Supervisor: Hans Kjeldsen and Simon Albrecht

Over the last 10 years, strong indications have been noticed that the known picture of the solar system with 8 planets is incomplete. Discovery of a ninth planet is expected, and it could happen any day.

To begin with, I will outline the recent history of efforts to discover new planets in the Solar System, starting in the 19th century. Secondly, I will introduce the audience to the characteristics of the so-called Trans-Neptunian Objects. Moreover, I will describe in detail the unusual spatial distribution of the orbits of these objects. Finally, I will explain that the distribution seems to be very well explained by existence of a planetary-mass object behind Neptune. In the end, listeners will hear about the range of expected parameters of the hypothetical ninth planet and its detection prospects.