Lo siento es difícil explicar este argumento en español por mí 
The International Astronomical Union (IAU) changed on the last 24th August the traditional definition of planet: this is the new one:
A planet is:
1) a celestial body which has its orbit around the sun
2) its mass is big enough to let it have a spheric shape
3) its orbit doesn't cross other orbits of other planets
so Plutón is:
1) a celestial body which has its orbit around the sun
2) its mass is big enough to let it have a spheric shape
3) its orbit crosses Neptune's orbit
the thing I didn't get is: why Neptune is still a planet? if Pluto isn't a planet anymore because it crosses Neptune's orbit, isn't true also the opposite? 
I'll try to get further informations