Pluto loses status as a planet - BBC
Here is the decision of the IAU.
They made the most conservative decision of preserving the appearance of the classic planets line-up... minus Pluto.
The final decision is based on the orbit-related argument that a planet must be the leader of its neighbourhood. If it is not a leader, it will be a new category of object.
Here are my first comments:
1. The definition will have to be rewritten in the next couple of years as more objects are discovered far away at the edge of the solar system. Bigger objects will be found. An object bigger than Mercury will eventually be found. This object may have an unstable orbit.
2. If, eventually, a small object is found to have a stable orbit (because it is truly the biggest one in the neighboorhood), its small size compared with other objects will make its planet-status sound ridiculous (and indeed it will be).
3. More extra-solar planets will be found... showing how orbit-based definition contradict common sense attitude to classify objects by their own characterists only
In the long run, the biggest casualty is science education, the IAU had a chance to show to the non-scientific world that our universe is very diverse. Stars have very diverse characteristics and very diverse environments, yet they are all called stars... no matter what and how they are orbiting.
Asteroids are called asteroids no matter what and how they are orbiting. The so-called Kuiper-Belt Objects will remain mysterious to most people as they are neither planets nor asteroids.
To summarize, the IAU has chosen to narrow down the definition of a planet instead of widening it and including more sub-groups to the current distinction between rocky planets and gas planets.
The main gain of the discussion was a genuine search for the common characteristics of planetary objects. Given the diversity of the universe, it's an impossible task.
A great merit of the new discussion is that it has become empirically easier to identify a planet from... something else. It will not change anything to the work of thousands of scientists who know that each object is unique and cannot easily fit one category or another.
The main casualty is education. We will have a "dull" narrow-minded vision of a simple solar system.
This is not the end of the story. Let us just wait and see.
Pluto's status has been contested for many years
About 2,500 experts were in Prague for the International Astronomical Union's (IAU) general assembly.
Astronomers rejected a proposal that would have retained Pluto as a planet and brought three other objects into the cosmic club.
Pluto has been considered a planet since its discovery in 1930 by the American Clyde Tombaugh.
The vote effectively means the ninth planet will now be airbrushed out of school and university textbooks.
The decision was made at a meeting of the International Astronomical Union (IAU) in the Czech capital Prague.
Pluto's status has been contested for many years as it is further away and considerably smaller than the eight other planets in our Solar System.
Since the early 1990s, astronomers have found several other objects of comparable size to Pluto in an outer region of the Solar System called the Kuiper Belt.
Some astronomers believe Pluto belongs with this population of small, icy "Trans-Neptunians", not with the objects we call planets.
Allowances were once made for Pluto on account of its size. At just 2,360km (1,467 miles) across, Pluto is significantly smaller than the other planets. But until recently, it was still the biggest known object in the Kuiper Belt.That changed with the discovery of 2003 UB313 by Professor Mike Brown and colleagues at the California Institute of Technology (Caltech). After being measured with the Hubble Space Telescope, it was shown to be some 3,000km (1,864 miles) in diameter, making it larger than the ninth planet.