Red and blue, now we see you

When the detection of a planet circling a star (51 Pegasi by name, 45 light years away) was announced in 1995, the discovery must have surprised those UFO abductees and Star Trek devotees who supposed that planets beyond the solar system were old hat. To be sure, occasional observers over the past 50 years had reported possible planets around other stars but independent confirmation never came. Not only was the 1995 detection by Geneva astronomers Michel Mayor and Didier Qucloz quickly confirmed but, by September 1996, the list of new planets had lengthened to eight.

Donald Goldsmith, an astronomer with a gift for science writing, recounts the happenings, which represent a resounding break with preceding decades of fruitless search. Those searching always understood that their best chances of discovery lay with large, Jupiter-sized planets circling parent stars still to be identified; they reasoned that, as Jupiter circles our Sun in about 12 years, observations sustained at least that long would be needed. The star, behaving like a dancer swinging his lightweight partner, would approach the observer ever so slowly and then recede. Detection was based on the expectation that the starlight emitted during approach would be slightly blue-shifted by the Doppler effect for perhaps six years and then be red-shifted for the rest of the planet's orbital period. Detection would depend on the star's slow colour changes to betray a planet's presence.

Most helpfully, the swaying of those waltzing stars was much larger than our solar system led one to expect and was detectable in much less than a year because, contrary to expectation, Jupiter-sized planets were found circling their stars at distances less than the Sun-Earth distance. The strong tugging on the star resulted in larger and more easily detected colour shifts; also orbits were completed in just days instead of years because a planet near its star hastens faster to counterbalance the greater attraction of gravity.

In later chapters the habitability of new planets and the origin of life is discussed. For years it was thought that oxygen and water were needed for life to begin; but the oxygen that animals rely on was not a prerequisite for life but was generated by primitive organisms. The presence of water at the dawn of life is implicit in Darwin's "warm little pond", from which life is said to have sprung. Revision of this picture has been under way since Thomas Gold showed that more stable conditions exist below the planetary surface than in a body of surface water that is subject to many disturbances. The subsurface rock, pervaded by bacteria that rival the surface animals and plants in total mass, is more likely to have harboured our remote ancestors. The deep bacteria gain their energy from a diet of pyrite and live in oil wells and ocean vents.

But need the subsurface life forms have originated on Earth? If they can live inside rock they are transportable through space in comets, asteroids or chunks of Mars rock that reached Earth after ejection by meteorite impact. Should life on Mars, whether extinct or not, prove to be related to Earth life, the grander question of whether life originated in the solar system or arrived from elsewhere on a comet will confront us.

Comets, planetary formation, the spinning infrared interferometer under development for direct viewing of nonsolar planets and many other topics related to the discovery of the new planets are discussed by Goldsmith, always in terms of basic principles of astronomy that are succinctly explained.

Ronald Bracewell is emeritus professor of electrical engineering, Stanford University, United States.