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When I was a student in the 1960s, anyone who speculated about life beyond Earth was considered a crackpot. Today, astrobiology is flourishing. It is a key component in the mission statements of NASA and the European Space Agency. All this despite the fact that we still do not have a shred of direct evidence that there is any life at all beyond our planet.

So what has changed? One factor is the discovery that life on Earth can survive in a much wider range of environments than scientists used to believe possible. There are microbes that thrive near scalding volcanic vents on the seabed in pitch darkness at temperatures well above the normal boiling point of water. Others are quite comfortable immersed in acid strong enough to burn human skin. Extremes of cold, saltiness and radiation attract their own weird microbial denizens. Studies of these “extremophiles” have encouraged scientists to extend the list of extraterrestrial locations at which some form of life might after all be possible.

Whatever their amazing powers of resilience, however, there is one thing extremophiles cannot do without – liquid water. That rules out most extraterrestrial real estate. But evidence for water elsewhere in the solar system has been steadily accumulating, with Mars and Europa looking like the best bets. The surface of Mars is now a freeze-dried desert, probably too harsh for even the hardiest extremophile. But in the far past it was a different story. The Red Planet evidently had rivers and lakes, perhaps even a small ocean, making surface life possible.

Deep beneath the Martian landscape, geothermal heat may have melted the permafrost to create briny aquifers that could still host life today. On Earth, microbes inhabit similar subsurface locales, making a living by turning dissolved gases and minerals directly into biomass using chemical energy rather than sunlight. A common by-product of this underground biology is methane gas. The recent discovery of methane in the Martian atmosphere has led to speculation that Martian microbes were responsible.

Astrobiology is still so new that few books have been written that capture the excitement and promise of this emerging discipline. The authors of Looking for Life, Searching the Solar System have pooled their expertise to produce an accurate, up-to-date and highly readable survey of the field. The book charts the story of life from its shadowy origins to an imagined future of human and robotic space exploration. It deals with a host of scientific, social, ethical and political issues along the way.

The search for life beyond Earth addresses the deepest of the deep questions of existence: are we alone in the universe? There are two distinct scenarios in which the answer might be no. The first is that life forms readily emerge from non-life given Earth-like conditions. The second is that there is a mechanism for transporting life from one cosmic location to another. Both scenarios have their advocates.

It is entirely possible that the origin of life was a stupendous chemical fluke unique to the Earth. After all, life is so complex that if chance had played the predominant role it is exceedingly unlikely that it would happen twice. On the other hand, the science of complexity is in its infancy. Could there be a “life principle” at work in nature that automatically steers matter and energy towards life? Though it is often declared that life is written into the laws of nature, nobody has yet demonstrated that the biological state of matter is an inevitable or even expected product of physics and chemistry.

“An ejected rock could cocoon viable microbes for millions of years, safely delivering them to another planet”

Given this state of uncertainty, the search for a second genesis – another place where life started from scratch – assumes paramount importance, for it would strengthen the case for a life principle, with the obvious corollary that the universe could be teeming with biological organisms. Mars offers the best hope for finding life beyond Earth, but here we hit a major snag. Mars and Earth are not biologically isolated. They have been trading material for billions of years in the form of rocks blasted from their surfaces by asteroid and comet impacts. Knowing that microbes like to live inside rocks, it becomes a virtual certainty that organisms will have hitched a ride from Mars to Earth or vice versa.

But could they have survived the journey? One of the authors, Gerda Horneck, has pioneered experiments to test how long microbes can endure space conditions. It turns out that the cold and vacuum are the least of their problems. Horneck’s studies of the resilience of bacterial spores in space showed that solar ultraviolet and cosmic radiation are the principal killers.

Horneck also discovered some extraordinary feats of endurance. For example, when coated in glucose, 80 per cent of Bacillus subtilis spores survived six years’ exposure. Microbes embedded deep inside rocks are even better protected. By screening out all but the most energetic radiation, an ejected rock could cocoon viable microbes for millions of years, delivering them safely to another planet.

Given these facts, the search for life on Mars takes on a double twist. On the one hand, there is now more reason than ever to suppose that we will discover traces of life there. On the other, it seems likely that life on Mars and Earth would turn out to be different branches of the same tree, rather than different trees.

An intriguing question, if there was planetary cross-fertilisation, is whether life started on Earth and spread to Mars, or the other way about. Mars was in some respects a more favourable planet for life early on. It cooled quicker, so it was ready for life sooner, and being smaller probably suffered less from the heavy bombardment that afflicted all the planets for the first 700 million years after the formation of the solar system.

It would be fascinating if it turned out that terrestrial life ultimately came from Mars. But it would do nothing to help answer the key question of whether life is widespread in the universe. Unless we find evidence for multiple genesis events on Earth and/or Mars, it could still be argued that life is a freak phenomenon confined to our cosmic neighbourhood.

What about the possibility that life spread across the galaxy by some form of panspermia process? This theory was championed by the late Fred Hoyle, and is supported most notably by Chandra Wickramasinghe. Ejected rocks do occasionally get flung out of the solar system, but the chances are slim that they will ever hit another Earth-like planet. If set adrift in space in enormous numbers, naked microbes might encounter other habitable worlds, but interstellar radiation would very probably kill them long before they reached their destination. Still, given the state of our ignorance, it would be rash to rule out the possibility.