Nasa's Curiosity, the rover now on Mars, may find evidence for creatures that lived early in Martian history; firm evidence for even the most primitive bugs would have huge import. There could be life in the ice-covered oceans of Jupiter's moon Europa. But what really fuels popular imagination is the prospect of advanced life – the "aliens" familiar from science fiction – and nobody expects a complex biosphere in those locations.
Suppose, however, we widen our gaze beyond our solar system. Astronomers have learnt that other stars have planets circling round them. Nasa's Kepler spacecraft monitors about 150,000 stars, measuring their brightness sensitively enough to detect the very slight dimming (about one part in 10,000) that occurs when an Earth-like planet transits a sun-like star. The data already reveals thousands of planets. But we'd really like to see these planets directly – not just their shadows – and that's hard. To realise just how hard, suppose an alien astronomer with a powerful telescope was viewing Earth from, say, 50 light years away. Our planet would seem, in Carl Sagan's phrase, a "pale blue dot", very close to a star (our sun) that outshines it many billion times – a firefly next to a searchlight. But if the hypothetical aliens could detect Earth, they could learn quite a bit about it. The shade of blue would be slightly different depending on whether the Pacific Ocean or the Eurasian land mass was facing them. They could infer the length of the day, the seasons, the gross topography and the climate.
In the 2020s, European astronomers hope to complete, in Chile, a telescope with a mosaic mirror 39 metres across. It will be powerful enough to analyse the faint light from planets orbiting other stars with enough precision to infer whether they harbour life.
We know too little about how life began on Earth to lay confident odds. It may have involved a fluke so rare that it happened only once in the entire galaxy. On the other hand, it may have been almost inevitable, given the right environment. In the next two decades we can expect progress in understanding the biochemistry of life's origins: this is one of the great unsolved problems – fascinating to the most Earth-focused biologist, but also crucial in guiding astronomers on how and where to search for alien life.
Even if simple life is common, it is, of course, a separate question whether it's likely to evolve into anything intelligent or complex. Indeed, evolutionists don't agree on how divergently our own biosphere could have developed if such contingencies as ice ages and meteorite impacts had happened differently. If, for instance, the dinosaurs hadn't been wiped out, the chain of mammalian evolution that led to humans might have been foreclosed and it's not clear whether another species would have taken our role.
Maybe the search for life shouldn't restrict attention to planets like Earth. Science fiction writers have other ideas: balloon-like creatures floating in the dense atmospheres of planets such as Jupiter, swarms of intelligent insects, nano-scale robots and more. (And it's often better to read first-rate science fiction than second-rate science – it's far more stimulating, and perhaps no more likely to be wrong.) The most durable form of "life" may be machines whose creators have long ago been usurped or become extinct.
The great physicist Enrico Fermi famously argued that advanced life must be rare. Many stars are a billion years older than our sun. Life, were it common, would have had a head start around these more ancient stars. Why then, Fermi argued, haven't aliens already come here? Why aren't their artefacts staring us in the face? There is substance to this argument. However, we mustn't be too anthropomorphic: some advanced life could be unimaginably different from that on Earth. Others could be living contemplative lives, perhaps deep under some planetary ocean, doing nothing to reveal their presence.
We may learn in the coming decades whether biological evolution is unique to the pale blue dot in the cosmos that is our home, or whether Darwin's writ runs through a wider universe that teems with life – even with intelligence.
Martin Rees is president of the Royal Society and also master of Trinity College, and professor of Cosmology and Astrophysics at the University of Cambridge.