In 2006, the International Astronomical Union (IAU) reclassified Pluto as a “dwarf planet”. Gone were the days of nine planets, while ahead lay contention about the criteria that resulted in Pluto’s demotion. Many astronomers and enthusiasts alike have voiced concerns about this change. Personally, I also question this change, in particular, the criteria.

Why was the change made? Over forty objects have been found that are similar to Pluto. The IAU’s position is that calling all these objects major planets is excessive, particularly as there are probably a great many more of these objects yet to be discovered. This is certainly worth considering. However, their criteria actually seems to hinder the very consistency they were aiming for.

Can a planet even be defined scientifically, rather than culturally? Article Pluto and the Developing Landscape of Our Solar System on the IAU’s website notes the following criteria for a major planet: “A "planet" is 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 neighborhood around its orbit.”

These criteria lack clarity, sparking much debate. The dwarf planet criteria differs in that such a planet does not have to clear the neighborhood around its orbit, and is not a satellite. Pluto satisfies this criteria. However, other planets aren’t clearing neighborhood objects either, and yet they get to keep the title. This is one of many examples of how these ambiguous definitions have raised questions about this change.

Many astronomers have given views on this matter. In chapter 9 of Astronomy for Dummies (published in 2005, a year before the change) author Stephen Maran suggested that Pluto be “grandfathered in”, should there be a change in planetary definitions. Obviously this didn’t happen, but I think it remains a meritorious fix.

Is there any point in the status debate? In his post Breaking News – Pluto not a planet, Bad Astronomy blogger Phil Plait described the debate as “a tempest in a teapot”, questioning its contribution to science. He also pointed out that, on the plus side, it was cultivating interest in the field: “…it’s great that folks are actually talking about astronomy. Maybe it’ll lead to some of them looking into it more, and that’s a good thing”.

I agree that it is good to be consistent, and obviously knowing what to call such objects can make life a bit easier. Learning about objects such as Pluto is fascinating, and investigating them can bring something useful to astronomy. But fighting over whether they should be labelled A or B? I don’t think that brings anything useful scientifically.

For more information, Scientists decide Pluto’s no longer a planet is an interesting article covering some of the technical problems in the criteria. The IAU’s Pluto and the Developing Landscape of Our Solar System article explains their position on Pluto and planetary definitions.

Sources

Pluto and the Developing Landscape of Our Solar System, International Astronomical Union website, accessed: 1st February 2009, no author specified.

Scientists decide Pluto’s no longer a planet, Space.com website, published 24th August 2006, accessed: 1st February 2009, author: Britt, R.R.

Far Out! Uranus, Neptune, Pluto , and Beyond (chapter 9, p.139), Astronomy for Dummies, published by Wiley in 2005, author: Maran, S.P.

Breaking News – Pluto not a planet!, Bad Astronomy (Discover Magazine) website, published: 24th August 2006, accessed: 1st February 2009, author: Plait, P.