To dream the impossible dream

The first exoplanet (a world orbiting another star) was discovered in 1992. Over 100 more are now known, and thousand...

The first exoplanet (a world orbiting another star) was discovered in 1992. Over 100 more are now known, and thousands of others that have been identified by space telescopes, such as NASA_s now-defunct Kepler mission, await confirmation by instruments on Earth.

Already several planets have been identified that are similar to Earth in terms of size, temperature and gravity. The statistics seem to suggest that Earth-like worlds are common in the galaxy. Estimates from the Kepler mission suggest that there may be 2 billion of them. The first detection of life on another planet could well happen within the coming century.

Lee Billings, an American science journalist, has written a definitive guide to astronomy_s hottest field. _Five Billion Years of Solitude_ is, in equal parts, a primer on the search for alien worlds, a biography of Earth and the life that inhabits it, and a story about how exoplanetology grew, and how, with the hour of its greatest triumph approaching, it fell short.

Mr Billings charts the formation and evolution of Earth, a complicated and fascinating tale of the interaction between geology, chemistry and biological life itself, which affects the planet_s atmosphere, oceans and rocks. He cites early speculations from the 1960s about just how common alien life might be, and describes a recent push by exoplanetologists, flush with their field_s success, to build colossal, high-tech space telescopes that would scan 1,000 of the stars closest to Earth looking for planetary companions.

The biggest and most capable machines would be able remotely to sniff a planet_s air, looking for a mix of gases, such as high concentrations of highly reactive oxygen, that would suggest habitation, and produce rough maps of planetary surfaces. The discovery of a world hosting alien life_even if there were no signs of intelligence_would be one of the greatest discoveries ever made, finally answering the question of whether humankind is alone in the cosmos. But this will have to wait, for Mr Billings also shows how the dream of building those telescopes foundered amid budget cuts, cost overruns at NASA and jealous opposition from elsewhere within astronomy.

The various disciplines that make up planetary science_astronomy, biology and geology_offer a wonderful, immense perspective. Gazing across the lightless light-years of space, or the silent aeons of geological strata revealed in a cliff face, encourages big thoughts.

Take, for instance, the fate of the only inhabited planet currently known to exist: Earth. Planetary science reveals that life on Earth is already middle-aged. It arose around 3 billion years ago and has roughly the same stretch ahead of it. Over that time the sun will slowly, almost imperceptibly, heat up. Eventually, it will become hot enough to scour Earth clean of life.

Complicated animals such as humans, animals and most plants have even less time remaining; perhaps a billion years or so. As the sun gradually brightens, the level of carbon dioxide in the air will fall, regardless of any temporary, millennia-long blips caused by footling things such as a civilisation based on fossil fuels. One day, far in the future, the plants that form the base of the food chain will find themselves no longer able to photosynthesise. When that happens, complex life will die out, and the bacteria will re-inherit Earth.

Armed with such long-term insight, a few of the scientists that Mr Billings writes about point out that, ultimately, humanity_s choice is between space flight and eventual extinction. It is a philosophy that is often kept hidden, he writes, for fear of cynical ridicule. But it is nevertheless common among those who dream of finding other life among the stars.

The Economist

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