The largest object found in our Solar System since Neptune was discovered in 1846, it was first seen in 2003 but has only now been confirmed as a planet.
Designated 2003 UB313, it is about 3,000km across, a world of rock and ice and somewhat larger than Pluto.
Scientists say it is three times as far away as Pluto, in an orbit at an angle to the orbits of the other planets.
Astronomers think that at some point in its history, Neptune likely flung it into its highly-inclined 44-degree orbit.
It is currently 97 Earth-Sun distances away - more than twice Pluto's average distance from the Sun.--