The Planets, by Dava Sobel

The little green spacecraft we all live on

The names of the planets, or most of them, have been fixed for centuries, but newly discovered objects must conform to a highly structured naming scheme. If it proves to be a planet, Xena's name will be changed to that of a classical god. Should it prove to be something more minor, for instance one of the hundred or so lesser lumps of rock and ice whirling beyond Pluto in the so-called Kuiper Belt, there is more latitude. The global catalogue of underworld deities provides most of the names for these objects but one, apparently, is known as "Smiley", after the Le Carré hero, although its official name is the more prosaic 1992QB1.

Naming conventions are just one of the many by-ways explored in this ruminative and frequently poetic book, a series of essays that takes a planet at a time and explores not just the scientific facts of its existence but its broader cultural meaning. That's a considerable undertaking, and some readers will feel short-changed when Sobel's interests do not correspond with their own. There must, for instance, be much more to say about images of the Moon in literature, or even pop song, than we get here.

It is science that excites Sobel. There's a real gee-whizzery about her treatment of telescopes and space probes and their many astonishing discoveries in recent years. Her treatment of the development of astronomy over the centuries is generous, if concise. Although the ancients had their confusions, notably the persistent idea that the planets moved in concentric crystal spheres around the Earth, they also achieved remarkable feats, finding the first six planets by tracing their wanderings against the fixed stars using the naked eye alone.

Copernicus, writing in the early 16th century, not only placed the Sun at the centre of the solar system, he proposed that the Sun was holding the planets there by a mysterious force. Isaac Newton, 150 years later, set out the workings of this force so skilfully that henceforth astronomers were able to detect the existence of planets even before they saw them, based solely on their gravitational effects. Thus Neptune was discovered a year before anyone managed to track it down with a telescope.

Lengthy and colourful depictions of the planets' surfaces, descriptions of their origins and dramatic histories, and explanations of their inner structures constitute a large part of this book. Admirers of Longitude, though, will be pleased to know that it contains a number of strong human stories, from those of the astronomers to those of the explorers who used their findings to guide them around the globe. But the most amusing anecdote appears only in a footnote. It seems US president Jimmy Carter once reported a mysterious light in the sky to the state police. It was the planet Venus.

Planets dominate people, as some claim they do in life. Sobel allows herself a teasing chapter on astrology, which would have pleased Galileo but will have shocked the contemporary astronomers who have supplied most of her data. It was probably worth doing just for that.

Other flights of fancy seem less well advised. One chapter is presented as an imaginary letter from one 19th-century female astronomer to another. And then there's a chapter narrated by a meteorite from Mars, an exercise familiar to any reader of the venerable "I am John's testicle" column in Readers' Digest.

While the book is dense with facts, its construction is too discursive and its chapters too disparate to make it any sort of reference work. Worse, there's no index. And for the common reader, the absence of a single narrative makes this book easier to put down than to pick up again.

But it is worth making the effort. It is hard to imagine a better picture of the dangerous and inhospitable nature of our solar system, where the existence of any form of life, let alone one capable of travelling to other worlds, is nothing less than a miracle. The book is also a timely reminder of the fragility of the little green spacecraft on which we are all passengers.