Within the last ten years astronomers have been finding more and more objects that are similar in size and composition to Pluto and orbit in the same part of our Solar System. Until 2003, the objects, such as Varuna,Quaoar, and Sedna, were all somewhat smaller than Pluto. Quaoar, for example, is about half the size of Pluto. Up to that point, astronomers could sort of draw the line at Pluto and say that it gets to be called a planet, but anything smaller does not.

In 2003, the jig was up because the object, now known as Eris, was found to be larger than Pluto, with an orbit that is sometimes closer to the Sun than Pluto. If Pluto is a planet, then so is Eris. Most astronomers think there are lots more objects such as Eris waiting to be discovered, so we could end up with dozens of new planets. Worse, insisting that only objects Pluto's size or larger could be planets would seem pretty artificial and it would be hard to justify keeping objects such as Quaoar out of the "planet club". Then we could end up with hundreds of objects, all called "Planets."

The International Astronomical Union, the body that is in charge of deciding what to call celestial objects, decided that the term "planet" needed to be reserved for objects that play major roles in our Solar System. They added a new criterion that an object has to meet to be called a "planet:" The object must dominate the region of space near its orbit by clearing any other objects from that region. Pluto does not do that. None of the other newly discovered objects meet the new criterion either.

Now we have a Solar System with 8 planets and perhaps a few hundred "dwarf planets." Pluto is one of the larger dwarf planets.