Astronomers detected Sedna using the Spitzer Space Telescope 6.2 billion miles (10 billion km) from Earth, but some say it's not really a planet.

Made of rock and ice, and about 1,200 (2,000 km) across, it could be bigger than Pluto, its closest neighbour.

One group of scientists don't even think Pluto is a real planet, but others say Sedna will change the way we think about our Solar System.

Sedna was named after the Inuit goddess of the Ocean.

It was discovered in a part of space called the Kuiper Belt, which is packed with hundreds of objects made of rock and ice.

It's thought that they are the left-overs from the creation of the Solar System.