Astronomers from all over the world, meeting in Prague in August 2006, have voted to no longer call Pluto a planet.
The scientists agreed that for a celestial body to qualify as a planet:
it must be in orbit around the Sun
it must be large enough that it takes on a nearly round shape
it has cleared its orbit of other objects
Pluto was disqualified because its highly elliptical orbit overlaps with that of Neptune. It will now join a new category of dwarf planets.
About 2,500 scientists meeting in Prague have adopted these new guidelines, and now we will have to talk about a solar system with only eight planets.