to rest. Alternative theories, on the other hand, require the existence of a hidden, coherent, and massive belt of icy planetesimals at hundreds of AU - a scenario that suffers from a number of astrophysical drawbacks. The P9 story thus continues to be in pretty good shape. Nevertheless, we have always felt the need to drive our understanding of the Planet Nine hypothesis a little bit further (and then a bit further after that). So, in collaboration with Juliette Becker and Fred Adams from University of Michigan (as well as Elizabeth Bailey here at Caltech and Alessandro Morbidelli from Nice observatory on earlier works), we spent the last couple years characterizing P9-induced dynamics from analytical grounds and trying to constrain the mass and orbit of Planet Nine to better precision.
The results of these endeavors are compiled in our new review article entitled “The Planet Nine Hypothesis,” published in Physics Reports today. Admittedly, in writing this manuscript, we ended up erring on the side of completeness over completion, so the paper is not exactly short. As a result, with an eye towards providing an “executive summary” of the results, in the next couple posts, I will highlight some of the main take-away points of the article, beginning with brief historical account of planetary predictions based on dynamical evidence.