Image courtesy NASA/JPL-Caltech.
On clear summer nights when I was quite young, I used to like to sit out on the lawn in an old chair and look up at the stars. There wasn't much light pollution back then, at the edge of the Cotswolds, so the sky was full of stars. And I'd wonder, as so many do, if a world much like Earth might be orbiting them, and if a boy much like me might be looking up at its night sky, and the insignificant star that was the Sun.
The profound question of whether we are alone in the Universe - if Earth and humankind are unique, or if there are many Earth-like worlds harbouring other forms of intelligent life - is the topic of Lee Billings's Five Billion Years of Solitude
. 4.6 billion years after it was formed, Earth sits at the centre of a small expanding sphere of radio noise that might be detected by other civilisations, and astronomers have begun to catalogue a vast variety of exoplanets. Could any of them harbour life? What would it look like if they did? And is there anyone else out there, as lonely as we are?
Billings frames the history of the search for extrasolar planets and plans to search for Earth-like worlds within biographical portraits of planet hunters, from Frank Drake to rising star Sara Seager, who plans to use relatively cheap nanosatellites to monitor single stars for signs of transiting planets. I would have preferred a little more science rather than noveletish descriptions of what Billings's interviewees happened to be doing and wearing when he met them, and because all of them are American the work of astronomers from other countries is somewhat scanted. Michel Mayer, who led the team which discovered the first exoplanet, is given only a passing mention; the work of the HARPS project, a collaboration between a Swiss team led by Mayer and the European Southern Observatory's telescope in Chile, is presented in terms of competition with an American team rather than in its own right.
But these are minor quibbles. Billings expertly anatomises the difficulties in detecting the faint jitters in the motion of stars or the minute dimming in their luminosity that signals the presence of exoplanets, evokes the teeming variety of exoplanets so far discovered and the problems astronomers hunting for Earth-sized exoplanets must overcome. He's very good, too, on the labyrinthine politics of NASA which have stalled the Terrestrial Planet Finder project, and the ongoing problems with the vastly expensive James Webb Space Telescope, the successor to Hubble. And his entanglement of the lives of the planet hunters with their work reminds us that their discoveries provide us with new and humbling perspectives on our place in the universe and the evanescence of our tenancy on this planet.
The universe is vast, and old. Billings uses the stump of a redwood tree in Frank Drake's backyard to provide a lovely and sobering lesson about deep time. Growth rings show that the tree was more than 2000 years old. During that time
'the Sun had scarcely budged in its 250-million-year orbit about the galactic center, and, considering its life span of billions of years, hadn't aged a day. Since their formation 4.6 billion years ago, our Sun and its planets have made perhaps eighteen galactic orbits - our solar system is eighteen "galactic years" old. When it was seventeen, redwood trees did not yet exist on Earth. When it was sixteen, simple organisms were taking their first tentative excursions from the sea to colonise the land. In fact, fossil evidence testified that for about fifteen of its eighteen galactic years, our planet had played host to little more than unicellular microbes and multicellular bacterial colonies, and was utterly devoid of anything so complicated as grass, trees, or animals, let alone beings capable of solving differential equations, building rockets, painting landscapes, writing symphonies, or feeling love.'
The first confirmed discovery of an exoplanet was made less only twenty
years ago. Although more than a thousand have been discovered since then, it's a microscopic sample of the trillions believed to exist in our galaxy. A
few are Earth-sized, but none found so far are known to be Earth-like, and we're still a long way from discovering evidence for life on another world, let alone any intelligent beings that might also be searching for traces of other life in the immense sea of stars. 'We're the product of millions of years of evolution,' Sara Seager says, 'but we don't have any time to waste.'