Is Pluto a Planet?

Is Pluto a planet? You'll probably remember that back in August the issue was debated (MP3 - 10.1MB) at the International Astronomical Union's General Assembly and a working definition of a planet was voted upon making Pluto a dwarf planet. The result left many people - including some planetary scientists - upset and several petitions (here, there and elsewhere) were set up in an attempt to re-instate Pluto to "full" planet-hood.

To many, the result of the vote was a terrible attack on poor little Pluto and they were astonished that anyone could change the number of planets in the solar system. After all, everyone knows that there have always been nine. Don't they? Actually, this isn't true and having an historical perspective on how we have got to the current number is a vital part of the debate. Over the past week I've been reading a great new book titled Is Pluto a Planet? by Professor David A. Weintraub (courtesy of the nice people at Princeton University Press), which gives a very thorough background to the history of the definition of a planet as well as listing and discussing all the possible scientific definitions.

Professor Weintraub starts at the beginning with what the ancients knew of as planets - the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn - and how they fitted into the universe. The book then charts the list of planets through the middle ages and into modern times adding and removing a whole suite of objects (comets, asteroids, Ceres, Triton, Planet X and Vulcan to name just a few) as they were discovered, and subsequently found not to be suitable for the label "planet", or in some cases not actually exist! The total number of planets has varied quite a bit with as few as six in 1800 to many as 12 by the 1840s.

Perhaps surprisingly given the title, Pluto doesn't really feature in the book until about half way through, but the first half is necessary to understand why Pluto got onto the planet list in 1930. In fact, I hadn't realised until I read this book that Pluto was actually the fourth planet to hold the title of "the ninth planet", the previous three being Jupiter's moon Europa, the asteroid Pellas and the intra-Mercurial Vulcan.

What the book demonstrates brilliantly is that time and time again philosophers and scientists have had to revise their definitions in light of new objects and new understanding. This constant adjustment is fascinating and really demonstrates how preconceived ideas about how the universe should (or is required to) be are challenged by new discoveries. The first good example in the book is Aristotle who was able to dominate our understanding of planets for over a thousand years until his ideas finally became untenable in the face of all the evidence. This is what science is about and this book shows that many times.

So, does Professor Weintraub answer the question he puts forward in the title? Well, he presents all the scientific arguments for and against and ultimately comes down on one side of the fence (to find out which side you'll have to read the book!). He makes many good points and he certainly made me question many of the planet definitions suggested back in August.

I do have some slight quibbles with the book: I thought that the Galilean moons were introduced a little too early on, I found one of the pictures showing Ursa Major a bit confusing and there was no discussion of the IAU vote in August. It would have been interesting to get Professor Weintraub's view on the IAU vote, but it is understandable that this was omitted as books usually have quite a long lead time before appearing in the shops. Still, these are only minor points and don't detract from the rest of the book which was a good read. If you want a good background to planets and the make-up of our solar system, I would definitely recommend that you read this. You'll probably learn something. I did.

Posted in astro blog by Stuart on Tuesday 31st Oct 2006 (22:35 GMT) | Permalink
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