Pluto has been demoted.
The celestial body, long known as one of the nine planets of the solar system, will now be considered a "dwarf planet," the General Assembly of the 2006 International Astronomical Union ruled in a vote Thursday in Prague, Czech Republic.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune will be defined as "classical planets."
Three other bodies had been contending for planetary status as well: Ceres, the largest-known asteroid; "Xena," the nickname for 2003 UB313; and Charon, which has been considered Pluto"s moon. Ceres and "Xena" will now share "dwarf planet" status with Pluto. Charon, it has been concluded, will be grouped with "small solar-system bodies."
The IAU said in a statement on Thursday that the definition for planet is now officially "a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and (c) has cleared the neighborhood around its orbit."