Under the new classification system, Pluto will be a "dwarf planet;" no longer considered a full-fledged planet, although the initial proposal on August 16 would have kept it so.

The vote by members of the International Astronomical Union (IAU) came after eight days of contentious debate that involved four separate proposals at the group's meeting in Prague.

The initial proposal, hammered out by a group of seven astronomers, historians and authors, attempted to preserve Pluto as a planet but was widely criticized for diluting the meaning of the word.

The category of "dwarf planet" is expected to include dozens of round objects already discovered beyond Neptune. Ultimately, hundreds will probably be found, astronomers say.

With these new rules in place:
Planets: The eight worlds from Mercury to Neptune.
Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite."
Small Solar System Bodies: All other objects orbiting the Sun.

In the early 1800s, an object, beyond Neptune, was discovered orbiting the Sun. Named Ceres, it would soon be demoted from planetary status. In the 1930s, the discovery of Pluto gave us a new planet once again. Although Pluto was thought to be much larger than it is, the word "planet" originally described wanderers of the sky that moved against the relatively fixed background of star. The debate on whether Pluto should be called a planet has continued since the late 1990s. Today's vote comes after two years of work on defining the word "planet." Astronomers at the IAU meeting debated the proposals right up to the moment of the vote.