Fifty years ago this summer, a new show appeared on American TV screens. These, the opening titles explained, were the voyages of the starship Enterprise; its mission — to explore strange new worlds, to seek out new life and new civilisations. Half a century later the Star Trek franchise still rumbles irrepressibly on, but now the first part of the Enterprise’s mission has moved firmly from the realms of science fiction into those of fact.

The change is profound: in 2016 we can look up at a night sky full of stars and know that almost every one of them has its own family of planets orbiting around it, just as the Earth and the other planets of our solar system orbit the Sun. This is not a new idea; as early as the 16th century the
unorthodox Dominican friar Giordano Bruno suggested that the stars were all suns, each with its own inhabited worlds. (He was later burned at the stake for an array of even more heretical opinions.) But it was only in the final decade of the 20th century that astronomers were able to confirm that such ‘extrasolar planets’ really did exist. Since that first detection, strange new worlds have been discovered at an ever-increasing rate. As of 18 July, the tally of confirmed extrasolar planets stood at 3,368 but, by the time you read this, the number will surely be even higher.

In The Search for Earth’s Twin, Stuart Clark charts the emergence of this new field of astronomical endeavour. Planet-hunting is now seen as one of the most fashionable and exciting pursuits in professional astronomy; but for the early pioneers — two small teams of Swiss and American scientists — it was a lonely and daring path to take, pitting them against a scientific orthodoxy that deemed their projects unlikely to succeed, and leaving them with a struggle for both funding and job security.

The initial scepticism of the scientific establishment was not without justification: finding planets around other stars is extremely hard. Planets emit no light of their own, and what little light they reflect is utterly swamped by the dazzle of their parent star. (Clark likens the challenge to trying to pick out a table tennis ball held next to a searchlight from a distance of several kilometres.) But, as often happens in science, the conceptual and technological breakthrough ultimately came via a creative, left-field approach. Instead of trying to see the planets directly, astronomers looked for more subtle signs of their presence: the tiny motions they induce in their parent star as they orbit around it, or the fleeting shadows they cast as they pass in front of it.

Clark is a novelist as well as a veteran science writer and he gives as much weight to the human elements of the exoplanet story as to its technical and scientific significance. The first discoveries, in the 1990s, provoked a media storm, but the new-found fame and academic credibility of the planet-hunting teams inevitably presented them with a fresh set of issues. The planets being found were strange indeed, and a weird new menagerie of ‘Hot Jupiters’, ‘lava planets’, ‘water worlds’ and ‘super-Earths’ challenged cosy preconceptions of what planets around other stars ought to be like. But for big players such as Nasa and the European Space Agency, the discoveries had PR value as well as scientific merit. Clark argues that this resulted in pressure to frame the scientific goals in ways that would enhance their public appeal: not simply to explore strange new worlds for their own sake, but as a media-pleasing quest to discover a planet that closely resembled the Earth.

Scientific narratives often mythologise the purity of the quest for knowledge, but in Clark’s account politics emerges time and again as the defining player: not just the fickle politics of government funding but also the human politics of science itself. Like everyone else, his cast of astronomers, engineers and administrators is prone to rivalry, group-think and the inertia of the status quo, especially when careers and reputations are at stake. But science, at its best, ultimately favours evidence and reason, and the story of exoplanets is one in which persistence and determination leads to remarkable triumphs.

The Search for Earth’s Twin gives us the story so far of a field whose real glory days are arguably still to come. In all the thousands of strange new worlds discovered to date, we have yet to find a planet that closely resembles our own. But the quest goes on, and the planet-hunters continue to boldly go where no one has gone before.

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