And 
then this week, the IAU (International Astronomical Union) decided to classify Pluto as a ‘dwarf planet’.
A summary of the (adopted by vote) resolutions can be found at the IAU site, where the honourable astronomers decided first to determine which types of planets there (resolution 5A):
RESOLUTION 5A
The IAU therefore resolves that “planets” and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A “planet”1 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 orbit.
(2) 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) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.
Resolution 6A concludes simple with ‘Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects’.
So long Pluto. We hardly knew ye. Well, in about 8 years, we may learn more about this dwarf planet.
Update 1: I suppose this is bad news for the astrology industry.
Update 2: The semantics of dwarf planets and dwarf humans.
Update 3 (09/07/06): New Scientist on Astrology and Pluto.
Update 4 (09/07/06): And Bad Astronomy Blog on this story.