Thousands of years ago, ancient Greeks looked up at the stars in the night sky. They noticed something strange. Some of the stars didn’t act like stars. They moved around. At different times of year, the sky watchers saw them in different places. The Greeks gave these mysterious objects a new name: planets. “Planet” means “wanderer.”

As years passed, people learned more about planets. They used telescopes to find new ones orbiting our sun. For a long time, we knew of nine planets in our solar system. You probably know them by heart: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. But then scientists discovered objects beyond Pluto in the Kuiper Belt. The Kuiper (KYE—per) Belt is a ring-shaped cloud. At least one of the objects inside it is even bigger than Pluto. Scientists call this object Eris. Could Eris be a planet too?

Scientists knew they had to make a decision. If Eris is a planet, many more planets might exist in the Kuiper Belt too. How could we ever keep track of them all? But if it isn’t a planet, maybe Pluto shouldn’t count as a planet either. In 2006, scientists took a vote. They agreed that in order to be called a planet, three things must be true of an object. First, the object must orbit the sun. Second, it has to be big—big enough for gravity to turn it into a ball. Third, the object must also be big enough to pull neighboring objects into itself, or slingshot them into outer space. According to the new definition, Pluto isn’t a planet. It isn’t big enough to get rid of neighboring objects. For that reason, scientists have put Pluto in a new category: “dwarf planet.” Sorry, Pluto!