Worlds within worlds

Next time there is a full moon take a look. The moon occupies half a degree of sky, or 1800 seconds of arc. The astronomers who hunt for planets beyond the solar system, described by Ken Croswell in Planet Quest, routinely measure thousandths of a second of arc, in other words an angle of about one two-millionths that of the full moon.

Or look at Alan Stern and Jacqueline Mitton's Pluto and Charon. As they point out, there are 15 million objects in the sky brighter than Pluto - but that did not stop Clyde Tombaugh from discovering Pluto in 1930. No wonder astronomy is one of the disciplines that used to be called the "exact sciences".

Both these books tell fascinating and overlapping stories that could not have been written before the late 20th century. For more than 400 years we have accepted that we live on a planet, one of a clutch orbiting the sun. Nevertheless, solar-system studies have been among the slower branches of astronomy until recent times. For there was little for observers to do, except peer at a few surface blotches displayed by planets and count their brighter satellites. Stern and Mitton point out that in the early 1940s Gerald Kuiper was the only astronomer in the United States working in planetary studies.

Now planets are all the rage. Apart from Pluto, every planet in the solar system has been visited by space probes, increasing our knowledge of them by a factor of billions in a few years. And Stern and Mitton explain entertainingly that new spacecraft technology and new thinking at Nasa, the US space agency, may bring about a Pluto mission early in the next century.

The story of our knowledge of Pluto is one in which luck as well as skill have played a part. Tombaugh found Pluto after many others had failed, including Milton Humason, one of the best observational astronomers of the century. Even the search for it was based on a false premise, that a large planet must be present in the outer solar system to perturb the orbits of Uranus and Neptune. This would take a planet bigger than the Earth, but Pluto has about 0.2 per cent the mass of Earth, far too little to affect the massive planets of the outer solar system.

Once discovered, Pluto revealed little of itself until the 1970s, when two things happened. Its spectrum was resolved and showed methane ice. And astronomer James Christy found a minute blip on images of Pluto - first thought to be a flaw in photographic plates (there had been such errors, including one confusion by the luckless Humason)- which proved to be its satellite, Charon. This discovery provided an indispensable aid for increasing our knowledge of Pluto.

Since then, things have moved on apace, again with a good dose of luck. Every 248 years, observers on Earth have the chance to see mutual occultations of Pluto and Charon, which are the ideal way to find out the characteristics of each object. Fortunately, these were about to start as Charon was discovered, and revealed a wealth of new data, including the fact that Charon has a surface of ice.

Stern and Mitton's account of this is well told, although a spot more editing might have eliminated "verticle", "shined" and "volcanos", or the two places in the book where the reader is asked to compare spectra in facing diagrams calibrated in different units, microns and ngstroms.

Nowadays we can make accurate maps of Pluto and the Hubble telescope can produce images of Pluto and Charon as separate objects. But all this effort is not simply about describing a couple of frozen pimples in the outer reaches of the solar system. More interesting is the message of Pluto and Charon about the solar system in general, to receive which we must abandon the idea that Pluto is a planet like Jupiter or the Earth. Also disproved is the idea that it is an escaped satellite of Neptune; in fact Triton, one of Neptune's satellites, turns out to be a captured planet. Rather, Pluto is the biggest member of a class of icy objects in the outer solar system which were once common and which the growth of Neptune prevented from achieving full planetary status. These objects, of which more are now being found, are vital to our understanding of the formation and development of the solar system. Stern and Mitton do a good job of telling us why.

Their discovery coincides with our growing ability to answer a question that had always seemed insoluble: do other stars than our sun have planets, and if so how common are they? Croswell's style of answer shows that he is a gifted writer. But his book starts badly, devoting more than 70 pages to a ramble through our own solar system that should have been compressed into a page and which slows the pace to a crawl.

When at last Planet Quest gets going, it tells some story. Take Barnard's star, the nearest to the sun after the Alpha Centauri group. Its apparent wobbles in its movement across the sky were once taken as proof positive that it has planets. The main proponent of this view, Peter van der Kamp, never got over the proof that the cause was systematic error on his part. Later Andrew Lyne of Jodrell Bank (one of the few non-Americans in these books) received a standing ovation at the American Astronomical Society for making an alteration to the end of his paper on a planet he thought he had discovered orbiting a pulsar (a burnt-out star) in 1991. He admitted that the planet never was, and had been produced by unsuspected errors in a computer programme.

Despite the distances involved, there are several ways of detecting "extrasolar" planets. The two now favoured involve looking for gravitational effects on its central star, which can be observed by charting its movement in space, and - the more successful method -hunting for the effect that the star's movement has on its spectrum. In future it may be possible to detect planets directly from their heat emissions, and to find planets in deep space far from any star.

But even the methods we already have show us that planets are reasonably common. The number of known solar systems is approaching double figures. With a little chutzpah it is even possible to classify extrasolar planets into Earth-like terrestrials, Jupiter types (Jupiter, or 47 Ursae Majoris B) or Pegasians, resembling those in the solar system of the star 51 Pegasi.

Reporters told about extrasolar planets naturally ask whether they have life. If they do, the odds are that it cannot be much more aggressive and dog-eat-dog than the world of professional astronomy as Croswell paints it, full of fierce competition and intense squabbles and jealousies. Both these books tell their scientific story well but are especially strong on the people who make the discoveries and how they do so. The late Carl Sagan pointed out how lucky he had been to live through the era of planetary discovery in the 1960s and 1970s. But today's findings suggest that Sagan would have found much to fascinate him now and no doubt in the coming decades.

Martin Ince is deputy editor, The THES.