Astrobiology E-Book

Andrew May

• Herausgeber: Icon Books
• Kategorie: Wissenschaft und neue Technologien
• Serie: Hot Science
• Sprache: Englisch
• Veröffentlichungsjahr: 2019

Seitenzahl: 215

Beschreibung

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?

Leseprobe

ASTROBIOLOGY

The Search for Life Elsewhere in the Universe

ANDREW MAY

CONTENTS

 Title Page 1Life Beyond Earth2Thinking About Aliens3Extraterrestrial Communication4Interstellar Engineering5Starting Small6Exoplanets7First Contact Further ReadingIndexAbout the AuthorCopyright

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 Earth4,500Formation of the Earth4,000First single-celled life forms1,700Microscopic multicellular organisms600Small marine animals; seaweed450Fish; land plants; insects350First land vertebrates (amphibians); trees180Jurassic dinosaurs; flowering plants50First lemur-like primates3.5First tool-making hominids0.3Homo 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 light?

As unimaginably ancient as 4.5 billion years sounds to us, the Earth is really quite young in a galactic context. The oldest Earth-like planets are likely to be around twice that age, while the average age is probably around 6 billion years. With a head start like that, the galaxy could be teeming with super-advanced aliens.

Fermi’s Paradox

Enrico Fermi was one of the most important scientists of the 20th century. He won the Nobel Prize in 1938 for his work on nuclear physics, and during the Second World War he was part of the team at the Los Alamos laboratory in New Mexico where America’s first atomic bombs were built. After the war, Fermi took up a professorship at the University of Chicago, but continued to make regular trips back to Los Alamos, where he acted as a consultant during the development of the ultimate Cold War weapon, the hydrogen bomb.

On one such visit in the summer of 1950, Fermi got into a lunchtime discussion with colleagues that had nothing to do with nuclear physics. This was just three years after the media had coined the term ‘flying saucer’ to describe alleged sightings of alien spacecraft, and the papers were still buzzing with stories about them. It seems this was the topic the Los Alamos physicists were discussing that day. They were particularly amused by a cartoon in the New Yorker magazine, which attributed a phenomenon described in a completely separate news story – mysteriously disappearing trash cans – to alien visitors.

As scientists, they were unconvinced by all the supposed evidence for flying saucers, because they could see there were always other, more likely explanations. At the same time, they were aware of the vast scale of the galaxy – both in terms of its enormous age and the sheer number of stars – and realised that, in fact, there ought to be extraterrestrials everywhere. Fermi summed up the problem with a simple question: ‘Where is everybody?’

It’s gone down in history as Fermi’s Paradox: the idea that space should be teeming with aliens, and yet we see no evidence of them. But is that really a paradox? To some people, it’s far from obvious that ‘space should be teeming with aliens’, while UFO believers would scoff at any suggestion that ‘we see no evidence of them’. More subtly, there’s an unspoken assumption: that if aliens do exist, we ought to see evidence of them – which is just as contentious. The plain fact is that one of these three statements must be false. There’s a flaw either in the logic that says extraterrestrial intelligence must be widespread, or in the assumption that we ought to be able to detect it, or in the assertion that there’s no evidence for it. We’ll need to learn a lot more about the subject before we can address the first two points – we’ll come back to them in Chapter 4 – but we can deal with that last point right away.

Is it really true that there isn’t a shred of observational evidence suggesting the existence of aliens? There’s certainly no shortage of people prepared to dispute that. The very term ‘unidentified flying object’ (UFO) has come to be synonymous with ‘extraterrestrial spacecraft’. Now, it’s a well-documented fact that witnesses – including professional pilots and astronauts – occasionally see flying objects they can’t identify. So no one can seriously deny that unidentified flying objects exist. Isn’t that the same as saying that no one can deny the existence of alien spacecraft? To anyone who makes the effort to parse each of those words separately (unidentified – flying – object), no, it’s not the same thing at all – but to others the two statements are completely equivalent.

There are plenty of other ‘mysteries of the unexplained’ that can be accounted for by aliens, too. Something crashed at Roswell, New Mexico in June 1947, and the military initially described it as a flying saucer – even if they later retracted the claim – so isn’t that proof enough? Who taught the ancient Egyptians how to build giant pyramids, if it wasn’t visitors from another planet? Even if aliens weren’t responsible for those disappearing trash cans in New York, surely they must have engineered the utterly inexplicable disappearance of that Malaysian airliner in 2014?

Questions like these are highly divisive between the scientific and UFO communities. To a scientist, they’re questions that can all be answered in more mundane, and far more likely, ways. To believers, the existence of alternative explanations is irrelevant. If it might be aliens, then it is aliens. The TV series The X-Files hit the nail on the head with the phrase ‘I want to believe’. If a person wants to believe a theory badly enough, they end up seeing evidence for it everywhere.‡

Having said that, it would be arrogant to suggest that just because an idea is ‘unscientific’ it’s necessarily wrong. That’s not something I would ever say about someone’s spiritual beliefs, because they’re almost always framed in a way that’s impossible to prove or disprove by scientific methods. The situation in the UFO community is very similar. Either by accident or design, they’ve built up an edifice of belief that can never be proved wrong. Their aliens are powerful enough to manipulate anything that might be used in evidence against them: our perceptions, memories, computer data – possibly even reality itself. They may even be deliberately mischievous, ensuring that believers see enough evidence to continue to believe, while sceptics and unbelievers see no evidence at all. As outrageous as that sounds to a rational mind, it’s not impossible – but it’s an idea that falls in the realm of philosophy, not science.

By its nature, science can only deal with objective, repeatable evidence. Since that’s what this book is about, we’ll have to wave goodbye to UFOs and their mischief-making occupants at this point. Fortunately, the question of whether or not they exist doesn’t alter the validity of what we’re going to say about the search for biosignatures and technosignatures.

The Way Forward

To have any hope of making progress, astrobiology has to focus on practicalities – constructing instruments, making observations, interpreting data – rather than just sitting and thinking about the subject, which is what philosophers and sci-fi writers do. There’s nothing wrong with speculation, but it only turns into real science if that speculation comes up with consequences that can be tested by observation. Going back to the Europa example, for instance, the notion of microorganisms living in a subsurface ocean there is speculation, but the idea that their chemical signature might be detectable to a spacecraft sitting on the surface is real science.

Then again, a book of this type can’t overlook the speculative side altogether, because the question of life beyond Earth is a multidisciplinary one that attracts the interest of philosophers, theologians, Hollywood screenwriters and tabloid journalists as well as scientists. So we’ll take a look at some of those broader aspects in the next chapter, and then visit them again at the end of book. The science can be found in Chapters 3 and 4, dealing with various technosignatures we might expect to see from advanced alien civilisations, and Chapters 5 and 6, on simpler lifeforms and their biosignatures.§

* ‘Endogenic’ is just a fancy way of saying ‘formed underground’. As for ‘biosignatures’ – hold that thought for a few pages and all will be explained.

† For that matter, even advanced biological species are likely to control their own evolution, via genetic engineering, rather than leaving it to Darwin.

‡ I wrote about this in my book Pseudoscience and Science Fiction – as the book’s title suggests, it’s a worldview that draws much more heavily on sci-fi and popular culture than on an understanding of science.

§ That might sound back-to-front, but the science of biosignatures is actually the newer, and more complex, of the two.

2

THINKING ABOUT ALIENS

The concept of extraterrestrial life is inextricably tied to our modern-day picture of the universe, in which the stars are distant suns, many of them – like our own Sun – with a retinue of planets orbiting around them. But people haven’t always seen things that way. Before the invention of the telescope, all the stars in the night sky were nothing more than tiny points of light, seemingly attached to the inside surface of a sphere that revolves around us. The only object that might conceivably have been another world like ours was the Moon. To the philosophers of ancient times, that made it the most likely place to find ‘extraterrestrial life’. The Greek historian Plutarch, writing almost 2,000 years ago, attributed the idea to followers of Pythagoras, who had lived several centuries earlier still:

The Pythagoreans affirm that the Moon appeareth terrestrial, for that she is inhabited round about, like as the Earth wherein we are, and peopled as it were with the greatest living creatures, and the fairest plants … There is nothing that doth prove and show directly, this habitation of men in the Moon to be impossible.

When the first telescopes were directed at the Moon in the 17th century, they revealed an intriguing landscape of mountains, valleys and craters that had only been hinted at by naked-eye observations. This prompted a young English clergyman, John Wilkins, to write a book called The Discovery of a World in the Moon. Published in 1638, the book drew numerous parallels between the Earth and the Moon, leading Wilkins to conclude that:

’Tis probable there may be inhabitants in this other world, but of what kind they are is uncertain.

That simply echoes the ancient Greeks, but Wilkins adds a remarkable insight of his own: ‘As their world is our moon, so our world is their moon.’ That was a hugely important step in our understanding of Earth’s place in the cosmos: that there is nothing unique or special about it. Wilkins made the same point even more clearly in his next book, published two years later. Its attention-grabbing title was A Discourse Concerning a New Planet.

Anyone familiar with the history of astronomy will know that no ‘new planet’, beyond those visible to the naked eye, was discovered prior to Uranus in 1781, a century after Wilkins’s time. But that’s not what he’s talking about. The point he’s making is clarified in the book’s subtitle: ‘Tending to prove that ’tis probable our Earth is one of the planets’.

The strikingly worded title page of a book by John Wilkins, written in 1640.

(Public domain image)

Today, it’s blindingly obvious that the Earth is a planet. It’s the very archetype of a planet, the one against which all the others are measured and contrasted. But things were different in Wilkins’s day. To most of his readers, the word ‘planet’ would have meant one of five points of light – Mercury, Venus, Mars, Jupiter or Saturn – in the night sky. The word comes from aster planetes, the Greek for ‘wandering star’. To the unaided eye, there is almost nothing to distinguish these planets from other bright stars, except for the fact that they slowly drift, from one night to the next, against the background of seemingly fixed stars.

The idea that Wilkins was promoting – that the Earth is just another of these planets, and that they all revolve around the Sun – had been developed a century earlier by Nicolaus Copernicus, an administrator at Frombork cathedral in Poland. It was one of the great scientific revolutions, displacing the Earth from its privileged position at the centre of the universe.* In more general terms, the ‘Copernican principle’ – that there is nothing special about our cosmic location – has become one of the fundamental tenets of astronomy. Yet Copernicus himself only took the first step in that direction. Like most of the ancient and medieval scholars who preceded him, he continued to picture the ‘fixed’ stars as being tiny objects attached to the interior of a sphere surrounding the Solar System, and nothing like our own Sun in terms of size and luminosity. It was left for others to dispel that illusion.

The Plurality of Worlds

Thomas Digges was born in 1546, three years after Copernicus published his revolutionary theory. The son of an astronomer, Digges encountered Copernicus’ work at an early age. By the time he was 30 he had produced the first account of it in the English language: A Perfect Description of the Celestial Orbs. But Digges went even further than Copernicus. He conjectured that the stars, rather than being fixed to a cosily Sun-centred sphere – ‘as in a nutshell’, as he put it – are spread out through an infinite universe.

Given this impressive insight, it’s sad that history has all but forgotten Digges. That can’t be said of a young playwright who once lodged in the same house as Digges in London’s Bishopsgate – one William Shakespeare. The two must have had some fascinating conversations, and Shakespeare drew on them many years later when he came to write one of his most famous works, Hamlet. At one point in Act 2 Scene 2 the title character, echoing Digges’ own words, says:

I could be bounded in a nutshell, and count myself a king of infinite space.

To modern ears that may sound like a glib cliché, but the notion of infinite space was cutting-edge science at the time it was written.

Another person who encountered Digges’ ideas was a loud and charismatic Italian named Giordano Bruno, who visited London in the 1580s. While there, he wrote De l’Infinito Universo e Mondi (‘On the Infinite Universe and Worlds’). As well as espousing Digges’ idea of other stars spread throughout the universe, he added another twist – those stars might have planets that are inhabited like our own:

It is impossible that a rational being, fairly vigilant, can imagine that these innumerable worlds, manifest as like to our own or even more magnificent, should be destitute of similar or even superior inhabitants.