Until its reclassification as a dwarf planet in 2006, Pluto was considered to be the ninth planet. However, it is now believed that there really is a ninth planet beyond the Kuiper belt – the cold scattered disc region towards the outer edge of the Solar System.
First proposed in 2015, Planet Nine is believed to exist because of its gravitational influence upon trans-Neptunian objects in the region, and has a diameter of two to four times that of Earth with an estimated mass of ten Earths.
It seems likely that it is the core of a primordial giant planet that formed close to the Sun during the birth of the Solar System along with the other eight known planets, but migrated out as Jupiter and Saturn gradually destabilised its orbit.
Planet Nine should be bright enough to reflect light from the Sun, but takes approximately 15,000 years to complete its elongated oblong orbit around the Sun. Therefore, it is difficult to differentiate it from background stars and so prove its existence.