Quaoar, which is about half the size of Pluto, turned up in 2002. Sedna came along the following year; it is roughly Quaoar’s size. And in 2004, Haumea, egg-shaped and even closer to Pluto’s size, came to light.
Then Eris showed up. Discovered in 2005, it has slightly more mass than Pluto and a diameter just a few dozen miles smaller. A crisis in confidence erupted among astronomers: If something about as big as Pluto is out there, who’s to say there might not be dozens more?
Astronomers mulled redefining the parameters of a planet. The International Astronomical Union (IAU) proposed a draft that would have made Eris, Ceres, and even Charon planets, but it got scrapped. Then, at its 2006 meeting, the IAU voted to define a planet as something that orbits the Sun, has enough mass for self-gravity to give it a nearly spherical shape, and clears the neighborhood around its orbit. If it meets just the first two conditions, it’s a dwarf planet.
Pluto did not make the cut. It’s worth noting — while remaining agnostic on the planet vs. not-a-planet debate — that astronomers considered the first few asteroids found in the early 19th century to be planets until they understood the sheer number of objects in the asteroid belt.