They are calling it Eris after the goddess of discord and strife.
The icy object, on the edge of our solar system, had been nicknamed Xena by discoverer Mike Brown who found it from California in 2003. But it stirred up a quarrel in the space community because it was found to be bigger than the then ninth planet Pluto.
What is more, it became clear that both worlds were among a number of similar bodies labelled Kuiper Belt Objects, left over from the formation of the solar system.
Astronomers had to decide whether to make them planets too or to take away Pluto’s planetary status. Last month in Prague they voted to axe Pluto and cut the solar system down to eight planets.
An IAU subcommittee, part of the Minor Planets Center, gave Eris its new name. It has a small moon which they are naming Dysnomia.