Astrobiology

The Search for Life Elsewhere in the Universe

Andrew May

• Editorial: Icon Books
• Año: 2019
• ISBN:
  9781785783432
• Categoría: Sciences physiques
• Categoría: Cosmologie

Extraterrestrial life is a common theme in
science fiction, but is it a serious prospect in the real world? Astrobiology
is the emerging field of science that seeks to answer this question.The possibility of life elsewhere in the cosmos
is one of the most profound subjects that human beings can ponder. Astrophysicist
Andrew May gives an expert overview of our current state of knowledge, looking
at how life started on Earth, the tell-tale 'signatures' it produces, and how
such signatures might be detected elsewhere in the Solar System or on the many 'exoplanets'
now being discovered by the Kepler and TESS missions.Along the way the book addresses key questions such as the riddle of Fermi's
paradox ('Where is everybody?') and the crucial role of DNA and water – they're
essential to 'life as we know it', but is the same true of alien life? And the really
big question: when we eventually find extraterrestrials, will they be friendly
or hostile?

1

LIFE BEYOND EARTH

In July 2018, the UK tabloid newspaper the Daily Express carried a story dramatically headlined ‘Aliens on Europa: NASA Hunts for Life Just 1 cm under Surface of Jupiter’s Moon’. To reinforce the message, it was accompanied by an eye-catching composite image. On one side of the graphic was a photograph of Europa: an enigmatic-looking world, nothing like our own Moon, with a smooth surface of solid ice, criss-crossed by dark cracks. On the other side of the image, an artist’s impression of a typical ‘alien’: grey-skinned but otherwise distinctly humanoid in appearance, with a high-domed, hairless forehead, large eyes and delicate features.

Confusingly, however, the article’s strapline read as follows:

Scientists hoping to find alien life on Jupiter’s moon Europa may not have far to search after a study revealed microbes could be surviving just one centimetre beneath the surface.

So what’s going on? Is NASA going to Europa to hunt for big-brained, humanoid aliens, or for tiny little microbes? Delving further into the fine print, it turns out the answer is neither. The Express article was prompted not by a space mission that’s about to take off, but by a clever piece of scientific deduction. It is widely accepted that, if life exists on Europa – and yes, we’re most likely talking about microscopic organisms here – it’s to be found in the ocean of liquid water believed to exist several kilometres below the icy surface. The new development the Express picked up on is the suggestion that chemical traces of life – for example proteins or complex DNA-like molecules – might be found near to the surface of the ice, making them much easier for a space probe to detect. This idea originated in a scientific study that had just been published in the journal Nature Astronomy, which came to the following conclusion:

These results indicate that future missions to Europa’s surface do not need to excavate material to great depths to investigate the composition of endogenic material and search for potential biosignatures.*

This is real science, and in principle it’s no bad thing that it found its way into a widely read tabloid like the Daily Express. But the way the newspaper chose to report it – and the way the popular media treats stories of this kind in general – is likely to leave readers more confused than enlightened. Are they saying that NASA believes there are humanoid aliens on Europa? Why go all the way to Europa when the same newspaper also frequently reports anecdotal sightings of humanoid aliens here on Earth? Is NASA on the point of sending a space probe to look for life on Europa, or is that just an idea for the future? Why do scientists keep going on about extraterrestrial microorganisms, when everyone knows that aliens are pretty much like us except for their big eyes and high foreheads?

All these things – and many more – will be clarified in the course of this book. Astrobiology is a wide-ranging subject, dealing with the possibility of life beyond Earth from every conceivable angle. To start with, however, let’s kick off with a much simpler question.

Is There Life on Earth?

From our perspective on the surface of the planet, it’s obvious there’s life on Earth. From out in space, too, it’s not that difficult to detect. The night side of the planet is lit up by city lights, there are thousands of small artificial satellites in orbit, the radio spectrum is buzzing with structured signals that have no natural explanation, and the atmosphere is laced with industrial pollutants.

But all those things have existed for just a century or so: a tiny fraction of the Earth’s lifetime, which is about 4.5 billion years. Nevertheless, life – at a less obvious level – has existed for a significant fraction of that time, perhaps as much as 4 billion years. Until just under a billion years ago, all of that life (and the vast majority of it even today) took the form of tiny single-celled organisms – the ‘microbes’ that scientists are so fond of talking about. The following table shows how, over the course of time, increasingly complex forms gradually evolved and were added to the mix of life on Earth.

Milestones in the evolution of life on Earth (all dates are approximate)

Time before present (in millions of years)	Evolution of life on Earth
4,500	Formation of the Earth
4,000	First single-celled life forms
1,700	Microscopic multicellular organisms
600	Small marine animals; seaweed
450	Fish; land plants; insects
350	First land vertebrates (amphibians); trees
180	Jurassic dinosaurs; flowering plants
50	First lemur-like primates
3.5	First tool-making hominids
0.3	Homo sapiens

This means the question of life on Earth is a matter of definition. To a scientist, ‘life’ includes any kind of living thing – even if it can only be seen through a high-power microscope. By that definition, Earth has been home to life for almost 90 per cent of its history. On the other hand, people brought up on a diet of sci-fi movies and tabloid stories about UFO encounters are more likely to equate ‘life’ with a technologically savvy civilisation – in which case that 90 per cent figure drops all the way down to 0.000002 per cent.

If we’re going to look for life on other Earth-like planets, what are the relative chances of finding it by those two definitions? We can make a rough estimate by picking random snapshots of the Earth at different points in its 4.5 billion-year history. On that basis, the chance of finding life – by the ufologist’s or sci-fi fan’s definition – is so tiny as to be virtually zero. By the scientist’s definition, on the other hand, the chances are pretty good.

So let’s look a bit more closely at that ‘scientist’s definition of life’. The nature of life turns out to be surprisingly difficult to pin down, and precise definitions tend to vary between specialists working in different branches of science. As far as astrobiology is concerned, a good starting point is the working definition devised by NASA in the 1990s:

Life is a self-sustaining chemical system capable of Darwinian evolution.

That’s refreshingly concise, but it packs a lot into a small number of words. The first part, ‘self-sustaining chemical system’ is clear enough. But the latter part, ‘capable of Darwinian evolution’, hides a lot of detail. It doesn’t just mean that our self-sustaining chemical system has to be able to evolve, or change its form over time. First, there’s an implicit assumption that the change occurs over successive generations, each of which is born, grows and dies. Then there’s that word ‘Darwinian’ – after Charles Darwin, the Victorian naturalist who did far more than suggest that living species evolve. He argued that they do this for a reason – to adapt to the changing circumstances of their environment – and that they do so by means of natural selection, or ‘survival of the fittest’.

The beauty of this definition is that it encompasses everything from the single-celled organisms that emerged on Earth 4 billion years ago – and may possibly be hiding under Europa’s ice sheets – via semi-civilised primates like ourselves, all the way up to super-advanced lifeforms we can hardly even imagine. Astrobiology – the subject of this book – deals with the possibility of life beyond Earth wherever it falls in that spectrum. As the ‘astro’ prefix implies, it’s essentially a sub-branch of astronomy, using the same sort of telescopes, space probes and theoretical techniques that astronomers apply to any other facet of outer space.

Earlier in this chapter (page 2) we saw a quote from a scientific paper featuring a lot of multisyllabic words. One of them, ‘biosignatures’, will turn out to be one of the most important words in this book. A moment ago, we saw how all the obvious ways an outside observer might detect life on Earth – artificial lights, satellites, radio signals, etc. – relate to our own civilisation. But there are other, subtler, ways of detecting more primitive lifeforms – and these are collectively known as biosignatures. Most importantly, living organisms produce, as waste products, tell-tale chemicals that would be very difficult to account for in terms of non-living processes. These chemical ‘signatures’ are potentially detectable to astrobiologists through telescopes or spacecraft-based sensors.

At the upper end of the spectrum of life, biosignatures are joined by ‘technosignatures’: detectable indications of a technologically advanced civilisation. As we’ll see later in this book, there are numerous possibilities here, but perhaps the most obvious – and the easiest for us to recognise as artificial – would be some kind of deliberate interstellar communication. In a historical context, the first practical efforts in astrobiology were aimed at detecting such communications, under the name of SETI – for ‘Search for Extraterrestrial Intelligence’. SETI is still going strong, although confusingly it uses the word ‘intelligence’ in a different way from people working in other branches of science.

To a biologist or psychologist, intelligence is the capacity for understanding and logical reasoning. By this definition, human beings were every bit as intelligent thousands of years ago as they are today. Yet from a remote-sensing point of view, they didn’t produce any detectable signatures that were noticeably different from far more primitive animals. So, as insulting as it is to, say, Alexander the Great or Lao Tzu or Akhenaten, they simply weren’t ‘intelligent’ by the standards of SETI researchers. They only produced biosignatures, not technosignatures.

Since I’ve started to quibble about other people’s choice of words, here’s another thing. Although SETI is a sub-branch of astrobiology, who’s to say that a SETI signal – if and when it’s detected – necessarily has a biological origin? It might be the work of an advanced AI – artificial intelligence – which has outlived its organic creators. Whether such an AI constitutes ‘life’ is a question for the philosophers – but we can say right away that it doesn’t conform to NASA’s definition. It’s not a ‘chemical system’, and it’s almost certainly the result of intelligently driven evolution rather than Darwinian natural selection.†

We can think of biosignatures and technosignatures as overlapping sets. The first is looking for biological life of any kind (technological or not), the second for technological civilisation of any kind (biological or not). Judging from the situation on Earth over the last several billion years, we might conclude that the first has a good chance of success, while the second is like searching for a very small needle in a very large haystack.

Fortunately, the prospect for technosignatures may not be as bleak as that. We’re forgetting that Earth has – hopefully – several billion years of existence ahead of it. Who knows what might happen in that time: a technological society that’s as far ahead of us as we are from the stone age, or a post-human world ruled by computers, or in which people have ‘uploaded’ themselves into digital form and can whizz around the galaxy at the speed of li...