The astronomers who in July announced the discovery of a "10th planet" in our Solar System say the object has a moon.
Currently, the planet has been dubbed Xena; the moon will be called Gabrielle until official names are agreed.
Both "codenames" come from characters in a US TV series, Xena: Warrior Princess.
"Since the day we discovered Xena, the big question has been whether or not it has a moon," Professor Brown said in a statement. "Having a moon is just inherently cool - and it is something that most self-respecting planets have, so it is good to see that this one does, too."