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February 2002

Book Review


Distant Wanderers - The Search for Planets beyond the Solar System 2002

Review by Case Rijsdijk, SAAO


The question "Are we alone?" has been asked for as long as people have been looking at the sky. Since we can now say with some confidence that there is no life, as we know it, in our solar system, the search must start beyond it. And this search must start by looking for planets around other stars.

Award winning science journalist, Bruce Dorminey, writes for the informed amateur and traces the recent history of the discovery of planets orbiting stars other than our Sun. In doing so he is faced with the difficult task of explaining how these 'planet hunters' know that some distant stars have planets orbiting them. Since it is not possible to 'see' these planets, even using the most powerful telescopes on Earth and in space today, other means have to be used. By interviewing and speaking to many of the astronomers involved in this exciting new aspect of astronomy he is able to introduce many of these difficult topics in an accessible and readable way.

Spectroscopy is the principal tool used by astronomers in their quest for extra-solar planets and the tug of massive, Jupiter-like planets will produce a periodic motion that is detectable using the Doppler shifted spectra in the parent stars. However, as Dorminey says, this technique will not lead to the discovery of small rocky planets such as Earth: other methods need to be used and he goes on to describe a few relatively new techniques including 'gravitational microlensing' and exo-planetary transits.

The first half of the book covers the discoveries: from the first planets orbiting a pulsar to the outstanding and prolific work of Marcy and Butler and the pioneering work of Walker, Mayor, Queloz and others. Any type of groundbreaking work is not without controversy and false starts. Those hunting for distant planets are no exception: Dorminey does not gloss over these and I found his coverage of the 'discovery' of planets around Barnard's Star both interesting and illuminating.

Of course as more and more extra-solar planets are discovered, the rate at present is around two or more each month, our perception of what planets are is being challenged. Dorminey spends quite some time discussing this: from relatively trivial matters of nomenclature to the difficulties of defining what a planet really is. He highlights the need for us to be able to distinguish between a Jupiter-like planet and a brown dwarf and if we are to really understand these new planets, then a better understanding of the physics of Jupiter itself is needed. This is particularly important as Jupiter is becoming the 'standard' with which the new planets are being compared: it is an intermediary standard.

This naturally leads onto the interesting, and often heated, debate on the evolutionary process involved in the formation of planets, proto-planetary disks and planetary systems similar to our own solar system. Dorminey explains that as new information comes in from the planet hunters so we have to change our view of how our solar system formed, and in turn a better understanding of that, means that we are in a better position to explain how these extra-solar planetary systems form.

Towards the end of the book he looks at what the future holds based on programmes that have been funded and are about to start. New techniques, including interferometry and adaptive optics at the Very Large Telescope, VLT, run by ESO, and the construction of larger ground based telescopes such as the Square Kilometre Array, SKA, the Southern African Large Telescope, SALT and the Large Binocular Telescope LBT will do ground breaking work. The launching of new satellites and space probes, such as Space Interferometry Mission, SIM, which uses new technology to measure and image directly some of the nearby extra-solar planets, will see much new information in the next decade.

Dorminey concludes this trail of discoveries with a rational look into the proverbial crystal ball and describes some really exotic ideas and instruments such as ESO's Overwhelmingly Large Telescope, OWL, a 100-m giant and NASA's Terrestrial Planet Finder, TPF. The latter is an ambitious mission consisting of four free-flying space probes each with a 3.5m mirror covering a baseline from 75 -1000m producing spectroscopy of unprecedented accuracy, capable of looking for the gases of life around these distant planets. Should even some of these see the light of distant stars, conclusive evidence of terrestrial planets will be found and maybe the question "Are we alone?" can finally be answered.

One could argue that Dormineys book is premature - there is still too much speculation and uncertainty about these distant wanderers. I would argue that it is a timely summation of the state of the search for extra-solar planets to date. He tells us who is doing the cutting edge research in this field, where it is happening, how it is being done and how it will be done. Bruce Dorminey's "Distant Wanderers" will certainly find a space on my bookshelf and I'll keep another space for the companion volume in ten years time!

Dorminey, Bruce Distant Wanderers - The Search for Planets beyond the Solar System 2002. XIV 226 pages Hardcover. Springer-Verlag ISBN 0-387-95074-5 US$29.95

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