In recent years new astronomical observations have led researchers to suggest that there may be another large planet in our Solar System, on a very elongated, elliptical orbit compared to the rest of the planets. Since nicknamed 'Planet Nine' (Planet X perhaps being too heavy with Zecharia Sitchin baggage for 'respectable' astronomers?), the mystery object is believed to currently be in the general direction of the constellation of Taurus (as viewed from Earth).
If you'd like to understand the discovery, and now search for, the mysterious Planet Nine, check out the video below by Mike 'Pluto Killer' Brown of Caltech, which is a part of a free online astronomy course "The Science of the Solar System", offered by Coursera. Brown gives the historical background to the search for distant planets, then describes how astronomers (including him) came to believe there was another large planet to be found, and how they are going to try and find it. A very interesting, concise explainer of the Planet Nine story, I definitely recommend checking it out: