On the word PLANET

False color image of Pluto's heart (Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

It's been more than ten years since a mysterious cabal of astronomers known as the International Astronomical Union (IAU) reclassified Pluto under a newly devised category of dwarf planet and curators of natural history museums have long since adjusted their exhibits to reflect an eight-planet solar system, but even after all this time, it apparently doesn't take much to reignite the debate surrounding the issue. In March of this year, a PhD student from Johns Hopkins University presented a poster at the Lunar and Planetary Science Conference in Texas that was enough to make headlines on both sides of the Atlantic because it proposed a revised definition of the word planet that would return Pluto to the category along with a further hundred or so other objects from our solar system.

From a linguistic perspective, the Pluto debate raises some very interesting questions. For instance, why did anyone think that the IAU had the power to change the meaning of an English word as opposed to merely establishing a terminological convention for technical discourse within the field of astronomy? Why is the reclassification of Pluto frequently described as a demotion as if it's some kind of honor for a celestial body to be called a planet? And in what sense has the meaning of the word planet actually changed in the minds of speakers as opposed to what they believe about how many planets there are? The discussion around Pluto's reclassification reveals a lot about how people think about the nature of both language and science, so I think it's worth digging a little deeper into these issues.

On falling apples and whether tomatoes really are fruit

Imagine a film of an apple accelerating toward the ground under the influence of gravity. If we played this film backwards, what would we see? It may seem odd, but a physicist will tell us that when the film is played backwards, the apple will continue to accelerate downwards! Regardless of whether the film is played forwards or backwards, the apple will accelerate downwards.

It is because physicists use the word accelerate with a slightly different meaning to the one in everyday use that an otherwise trivial fact about the universe can be cast as such a curious statement.