Three years ago, Konstantin Batygin and Michael E. Brown at the California Institute of Technology announced a theory suggesting the existence of a ninth planet in the Solar System. Earlier this year, they released two new papers providing further evidence. Published on January 22, their first paper, “Orbital Clustering in the Distant Solar System”, claimed that an unseen planet’s gravitational pull affects the clustering of objects in the Kuiper Belt. Brown and Batygin assessed whether an observational bias was involved in the finding and concluded that there was only a probability of one in 500 that the finding was illegitimate.

A second paper named “The Planet Nine Hypothesis” included computer models of the Solar System’s evolution, providing an estimate that Planet Nine might be smaller than and closer to the Sun that was previously hypothesized, with a size five times bigger than that of Earth and an orbital semimajor axis of 400 astronomical units (AU). While it’s existence hasn’t been explicitly proven, Batygin optimistically states that the hypothesis is “observationally testable”, bringing potential for future discoveries.