Actually this is not a redefinition of the planet concept, the truth is that we have never had a definition at all! The reason is that of the current nine accepted planets six were known since prehistoric times (when people used FORTRAN and hunted woolly mammooths) because they are easy noticeable in the sky (changing it's position everyday and not blinking as stars). When W. Herschel discovered Neptune and the posterior finding of Uranus (which was predicted by theoretical calculations!!!) it was clear that they were very similar to the known planets so there wasn't much contreversy.
Using the same theoretical arguments it was expected that a ninth planet would eventually emerge and Clyde Tombaugh eventually "found" it at Lowell Observatory in 1930. It was only decades later when it's "moon" Charon was found and it became possible to approximate Pluto's mass, this mass is so small that it has a negligible effect on the orbits of the other planets (another interesting feature of the Pluto-Charon system is that the center of gravity of this system is outside of Pluto, so both orbit a common center somewhere between them) so the question remained open and the existence of a "Planet X" was expected (at least by a few astronomers). Today we know that the data used for the "theoretical" prediction of Pluto was simply wrong and there is nothing as a "Planet X", although high precission measure of the orbits of the probes Pioneer 10 and 11 seem to point to some source of perturbation a major planet is not in the possible explanantions.
Recently the problem of defining a planet has emerged from two principal sources:
- The extrasolar planets found using diverse techniques seem to be way more massive than Jupiter (the most massive planet in the solar system) and some of this planets were found so close to it's star that it challenged the model of planet formation of that time and it's still a bit problematic. It was hard to know where planets end and stars begin.
This has generated much controversy about what is a planet, as our current list of planets seems to include an object that is esentially a big asteroid (Pluto) and some really big extrasolar planets that are almost stars. So the IAU has proposed the following definition:
A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.
Using this definition the solar system would increase inmediately to 12 (planetary) members, and the new folks in town will be: Ceres, Charon (so the Pluto-Charon system would be considered a double planet) and Xena. There are other obvious candidates like Quoar that would enter the planet definition as soon as astronomers confirm their properties. The definition also includes a new class of planet: pluton that applies to all planets similar to (ughh) Pluto.
These are the "new" planets, one of them (Ceres) has been considered for long time as an asteroid of the asteroid belt between Mars and Jupiter.
In my personal opinion this "new" definition is nonsense, despite it is based on a scientific and unambigous definition it simply adopts a "leave no big icy rock behind" and this "new" solar system is way too messy (specially for the public, after all the definition isn't really that much relevant for astronomers), the solar system will have eventually hundreds of planets that have little (if any) in common like Ceres and Jupiter. Not only that, this definition is completely ambigous on the other end of the spectrum and leaves the separation between brown dwarfs and big planets as blurry as it can be. It is expected that the resolution will be decided on August 24th. Many of my peers are in Prague waiting impatiently the result of this resolution, I will inform you about it as soon as I can.
Look at some of the new candidates for planets, all them are very small, actually some of the moons of Jupiter and Saturn have the double (or more) of its size. Just compare the size of Vesta and Pallas (in the asteroid belt) with the size of the earth.