Astronomers in the US have announced that they have detected the 10th planet in our Solar System.
The object was first spotted in 2003, but scientists have only just confirmed it is definitely a planet.
The planet, which is actually called 2003 UB313, is about 3,000km across. Experts say it's bigger than Pluto, and three times further away from Earth.
The planet is the biggest object found in our Solar System since Neptune was discovered in 1846
The planet was first seen on 21 October 2003, but astronomers did not see it move in the sky until 15 months later on 8 January 2005.
The finding is big news for scientists, as until now they had believed there were only nine planets in our system that orbits the sun.