“MY VERY educated mother just served us nine pizzas.” Many American school children are taught this mnemonic to help them remember the order of the planets of the solar system. Soon though, this may change because, on July 29th, a team of astronomers announced the discovery of a very distant celestial body larger than Pluto. The researchers claim that the new body—which they are informally calling Xena—should be classified as a planet.

The new body—temporarily named 2003 UB313—orbits the Sun once every 560 years. It is currently over 14 billion kilometres away, about three times farther out than Pluto, making it the most distant object ever discovered in the solar system. The researchers think it is part of the Kuiper belt, a ring of rocky objects that extends beyond Neptune.

Mike Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University discovered the object in data recorded at the Palomar Observatory in San Diego in October 2003, but its motion did not become apparent until they reanalysed the data in January 2005.

The question of whether or not the new body should be considered a planet has rekindled the debate over what exactly counts as a planet. A handful of objects of similar size to, but smaller than, Pluto have been discovered in the Kuiper belt over the past few years. These have not been considered planets, mainly because they were smaller than Pluto. But 2003 UB313 is larger than Pluto. If Pluto is a planet, shouldn't it be as well?

The case is not so clear cut. Many astronomers argue that Pluto should not be considered a planet. It is more like a large asteroid, they hold. Meanwhile, Dr Brown asserts that as Pluto has historically been considered a planet, anything larger should also be considered one.

Ultimately, the International Astronomical Union, a group of professional astronomers, will end this existential angst. Dr Brown expects the process to take months, and the team is not allowed to reveal its suggested name until then. Since most Greek and Roman names have already been used, he and his colleagues have previously drawn upon Native American and Inuit mythology for names. He will only hint that the new name comes from a different tradition altogether.

Time will tell whether mother will be serving “nine polished xylophones”, “nine pizzas” or just “noodles”.