Are We Alone-Finding Life On Other Planets

Are we alone in the universe?
By Clive Cookson
Published: March 19 2010 22:18 | Last updated: March 19 2010 22:18

The Allen Telescope Array in California is used both for radio astronomy and the search for extraterrestrial intelligence
The Eerie Silence: Are We Alone in the Universe?
By Paul Davies
Allen Lane £20, 260 pages
FT Bookshop price: £16

How to Find a Habitable Planet
By James Kasting
Princeton University Press, £20.95, 326 pages
FT Bookshop price: £16.76

We Are Not Alone: Why We Have Already Found Extraterrestrial Life
By Dirk Schulze-Makuch and David Darling
OneWorld, £12.99 186 pages
FT Bookshop price: £10.39

Life, the Universe ... and the Scientific Method
By Steven Benner
FfAME Press, $35 302 pages

On a misty morning in April 1960, a young astronomer called Frank Drake pointed the Green Bank radiotelescope in West Virginia at a nearby star, Tau Ceti. He listened for signals from any alien civilisation that might inhabit its planetary system. Silence. Then Drake reoriented the 26-metre dish towards a second star, Epsilon Eridani. The startled astronomer picked up a strong radio signal that could not come from a natural source. But before he could start decoding the alien message, he learnt that it actually emanated from a secret terrestrial source: a defence radar establishment.

Fifty years later, Drake is the grand old man of the field he founded, which quickly became known as Seti, the search for extraterrestrial intelligence. The amount of data has expanded immensely but the story of Seti is still radio silence, punctuated by occasional false alarms.

Indeed, not only have astronomers failed to find evidence of alien intelligence, they still do not know for sure that any life exists beyond Earth, even simple micro-organisms. Yet, as a crop of new books shows, astrobiology – the scientific search for extraterrestrial life, simple or advanced – has never been more active. Evidence is growing that many stars in our galaxy have planets on which life might have originated. At the same time, new technology should make it easier to detect alien life if it exists – in primitive form in our solar system or more advanced on other star systems.

The authors take markedly different approaches to the subject, so there is little overlap between their four books. However they are all mainstream scientists; there is nothing here about UFOs or alien abductions. Paul Davies, a physics professor and the best-known author in the group, concentrates on Seti. James Kasting, a geoscience professor, looks at the search for habitable planets in orbit around other stars. Dirk Schulze-Makuch, an astrobiology professor, and David Darling, a science writer, focus on life within the solar system. And Steven Benner, a biochemist, takes a broader view of the scientific and philosophical issues involved in the origins and evolution of life.

Soon after beginning his search, Frank Drake formulated Seti’s key equation: N = RfpneflfifcL. To estimate N, the number of advanced civilisations with which we might communicate, you need to multiply seven terms: the number of stars in our galaxy; the fraction of stars with planets; the fraction of planets that are Earth-like and “habitable”; the fraction of habitable planets on which life gets going; the fraction of inhabited planets on which intelligent life evolves; the fraction of intelligent civilisations than can communicate over interstellar distances; and the average lifetime of a communicating civilisation. Apart from the number of stars, which can be estimated reasonably well at around 400bn, the terms in the Drake Equation remain almost as uncertain today as when it was formulated.

As Kasting shows in his technical but readable guide, How to Find a Habitable Planet, we are making rapid progress in discovering planets around distant stars – almost 500 so far. Current technology does not yet enable astronomers to detect planets similar in size and composition to Earth; all the ones found so far are much larger than Earth. But according to current theories of planetary formation, these giants must be accompanied by smaller habitable planets – 4bn of them in our galaxy, Kasting estimates. A new generation of space telescopes will narrow down that estimate. By 2013 the Kepler observatory, launched by Nasa a year ago, should give astronomers a good idea of how common Earth-like planets are.

But, as Kasting tells us, we shall have to wait another two decades – depending how generous governments are in their funding of space agencies – before telescopes are sensitive enough to detect planets that actually harbour life. An atmosphere rich in oxygen and certain other gases would be a sure sign of biology. There is even the possibility of eventually taking pictures of planets to look for an Earth-like surface of oceans and continents.

Despite the buzz about astrobiology, there is still no scientific consensus about how life started on Earth, let alone elsewhere. The general presumption is that the carbon-based (organic) compounds on which terrestrial life is based formed through abiotic chemical reactions on the hot, wet, electrically charged surface of the young planet about 3.5bn years ago. Scientists who simulate the conditions of early Earth in a laboratory can make some simple organic chemicals that may be precursors of life. But it is a huge and unknown step from there to the large, replicating and evolving molecules that might be regarded as alive.

The best account of possible origins of life on Earth and beyond appears in Life, the Universe ... and the Scientific Method. Many authors have explored the issue, and Benner valiantly read 62 previous books on the question to get a feel for scientific opinion before writing his own.

“Some books treated life as something easy to originate as an inevitable consequence of the laws of physics and chemistry,” he reports. “They viewed life as abundant in the cosmos. Reading these books, I began to believe that aliens are everywhere.” On the other side of the chasm were authors who “provided pages of reasons why life could not easily emerge by any known process. Their authors saw the emergence of life as a highly improbable event, suggesting that life in the cosmos should be scarce. Reading these books, I felt lucky to be here myself”.

Without coming down firmly on either side of the argument, Benner describes a plausible chemical scenario for the formation of life, in which minerals could act as a template for molecules of RNA (ribonucleic acid) to form and replicate themselves. Today RNA acts as an intermediate between DNA (deoxyribonucleic acid), the store of genetic information, and proteins, the molecules that do most of the work in living organisms. According to the “RNA world” hypothesis, simple life on Earth began with self-replicating RNA, which evolved into cellular microbes as proteins and DNA were added to the equation. So far, scientists have assumed that life on Earth started once only. Certainly the biochemical evidence, notably the existence of a single genetic code, suggests that all creatures great and small are descended from a common ancestor.

But Paul Davies suspects that a second genesis may have given rise to a “shadow biosphere” consisting of microbes with distinctly different biochemistry, which could be still lurking in some extreme environments – such as deep within rock strata.

A shadow terrestrial biosphere is certainly worth a scientific search. As Davies writes in The Eerie Silence: Are We Alone in the Universe?: “If life started more than once on Earth, we could be virtually certain that the universe is teeming with it. Unless there is something very peculiar about our planet, it is inconceivable that life would have begun twice on one Earth-like planet but hardly ever on all the rest.”

Some extraterrestrial life enthusiasts, however, feel they already have convincing evidence for its existence. They include Dirk Schulze-Makuch and David Darling, as the title of their provocative – but not wholly convincing – book makes clear. We Are Not Alone concentrates on our own solar system. The authors conclude that three sources of evidence prove the existence of microbial life on Mars. One is the experiments carried out on the Martian surface in 1976 by the US Viking Landers; at the time Nasa said the intriguing soil chemistry could be explained by non-biological processes but Schulze-Makuch and Darling make a good case for microbial involvement. They also believe that methane gas, recently detected in the Martian atmosphere, is a sign of life – and that fossilised microbial remains exist in Martian meteorites, chunks of rock blasted off the planet by asteroid impacts that ended up on Earth.

Because the planets can exchange material via meteorites, the discovery of life on Mars would not necessarily require a second genesis. Earth could have seeded Mars or vice versa. But Schulze-Makuch and Darling are also confident – without any firm evidence, it must be said – that life of a very different kind exists in the outer solar system, on moons of Jupiter and Saturn, where it could not have realistically travelled from Earth or Mars.

Once life does get started, what are the chances that intelligence will evolve? On Earth it took around 3.5bn years, in what appear by galactic standards to have been remarkably benign conditions. Both Kasting and Davies discuss many special factors that may have made Earth a uniquely stable environment for complex life to develop. These include plate tectonics – a geological mixing process that plays a vital role in the development of a life-friendly atmosphere – and the stabilisation of Earth’s spin axis by the moon. Sheer luck must have played a role, too, in preventing a life-ending impact with an asteroid or comet larger than the one that put paid to the dinosaurs 65m years ago.

In the end Kasting rejects what has become known as the Rare Earth hypothesis, which holds that complex life is extremely scarce throughout the galaxy. Davies, however, concludes “as a scientist” that “we are probably the only intelligent beings in the universe ... I arrive at this dismal conclusion because I see so many contingent features involved in the origin and evolution of life, and because I have yet to see a convincing theoretical argument for a universal principle of increasing organised complexity”.

On the other hand, Davies “as a philosopher and human being” takes a very different view. He strongly supports Seti – though he wants it to move beyond ever more powerful searches for extraterrestrial radio signals and laser flashes to look for other signs of aliens in the galaxy – and he even has an official role, as chairman of the Seti Post-Detection Taskforce, which decides how astronomers should respond to an alien message. Davies is the most engaging of writers and his contradictory views encapsulate those in the field.

Today, enthusiasts can still argue that we just have not looked hard enough for extraterrestrial life. But data and analysis are accumulating at an exponential rate. After another 50 years we should know enough to choose between the three current possibilities. Most exhilarating would be to find a universe full of intelligence, promising a bright future for mankind. The most depressing finding would be that complex life is widespread but intelligence is confined to Earth, because this would imply that advanced civilisations got snuffed out quite quickly by warfare, environmental destruction or technological accidents. The third possibility, that Earth is a unique oasis of life, might be quite uplifting. Davies says: “It would provide us with the truly cosmological mission of perpetuating a precious phenomenon – the flame of reason.”