IAU definition of planet:
The definition of “planet” set in 2006 by the International Astronomical Union (IAU) states that in the Solar System a planet is a celestial body that:
1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has “cleared the neighbourhood” around its orbit.
A non-satellite body fulfilling only the first two of these criteria is classified as a “dwarf planet”, which is not a type of planet, while a non-satellite body fulfilling only the first criterion is termed a “small solar system body” (SSSB). Initial drafts planned to include dwarf planets as a subcategory of planets, but because this could potentially have led to the addition of several dozens of planets in the Solar System, this draft was eventually dropped. In 2006, it would only have led to the addition of three (Ceres, Eris and Makemake) and the reclassification of one (Pluto). The definition was a controversial one and has drawn both support and criticism from different astronomers, but has remained in use.
According to the definition, there are currently eight planets and five dwarf planets known in the Solar System. The definition distinguishes planets from smaller bodies and is not useful outside the Solar System, where smaller bodies cannot be found yet. Extrasolar planets, or exoplanets, are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger.
Reasons for the debate:
Before the discoveries of the early 21st century, astronomers had no real need for a formal definition for planets. With the discovery of Pluto in 1930, astronomers considered the solar system to have nine planets, along with thousands of smaller bodies such as asteroids and comets. Pluto was thought to be larger than Mercury.
In 1978, the discovery of Pluto’s moon Charon radically changed this picture. By measuring Charon’s orbital period, astronomers could accurately calculate Pluto’s mass for the first time, which they found to be much smaller than expected.[1] Pluto’s mass was roughly one twentieth of Mercury’s, making it by far the smallest planet, smaller even than the Earth’s Moon, although it was still over ten times as massive as the largest asteroid, Ceres.
In the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt Objects, or KBOs.[2] Many of these shared some of Pluto’s key orbital characteristics and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, and some astronomers stopped referring to Pluto as a planet.[3] Pluto’s eccentric and inclined orbit, while very unusual for a planet, fits in well with the other KBOs. New York City’s newly renovated Hayden Planetarium did not include Pluto in its exhibit of the planets when it reopened as the Rose Center for Earth and Space in 2000.[4]
Starting in 2000, with the discovery of at least three bodies (Quaoar, Sedna, and Eris) all comparable to Pluto in terms of size and orbit, it became clear that either they all had to be called planets or Pluto would have to be reclassified. Astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other solar systems. In 2006, the matter came to a head with the measurement of the size of 2003 UB313. Eris (as it is now known) turned out to be slightly larger than Pluto, and so was thought to be equally deserving of the status of ‘planet’.[3]
http://en.wikipedia.org/wiki/2006_definition_of_planet
Thans ETL.
Mike Brown on the planet debate:
http://web.gps.caltech.edu/~mbrown/whatsaplanet/
http://www.nytimes.com/2005/08/23/science/space/23profile.html
dwarf planets:
http://www.gps.caltech.edu/~mbrown/dwarfplanets.html
Eris:
http://web.gps.caltech.edu/~mbrown/planetlila/
Quaoar:
http://www.chadtrujillo.com/quaoar/
Sedna:
http://web.gps.caltech.edu/~mbrown/sedna/
Not true. Pluto could be "grandfathered" and then the standards could be tightened. It would always remain an interesting lesson on early astronomy.