Illustration: Pluto and its large "moon" Charon
An artist's image shows Pluto (center) and its large "moon" Charon (right) from the surface of one of Pluto's two smaller satellites. The other small moon can be seen on the left.

Image courtesy NASA, ESA and G. Bacon (STScl)

In the game Pluto's Secret, Nat tells his friend Geo that he heard that Pluto is no longer a planet. Is Nat right? Is Pluto no longer a planet? There's debate in the scientific world.  National Geographic News says that, according to the International Astronomical Union, a full-fledged planet is an object that orbits the sun and is large enough to have become round due to the force of its own gravity. Because Pluto doesn't meet these standards, the IAU classifies Pluto as a dwarf planet.

Not everyone agrees that this is a good way to decide, though. Andy Cheng, a planetary scientist at Johns Hopkins University, says that the new rules aren't clear enough and asks the question "how round is round? ...I'll still continue to maintain that Pluto is a planet," he said.

Owen Gingerich is an astronomer and historian at Harvard University in Cambridge, Massachusetts, and head of the IAU committee proposing the definition. He favored a special distinction for Pluto. Gingerich supported a proposal to call the big eight planets classical planets—as opposed to just plain "planets"—and Pluto and the others dwarf planets, so there would be two classes of planets. He believes that reclassifying Pluto as a dwarf planet is not "sensitive to the historical and cultural role that Pluto has played."

The argument continues. In the meantime, however, Nat and Geo are correct—new textbooks will list Pluto as being a dwarf planet.