Poor little Pluto. Five years ago, after a lengthy, contentious debate among astronomers, the International Astronomical Union announced its new criteria for what it takes to qualify as a planet.
Pluto didn't make the cut.
To be considered a true planet, a non-satellite, celestial body must:
(A) be in orbit around the Sun.
(B) have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
(C) have cleared the neighborhood around its orbit.
If it meets only the first two conditions, it is classified as a "dwarf planet." If it fulfills only the first criterion, it is defined as a "Small Solar System Body" (SSSB). [Meet the Solar System's Dwarf Planets]
Pluto, for example, is surrounded by several other non-satellite objects, so it has not met the third criterion. In addition to Pluto, there are currently four other known dwarf planets: Ceres, Eris, Haumea and Makemake (although only Pluto and Ceres have been observed in enough detail to confirm they meet dwarf planet criteria).
When Pluto was discovered in 1930, it was thought to be larger than Mercury. That changed in 1978, when astronomers discovered Pluto's moon Charon, which made it possible to more accurately measure Pluto's mass, pegging it at about one-twentieth the mass of Mercury.
This, along with some of Pluto's other eccentricities, got astronomers talking about demoting Pluto. In the 2000s, astronomers began discovering other objects in the Kuiper Belt, and beyond, that were nearly as large as — or, in the case of Eris, larger than — Pluto and shared some of its orbital characteristics. [Will Pluto Ever Hit Neptune?]
In 2006, during the IAU's general assembly, astronomers proposed including Charon, Eris and Ceres in the list of planets. This did not go over well, and after several other drafts and heated debates, the IAU settled on the definition above, and Pluto was declared a dwarf planet. Astronomers estimate that there are upwards of 200 bodies in the solar system that meet dwarf planet criteria; so far, 40 such bodies are known.
Interestingly, astronomers have been through this type of reclassification before. When Ceres was discovered in 1801, it was declared the "missing planet" between Mars and Jupiter. Within a few years, though, astronomers discovered two more objects of comparable size. The discoveries continued, and by 1851 the count of "planets" had grown to 23. Astronomers were certain that they would keep finding bodies of similar size, and decided to redefine what it meant to be a planet; Ceres and the other new objects became known as "asteroids."