Planet Nine (or Planet 9) is a theoretical additional planet beyond Neptune, under consideration because of solar system orbit characteristics. In the last decade, a theory has grown from studying the orbits of many minor planets beyond Neptune (i.e., K Belt objects) and reverse-engineering how the orbits could come to be as they are, Of interest are their orbital inclinations and comparing the probabilities of developing them against the observed population counts. Some earlier planets were discovered through observation of orbits of planets, but the current work is based not upon a feature of an individual orbit but of orbits of whole populations of objects. Simulations of long-term orbit histories are tried to explore what could create the current situation.
The suggested planet would have a high orbital inclination (30°), a substantial eccentricity, in an orbit of about 700 AU, and a mass of about 10 Earth masses. The possibility of a gas giant at that distance is considered eliminated.
In the past, there have been other theories. Obviously, Pluto was considered planet 9 for a while after its discovery (and similarly, Ceres was considered a planet for some time after its discovery in the early 1800). In the 20th century, theories have been developed about an additional planet (a 10th planet). With the current theory of a 9th planet, there are also more speculative theories about a Planet 10: at 62 AU with a Mars mass and an orbital inclination of about 7°.