The IAU have just in the last few hours voted on the definition of the word planet. The new definition removes Pluto from the family of planets and places it into a new category of dwarf-planets.
Here is 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 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) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
And resolution 6a:
The IAU further resolves:
Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
The previous resolution that got all the news headlines last week, was shot down 2 days later. Under this resolution we would also include Ceres, Charon and 2003 UB313.
This was deeply unpopular with a lot of astronomers including myself, although 12 doesn't sound like too bad a number it would grow rapidly and many trans-Neptunian objects would find themselves being classed as planets, even our Moon under that definition would become a planet in approximately 3 billion years.
So there it is, we have a precise scientific definition for the word 'planet' and we have a solar system with 8 planets; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.