Images: It's official--Pluto's out

An International Astronomical Union's committee had proposed a new definition of a planet as an object that orbits the Sun but is massive enough for its own gravity to put it into a spherical shape. Other scientists argued that the three new proposed planets and Pluto were too small and insignificant to be considered full-fledged planets.

The dozen planets would have included Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon, Ceres and Xena.

This is an artist's montage of an "expanded" solar system.

Photo by NASA