The IAU’s planet definition ejects Pluto and the other dwarf planets of the solar system from the list of worlds that the IAU considers to be planets – this despite the IAU confusingly adopting the term “dwarf planet” for these worlds. ~ My conclusion is that the IAU definition is not only unworkable and unteachable, but so scientifically flawed and internally contradictory that it cannot be strongly defended against claims of scientific sloppiness, “ir-rigor,” and cogent classification.
The New Horizons project, like a growing number of the public, and many hundreds if not thousands of professional research astronomers and planetary scientists, will not recognize the IAU’s planet definition resolution of Aug. 24, 2006.
As the leader of New Horizons, I believe this is the right course for us to take, for a number of reasons. One is that the IAU definition is too controversial. Another is that it is losing, rather than gaining, support, and is likely to be largely irrelevant in the long term. A third is, despite the fact that the IAU has put its reputation behind such words, we should not support a technically and linguistically flawed planet definition: doing so would only further the damage to astronomy already done by this definition.
Some IAU astronomers are challenged by the implications of this recent realization, which portends that the Earth is more a misfit object in the solar system than is Pluto, and that dwarf planets, like dwarf stars and dwarf galaxies, are the most representative members of their astronomical genus.
So on this Web site and in documents, discussions and other aspects of the New Horizons mission, we will continue to refer to Pluto as the ninth planet. I think most of you will agree with that decision and cheer us on.
As I’ve said before in print, were location and context valid in biology the way that some want it to matter in planet definitions, a cowboy would become a cow when he herds his cattle.
I’ve got no problem with Pluto being a dwarf planet.