So, in school (that's a long time ago) they have been teaching us there are 9 planets in our solar system.

Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto

But every now and then I keep reading stories about another "dwarf planet" (Eris, discovered in 2005) that - depending on what source tells the story - is another planet according to the astronomical definition, while other sources say that it isn't a planet. Some even say Pluto isn't a planet anymore either.
The result: I'm confused due to the contradicting stories. Even Wikipedia isn't clear about Eris and only writes (emphasis mine):

NASA initially described it as the Solar System's tenth planet.

Initially? So, is it a 10th planet or not? Fact is, there is another "something" out there and it surely seems to look like a planet. Yet, some people keep stating there are 9 planets in our solar system, while others say there are more than 9 planets, and then again there are people stating that the latest definition of "planet" has kicked out Pluto too so there are actually fewer than 9 planets in our solar system.
Trying to get a definite, official, and astronomically correct answer I can actually rely on, I'm therefore asking: How many planets are there in this solar system?
EDIT
The "Definition of planet" at Wikipedia doesn't really help either, as it states:

Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognize Pluto's historical identity as a planet by "grandfathering" it into the planet list.*
* Dr. Bonnie Buratti (2005), "Topic — First Mission to Pluto and the Kuiper Belt; "From Darkness to Light: The Exploration of the Planet Pluto"", Jet Propulsion Laboratory. Retrieved 2007-02-22.

So, if you link somewhere to provide proof, it would be great if you could point me to a more trusted source than Wikipedia. Ideally, an astronomical trusted source and/or paper.

Since we're talking about terminology, we need to remember that none of this really matters, outside of clarity when communicating. Still, some people tend to have rather strong opinions on it, thus confusion about how many planets are really in the solar system arises.

The people

The most trusted source in Astronomy would have to be the people that set the generally accepted rules. The IAU (International Astronomical Union) has been in existence since 1919 and is comprised of 10814 Individual Members in 93 different countries worldwide. Of those countries, 73 are National Members.


  The key activity of the IAU is the organization of scientific meetings. Every year the IAU sponsors nine international IAU Symposia. The IAU Symposium Proceedings series is the flagship of the IAU publications. Every three years the IAU holds a General Assembly, which offers six IAU Symposia, some 25 Joint Discussions and Special Sessions, and individual business and scientific meetings of Divisions, Commissions, and Working Groups. The proceedings of Joint Discussions and Special Sessions are published in the Highlights of Astronomy series. The reports of the GA business meetings are published in the Transactions of the IAU - B series.


The definition

At the 2006 IAU General Assembly in Prague, the accepted definition of a planet was debated vigorously. The outcome of the meeting was the currently accepted definition of a planet:


  A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.


What about Pluto?

With this in mind, the group decided on Pluto's fate. From page 2 of this official resolution document:


  The IAU further resolves: 
  
  Pluto is a "dwarf planet" by the above definition and is recognized as the 
  prototype of a new category of Trans-Neptunian Objects. 


The reason given in the FAQ on this page:


  Q: Why is Pluto now called a dwarf planet? 
  
  A: Pluto now falls into the dwarf planet category on account of its size and the fact that it resides within a zone of other similarly-sized objects known as the transneptunian region. 


Basically, they decided that it isn't officially a planet anymore because it didn't match criteria (c): has cleared the neighbourhood around its orbit. It hasn't done this, because it 'resides within a zone of other similarly-sized objects'. Therefore, it hasn't cleared its neighborhood.

Soo... what about the number of planets?


  Q: Based on this new definition, how many planets are there in our Solar System?
  
  A: There are eight planets in our Solar System; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Mnemonic: My Very Educated Mother Just Served Us Nachos.


But that's if you don't count Dwarf planets - if you do count them, you end up with five more:


Ceres
Pluto
Eris
Makemake
Haumea


So there are 8 planets in the solar system if you don't count Dwarfs, 13 if you do.
