Trujillo and Sheppard (2014) noted some peculiar orbital features of trans-Neptunian objects with orbits well detached from those of the outer planets, and suggested the possible existence of a planet in an outer orbit in order to explain them. More recently, Batygin and Brown (2016) worked out the dynamical mechanism by which a trans-neptunian planet, with suitable mass and orbit, might be the cause of these orbital features. After the paper by Batygin and Brown the hunt for "planet 9" has started, much in line with the course of events that led to the discovery of Neptune in the XIXth century, but without success so far. I will review and discuss the state of the issue.