Ci-dessous, excellent article du professeur Chandra Wickramasinghe, en charge de la recherche en astrobiologie à l'Université de Buckingham :
EVIDENCE OF LIFE OUTSIDE THE EARTH
May 7, 2016, 7:49 pm
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by Professor Chandra Wickramasinghe
The answer to this age-old question may be closer to answering now than it has been for centuries.
From time immemorial our ancestors would have gazed at the magnificent spectacle of the Milky Way arching across the night sky and asked the question "Are we alone in the vast cosmos?" The same question continues to be asked in the present day.
If it can be firmly established that we are not alone in the Universe the implications for humanity will be profound. It could be even more important if it is shown that alien life in the form of microorganisms exist in our midst, perhaps continuously raining down on our planet. In either case, whether as alien microbes at home or alien intelligence on distant planets, the realisation that we will mark an important turning point in human history.
The much publicised scientific developments recent months – the Rosetta Mission to comet 67P/C-G, the New Horizons Mission to Pluto, a Russian billionaire’s support for SETI, and the Kepler Mission discovery of an "Earth twin" orbiting a distant star – all spell out a single cosmic truth. Homo Sapiens as a sentient species appears to be hard-wired to seek out its cosmic origins, perhaps intuitively sensing that we cannot be alone.
First and foremost we must ask the question: How did life arise? Not just on the Earth, but anywhere in the Universe? Does life emerge spontaneously on every Earth-like planet by processes involving well-attested laws of physics and chemistry? Or did the first-ever origin of life involve an extraordinary, even miraculous intervention? These questions are beginning to acquire a new sense of urgency in recent times.
The first requirement for the emergence of creatures like ourselves would be for the existence of rocky planets with water and an atmosphere generally similar to Earth. In 1995 Cambridge-based astronomer Didier Queloz together with Michel Mayor discovered the first planets outside our solar system. The first of these so-called exoplanets orbited a star 50 light years away in the constellation of Pegasus; it was a giant planet with a mass similar to Jupiter located too close to its parent star for any life to be possible. In 2009 NASA launched its orbiting Kepler telescope, which was specifically designed to discover planets which are the size of Earth. The detection process involved tracking down minute blinks (dimming) in the star’s light when a planet transited periodically in front of it during its orbit.
Within a few months of its launch the Kepler project, with a team led by William Borucki discovered 5 new planets with sizes ranging from that of Jupiter to Neptune and slightly smaller. The tally of these so-called exoplanets has steadily increased including amongst the detections a few Earth-like planets on which life may be possible. The most recent to hit headlines is Kepler 452b, a planet slightly larger than the Earth and orbiting around a sun-like star within its habitable zone, a region where liquid water on the planet’s surface and an atmosphere is possible. This new discovery has sparked off a huge wave of popular interest in the possibility of life existing outside our Earth .
Extrapolating from the sample of present detections the estimated total number of habitable planets in the galaxy is reckoned to be in excess of 144 billion! Most of these planets orbit very long-lived red-dwarf stars that are nearly twice as old as the sun. On many of these planets one might speculate that life may have begun, evolved, and perhaps long since disappeared.
Another related enterprise that has captured the news recently is the search for extraterrestrial intelligence using arrays of radio telescopes to scan the skies for evidence of intelligent signals. Over half a century ago Philip Morrison and Giuseppe Cocconi first drew attention to the possibility of searching the microwave spectrum of cosmic sources for intelligent signals and suggested particular frequencies as well as a set of potential targets. The SETI program (Search for Extraterrestrial Intelligence) began tentatively in 1960 and was first supported by NASA, and later by a host of private or semi-private entrepreneurs. With the exception of a single brief and mysterious "Wow!" signal discovered in August 1977 there has been a deathly silence across all of the prospective sources that been scanned. There could be a case for saying that the lack of progress in this venture was the result of organisations like NASA backing off. This may have been the thinking behind Russian billionaire Yuri Milner’s 100 billion dollar SETI initiative that has just been announced with much pomp. Buying more telescope time, increasing the range of wavelengths, enhancing detector sensitivity and extending sky coverage have been argued as prerequisites if a breakthrough within a decade is to be achieved. But a positive result from SETI would be contingent on the emergence and widespread dispersal of primitive life capable of evolving into intelligent creatures. How often does this happen?
The idea that microbial life springs up de novo on billions of Earth-like habitable planets is an unproven, and most likely erroneous proposition. Such a belief is an extension of the canonical "primordial soup theory" for life’s beginnings on the Earth, which is a dogma with no hard evidence to support it. If there was a deep principle of nature that drove inorganic systems towards the emergence of primitive life, the evidence for this would have long since been discovered in the laboratory. With a whole raft of calculations showing grotesquely small a priori probabilities for the transition from non-life to life only two options remain. The origin of life was an extremely improbable event that certainly occurred on Earth (because we are here!) but will effectively not be reproduced elsewhere. In that case we would indeed be hopelessly alone. Or, a very much vaster cosmic system than was available on Earth, and a longer timescale was involved in an initial origination event, after which life was transferred to Earth and elsewhere by processes that present writer and the late Sir Fred Hoyle discussed many years ago - panspermia.
The discovery of microorganisms occupying the harshest environments on Earth continues to provide support for this point of view. Transfers of microbial life from one cosmic habitat to another requires endurance to space conditions for millions of years. The closest terrestrial analogue to this latter situation exists for microbes exposed to the natural radioactivity of the Earth. Quite remarkably microbial survival under such conditions is now well documented. Dormant microorganisms in the guts of insects trapped in amber have been revived after 25-40 million years. More direct experiments exposing bacteria and viruses to space conditions and discovering high rates of survival continue. Viruses mounted on the outer surface of a Russian sounding rocket and fired through the atmosphere were recently found to survive. All this goes to show that arguments used in the past to ‘disprove’ panspermia on the grounds of survivability during interstellar transport are seriously flawed.
Another lead in this story has come from the study of interstellar dust clouds that has been conducted over several decades. The list of organic molecules present in interstellar clouds has increased dramatically in number since their first discovery in the 1970’s and so also has their degree of complexity. Decisive evidence for complex aromatic and aliphatic carbon-based molecules (ring molecules and long chain molecules) exists everywhere in our galaxy, and even beyond in galaxies as far away as 8 billion light years. Whilst all such data still tends to be interpreted cautiously avoiding "biology" with the suggestion that we may be witnessing "primordial soup-type events" on a cosmic scale, it is cosmic biology that remains by far the most attractive logical option. This is further evidence of panspermia in action - the complex organic molecules in interstellar space being degradation produces of iterant bacteria and viruses.
Comets in our solar system have been the target of several space missions since 1986 following ESA’s Giotto successful mission to Halley’s comet. The Giotto mission showed clearly that the prevailing theory that comets are dirty snowballs had to be abandoned in favour of comets rich in organic molecules, and most likely also containing viable bacteria and viruses. More recent explorations of comets, culminating in the Rosetta Mission to Comet 67P/C-G, have yielded a formidable body of evidence all showing consistency with the existence in comets of the seeds of life. Interestingly a comet called Lovejoy has recently been found to be releasing methyl alcohol at the rate of some 300 bottles of wine every second. Undoubtedly the product of bacterial fermentation.
The reluctance of some scientists to endorse these discoveries lies not in the quality of the data but in a desire to maintain a conservative position in relation to life on Earth and its purely terrestrial origins. It is perhaps only in this way that public funding of their research projects (and livelihoods!) can be assured. Although the Earth was demoted from its privileged position physical centrality in the Universe over 500 years ago (and not without anguish) the trend to regard life as being centred on our home planet has persisted almost to the present day. But a paradigm shift with far-reaching consequences is imminent now and public support seems also to be growing.
During the past decade tantalising evidence of microorganisms currently entering Earth has accumulated, but has been largely ignored and not pursued. The currently available data was acquired from relatively inexpensive projects that involve balloon flights to the stratosphere and recovery of infalling cometary dust. The first in a series of such experiments was conducted by the Indian Space Research Organisation in 2001 and 2006 with staggering results - indicating an inflow of microorganisms at the rate of a tenth of tonne per year. Some years later a team of investigators in the University of Sheffield led by Milton Wainwright obtained very similar results. It is obviously of the utmost importance that these experiments are repeated by independent bodies but this has not happened so far. More expensive and sophisticated investigations need to be carried out even on the samples collected so far, if we are to prove beyond doubt that these microbes are unequivocally alien. The sad truth is that funding for such vitally important experiments is well nigh impossible to secure. Compared with other Space Projects for solar system exploration the budgets involved here are trivial and the scientific and societal pay-off could be huge. Our ultimate goal must be to confirm that Darwinian evolution takes place not just within a closed biosphere on our minuscule planet Earth but extends over a vast and connected volume of the cosmos.
Over the past few years there has been a gradual realisation that life must be a truly cosmic phenomenon; and many people who were antagonistic to this idea in the past are beginning to voice contrary opinions about what should be done to cope with the realisation that life exists outside the Earth. In Davos, Switzerland in 2013, the world’s business leaders and politicians met to discuss global risks and challenges that would confront humanity in the next 10 years. One of the top 5 global "risks" to be identified was the discovery of extraterrestrial life. This discovery it is reckoned would profoundly influence the entire future of humankind. The prevalence of life of any kind outside our cosy Earth raises issues connected not only with science, but with psychology, sociology and even religion. To some religious groups the realisation that the site of our "creation" was located outside the Earth may cause conflicts with theology. Earth-centred theologies and philosophies may need to be revised.
The discovery of intelligent life outside Earth, if that happens, poses the most serious problems of all, calling for fundamental revisions and readjustments of our perceptions about ourselves. Even the mere proof that such extraterrestrial intelligence exists will seriously erode our perceived position of unrivalled supremacy in the world. And if extraterrestrial intelligence is indeed found to be resident nearby, and contact thought imminent, the situation might become analogous to the fear that primitive tribes may have had regarding the prospect of encounters with more civilised conquerors.
There is, however, a practical application that follows if ongoing input of viruses and bacteria is confirmed. In the near future it will become clear that bacteria and viruses coming to the Earth from outside could sometimes pose serious threats of pandemic disease, not only to humans, but also to plants and animals. This is connected with an idea Fred Hoyle and I explored as early as 1979 - that most of the pandemics throughout history were driven from space with the arrival of new viruses and bacteria. With all the data that is currently available across a wide spectrum of disciplines, I believe there is an urgent need for the possibility of bacterial and viral ingress from space to be taken seriously.
Professor Chandra Wickramasinghe is Director of the Buckingham Centre for Astrobiology, and Honorary Professor at the University of Buckingham and a Visiting By-Fellow of Churchill College, Cambridge. He is also a Visiting Professor at the University of Peradeniya.
He is the author of over 25 books and 350 scientific papers, 65 in the journal Nature. He latest books are, Comets and the Origin of Life (2011), A Journey With Fred Hoyle (2103), The Quest for Our Cosmic Ancestry (2014), Where did we come from? (2015)
