We now know that the Kuiper Belt is about three times the surface area of everything inside it, and it likely contains hundreds of Pluto-like objects. As Stern points out, it's ridiculous to assume that any object in the Kuiper Belt could clear its orbital zone-the third criterion for a planet-given the size of the region and its abundance of objects. "If you put Earth out in the Kuiper Belt, it couldn't clear it either," he said. "Does that mean Earth's not a planet?"
What exactly does it mean for an object to "clear its zone"? Stern says it's ambiguous: "The intended meaning is that its gravity is enough to sweep everything else where it's orbiting away. But in reality, every planet in the solar system has other small bodies next to it that it has not cleared. So, this lousy definition from non-specialists rules out everything. It rules out Jupiter, because Jupiter has Trojan asteroids. It rules out Neptune, because Pluto crosses Neptune's orbit. It's just so poorly thought-out that it's laughable."