Pluto no longer a planet

GET out your astronomy books and cross out Pluto from the list of planets. (see the fullsize image here)

Pluto has been demoted to dwarf planet status under the International Astronomical Union (IAU)’s new resolution on the definition of a planet.

IAU’s resolution 5A defines planets and other bodies in our Solar System into three distinct categories:

• A “planet” is 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 neighbourhood around its orb

• A “dwarf planet” is 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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite

• All other objects except satellites orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies.”

Majority of the the 2,500 astronomers who attended the IAU’s 26th General Assembly agreed that Pluto is not a planet because its orbit does not clear the neighborhood.

The IAU also decided that planet and dwarf planets “are two distinct classes of objects.”

Resolution 6A defines Pluto-class objects in the dwarf planet category. These include Ceres, Pluto and 2003 UB313 (a temporary name). The IAU is expected to announce more dwarf planets in the future.

There are currently a dozen candidate dwarf planets listed on the IAU’s dwarf planet watchlist, which continually changes as new objects are discovered and the “physics of the existing candidates becomes better known.”