The changing face of ET

Whether there is a plurality of worlds that are in some way like ours is a topic of public interest, as regularly evidenced in the news media. An Antarctic meteorite, seemingly from Mars and containing fossils, Jupiter-sized planets reported revolving close in around nearby stars, radio searches for signs of communications activity around other stars, and other items, are discussed in the news media in response to curiosity about our place in the universe.

Though topics change from decade to decade, Steven J. Dick, who is an historian of science at the US Naval Observatory, traces the continuity of public fascination with the theme of extraterrestrial life in a scholarly and most readable book with something of interest for everybody. After reciting the views and controversies of philosophers and theologians from ancient times, Dick moves on to Alfred Wallace, well-known as the co-discoverer of evolution but less remembered for his 1903 book Man's Place in the Universe. In answer to the question, "Are we alone", his response was "Yes". He staked out an early modern position in favour of uniqueness, which was vigorously disputed, but then received support in the 1920s from Sir James Jeans, according to whom the planetary system originated in a collision of our sun with a passing star, an occurrence of extreme rarity. The pendulum then swung back in the direction of the principle of mediocrity: it is unthinkable that we should be the only beings, even if the probability of suitable conditions for life to evolve is vanishingly small. This pluralist view ruled throughout most of the 20th century (and was expressed in antiquity) but not universally.

Populations that reached our own space-age level long ago might already have arrived here. A serious challenge to pluralism has therefore been mounted on the grounds that they have not done so. Surely the scientific attitude is to admit our ignorance and to ask what we can do to find out.

There is plenty of action. Scanning the skies for radio messages from planets of other stars and spectroscopic studies of the Doppler shift of light from stars are revealing the existence of planetary companions, while plans for space exploration include ambitious instruments for mapping the infrared emission from dust clouds in which planets might be seeable. A nulling interferometer that can suppress the starlight glare while accepting the faint planetary glow is a candidate and has passed its first test in a ground-based observation.

A chapter is devoted to life in our solar system, from Percival Lowell's time up to the present, with interesting superpositions of Lowell's drawings of purported Martian canals on top of modern photography from a Mariner spacecraft in 1971. No canals were seen. Nevertheless, looking for signs of life in the Martian soil was the primary goal of the Viking landers of 1976 and is about to be resumed.

Jupiter and Venus are thought to be less favourable to life than Mars would be, except by science fiction writers, but the moon was a serious candidate, as witnessed by precautions taken to prevent contamination of the moon by earthly organisms and vice versa. Mars and the moon have had surface water in the past and it is conceivable that, like the earth, they might harbour sub-surface populations of bacteria. In that case, DNA studies would bear on the possibility that life can spread through space, possibly living within solid bodies like meteorites or comets.

Other chapters deal with the UFO phenomenon, the origin of life, Seti (search for extraterrestrial life by radio), the sociology of science, and philosophical views of astro-theology and the meaning of life. Throughout the book, the most meticulous recounting of history is presented, with many footnotes which, as often happens, one tends to read first. The author's assessments are unobtrusive; the general impression is one of even-handed reporting of the contributions of the participants in an ongoing debate about how and why we are here.

Ronald Bracewell is author of The Galactic Club: Intelligent Life in Outer Space.