I'm surprised this hasn't been mentioned already, but then
again, my search-fu is weak so it might have been. Over the past
week a commitee of the International Astronomical Union (IAC)
have been meeting in Prague to come up with an official
definition of a planet. The idea was to finally resolve whether
of not Pluto was a planet or just a sizable lump of rock. On
Wednesday(-ish) they released their proposal. It's based on the
gravity of the object in question and basically says a planet is
round and orbits a star (and not another planet, 'cause that
would make it a satellite). This leads to their recommendation
that the Solar system be increased to 12 planets. The new ones
being Ceres, largest rock in the asteroid belt; Charon, Pluto's
'moon'; and UB313 (aka 'Xena'), which is beyond Pluto. The
interesting thing is that Pluto and Charon form a 'double
planet' where the centre of rotation is between the two, so
neither is the 'main' planet. There are those opposed to the
idea, including the discoverer of Xena,