Why did they downgrade Pluto?

Answer 1

Pluto was not exactly down graded, it was reclassified. The reason for this is since Pluto was discovered, we have learned more about our solar system and discovered more objects (some larger than Pluto) that orbit the Sun. This new knowledge means that it is more sensible to group Pluto in with these new discoveries than to classify it as a planet.

---

Pluto was the "Ninth Planet", and there was incredible competition among astronomers to find the "Tenth Planet". One team had even picked out a name - "Xena" - as being perhaps appropriate. (That the entire team were big fans of Lucy Lawless and the "Xena - Warrior Princess" TV program probably goes without saying.)

Several candidate bodies had been identified, but the International Astronomical Union decided to come up with a formalized definition of "planet", which had never existed before. Each of the potential "Tenth Planet" candidates were as large as or larger than Pluto, so the IAU devised a definition that excluded them all, and stripped Pluto of its planetary status at the same time.

The first two requirements are pretty obvious; it has to orbit the Sun and be massive enough for gravity to have crushed it into a more-or-less spherical shape.

But the third criteria, that it have cleared its orbit of other objects, could exclude the Earth from planet-hood as well. As we've seen over the past few years and especially on February 15, 2013, there are a LOT of other objects in our orbit. Case in point; the asteroid Apophis, which comes fairly close to the Earth every seven years.

Astronomers are enormous geeks, as you can probably tell. "Apophis", in addition to being a minor Egyptian diety, was the prime villain in the TV series "Stargate - SG1", which was a favorite of the team that discovered it.