It has just been decided by the International Astronomical Union (IAU), that Pluto has lost its status as a planet.
A “planet” is now defined as (IAU):
…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.
Pluto fails on the third criteria, (c).
Well, I’m sure it’s not the end of days for astrologers (and other crackpots) yet – for example, you can still obtain a free computer-generated horoscope here.
Update: there is a critical discussion about the definition of a “planet”, and whether we should really care much about such a definition, at Bad Astronomy; and Alan Stern, leader of NASA’s New Horizon’s mission to Pluto, thinks that the definition stinks.