This article from the IAU states

The first draft proposal for the definition of a planet was debated vigorously by astronomers at the 2006 IAU General Assembly in Prague and a new version slowly took shape. This new version was more acceptable to the majority

What were the other proposals for the definition of a planet? What was changed between the first draft and the final version that was voted on?


1 Answer 1


The IAU website has this draft definition of "planet" and "plutons" page, published on 16 August 2006 in Prague. It says (emphasis mine):

The world's astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the difference between "planets" and the smaller "solar system bodies" such as comets and asteroids. If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, our Solar System will include 12 planets, with more to come: eight classical planets that dominate the system, three planets in a new and growing category of "plutons" - Pluto-like objects - and Ceres. Pluto remains a planet and is the prototype for the new category of "plutons."

The proposed 12 planets Solar System looked like this (credit: The International Astronomical Union/Martin Kornmesser):

enter image description here

The final press release, published on 24 August 2006, says:

The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as 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.

This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name).

Which looked like this (credit: The International Astronomical Union/Martin Kornmesser):

enter image description here

So in 8 days, the IAU switched from a 12 planets system, with a planet subcategory called "plutons", to a 8 planets system, with an dwarf planet category distinct from the planet category. How did that happen? It's probably something only a General Assembly attendant who witnessed the discussion would know. It must have been quite a debate!


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