It's a very cold frozen world, with a temperature of -240 degrees Celsius. It takes 560 Earthly years to circle the sun once and it's more than 90 times farther from the sun than our planet is.
The planet is three times as far from the sun as Pluto and it's also bigger than Pluto, the smallest planet in our solar system. The planet is about 36 percent bigger than Pluto.
"To think that you could actually find something in our own solar system bigger than anything that had been seen in 150 years, just is sort of an amazing prospect," said Michael Brown, professor of planetary astronomy at the California Institute of Technology.
Brown is the astronomer who headed the discovery team. He says that if other scientists agree, it's time to start rewriting the textbooks: The sun now has 10 planets.