Just when you thought you knew the solar system...

Some long term readers may recall a heated debate twelve years ago over the nature of Pluto. When it was first discovered over a century ago, astronomers named it and labelled it the ninth planet. Generations of students learned the names of the planets, and wrote endless reports about the ninth planet. A few years ago the New Horizons probe sent back detailed images of its surface, taken from orbit during a flyby as the probe left the solar system. It is a planet.

However over the last few decades, astronomers have discovered hundreds of other objects floating in space near Pluto, and many of these are comparable in size to our smallest planet. It is now believed that there are objects in the Kuiper belt that may actually be more planet-like than Pluto. And that resulted in a campaign by some astronomers to have Pluto stripped of its planetary status.

The problem is that there is no definition of a planet that includes all of the other planets in our Solar System but excludes all of these objects on the edges. In the end, astronomers agreed that a planet needs to meet three criteria  - it must be spherical under its own gravity (to exclude rocks and debris), it must primarily be orbiting the Sun rather than orbiting another large object (to exclude the various moons), and it must have cleared everything else from its orbit. It is this third requirement that caused the controversy, and many in the astronomy community were opposed to this third criteria. It excludes Pluto and the other distant objects, since they are part of the Kuiper belt and not isolated, but it also could mean that when the Earth or even the massive planets such as Saturn and Jupiter are passing through meteors and asteroids, they could suddenly lose their planetary status as well! 

And this situation was not helped by the way the change was made, with a single astronomy organization (The International Astronomical Union) holding an unannounced and not publicized vote among less than four hundred astronomers - most of whom were not specialists in planetary science - and taking those results to be permanently binding (It also doesn't help that one of the key campaigners to make this change has built his entire career around writing books and doing interviews in which he boasts about "killing Pluto". There is no place in scientific research for that kind of boasting and gloating.). Following that disaster, a number of politicians and scientific organizations held their own votes and declared that they would still consider Pluto a planet. And the debate rages on to this day.

However a new paper that was published last week has added further fuel to the proverbial fire. 

This new result has nothing to do with any new scientific discovery or new astronomical observations, but is rather an argument based on how the word "planet" developed over the last two hundred years. In searching through academic literature from that time frame, the authors of this study uncovered at least one hundred references to a "planet" in which the word was applied to an object which violates at least one of the IAUs three requirements. As recently as the 1950s, professional astronomers were still writing research papers to be published in major, peer reviewed journals, which referred to such things as asteroids as "small planets". 

And that has renewed the arguments by many in the scientific community and society at large that the method of naming things and defining terms be changed. Many people feel that when the IAU held a small vote amongst a small group of their own membership, and used that to permanently change the definition of a word whose meaning dates back for many centuries, they have harmed the reputation of the scientific community by replacing logic and reason with what is in the end little more than a popularity contest.

In the end, science must always be decided by facts and logical reasoning, and not by popular vote. And in the case of the IAU and Pluto, the facts are that Pluto is and always will be a planet!