If you answered nine then you would be wrong.The International Astronomical Union (IAU) have today decided that Pluto is not a planet.
A proposal was tabled at the IAU General Assembly meeting in Prague that officially definined the word "planet".
The debate was brought to a head by the discovery of a potential 10th planet, temporarily named 2003 UB313 in January 2005. This new candidate planet is bigger than Pluto and the question that faced the IAU was whether to make this new discovery a planet.
Pluto has been considered a planet since its discovery in 1930 by the American Clyde Tombaugh.
