At the Edge of the Solar System
Alain Doressoundiram & Emmanuel Lellouch
205 pages including index
published in 2008

In 2006 the International Astronomical Union demoted Pluto, long the ninth and last planet in our Solar System from being a planet into a socalled dwarf planet, a new category not just meant for Pluto, but also a half dozen other planets that had been recently discovered at the edge of the Solar System. With the number of planets rapidly rising and estimates raging from a 200 to 2,000 more to be discovered as well as the general feeling that Pluto, only one fifth the mass of the Moon just did not fit in with the rest of the classical planets, this new categorisation was needed, halfway between true planets and asteroids or comets, now classified as small Solar System bodies.

Surprisingly for such a dry subject, the reclassification of Pluto led to a huge amount of media coverage and some controversy; many people, including myself, saw the argument as somewhat specious or had a sentimental attachment to the idea of the classical nine planets. They now were confronted with the reality of the Solar System being massively more complex than they had suspected, with our knowledge of the very edges of it having expanded massively since even the late seventies. Which is where At the Edge of the Solar System: Icy New Worlds Unveiled comes in: an introductionary text book about these discoveries and how they were made.

Though a relatively short book, the main text only being 178 pages long, it packs in a lot of material. It's a good overview not just of what we know is in the trans-neptunian Solar System but also how we got to know this. Much of what oressoundiram and Lellouch talk about I knew at least a bit of already, but the way they have put it all together made me understand it better. It's laid out as a text book, with frequent explanation boxes to go deeper into scientific concepts or astronomical techniques mentioned in the main text, without breaking up its flow.

The book starts with a very short history of how our ideas evolved from antiquity to the year that Pluto was discovered, 1930, focusing on how measured inaccurancies in the predicted orbit of each successive outer planet led to the discovery of Neptune, Uranus and ultimately Pluto. In the second chapter the focus lies on Pluto itself and what eighty years of observation taught us about it: not very much, due to the huge distance it's from us. It was only in 1978 that somebody actually noticed that Pluto, which we thought had a mass greater than Mercury, was actually a double system with a moon almost as big as itself, Charon. As we slowly got to know more about both Neptune, especially its moon Triton and Pluto/Charon, the more something seemed off about calling Pluto a planet.

From there it's a logical step to look back at how the Solar System might've started and what that may imply for trans-neptunian space, as well as where comets come from. Then it's back to the outer rim and the Kuiper Belt, originally a theoretical concept for the origin of comets, a huge second asteroid/cometoid belt of icy worlds which Pluto is in the middle off. As more and more objects, comets, plutinos and other exotic worlds were being discovered, the belt moved more and more from theory into reality, the more so as more proper, Pluto like planets were being discovered in the nineties and early twentyfirst century.

In the fifth chapter the writers go deeper into those newly discovered worlds, what they look like (as far as we know) and what else we can tell about them from Earthbound observation. The sixth chapter meanwhile is all about the Pluto controversy. As the discoveries detailed in the previous chapters undermined Pluto's uniqueness the need for a reclassification grew and ultimately ended as I described above. There is something to say for this, even if you can't help but suspect part of the motive for it is to keep the number of proper planets down, as there indeed isn't any good reason to keep Pluto one, but none of the other worlds, when some are barely smaller, some as big as and some even are maybe larger than Pluto itself.

Finally the book ends with a short chapter on the history of the outer Solar System as far as we know it and what we don't quite have figured out yet about it, the biggest of which is why the Kuiper belt ends so abruptly. Is that just a question of more observation or could there be a true mechanism in the history of the early Solar System present that could explain this. The very last chapter then takes a quick look at the near future and the projects that are under development to give us a much better view of the outer Solar System and which might help us solve these problems.

I read At the Edge of the Solar System: Icy New Worlds Unveiled in just a few days, mainly while commuting to and from work. Reading any scientific book that way can be a recipe for disaster as you can't get in the flow of it, but the way Doressoundiram and Lellouch have written it made it a breeze to get through, while still keeping a relatively large information density. If you're interested in Pluto and the outer Solar System but not that familiar with it, this is a good introduction.