Human Universe - Brian Cox, Andrew Cohen (2014)

ARE WE ALONE?

Sometimes I think we are alone in the universe

and sometimes I think we’re not.

In either case the idea is quite staggering.

Arthur C. Clarke

SCIENCE FACT OR FICTION?

There are questions to which knowing the answers would have a profound cultural effect. The question of our solitude is one. Are we alone in the universe – yes or no? One of these is true. The question as posed isn’t a good one, however, because it is impossible to answer in the affirmative. We have no chance, even in principle, of exploring the entire universe, which extends way beyond the visible horizon 46 billion light years away. The answer can therefore never be yes with certainty. Indeed, if the universe is infinite in extent, we have our answer! No, we are not alone. The laws of nature self-evidently allow life to exist, and no matter how improbable, life must have arisen an infinite number of times. In itself, this is quite a challenging statement, and we will explore it in more detail later on. But this isn’t really what most of us want to know.

I’ve always been interested in aliens – the ones that fly spaceships around – and I want to talk to one. On a winter afternoon in 1977 I stood in a queue that went around three sides of the Odeon cinema in Oldham with my dad, shuffling through half-frozen puddles to see Star Wars, and spent the next decade building Millennium Falcons out of Lego. At some point in 1979 I picked up a magazine about Alien, and moved on to Nostromo, which required more bricks. To my delight I saw Alien when I was 11 years old at Friday Evening Film Society at school, and it didn’t put me off. I just realised I really liked the spaceships, and didn’t care much about the organic stuff. Everyone should see Alien at 11. To hell with the ratings; terror, technology and Sigourney Weaver are good for the soul.

Science fiction was a natural home for my imagination. I’d been interested in astronomy for a while, I’m not sure why, but the study of the stars seemed clean and precise and romantic; something done on cold nights before Christmas with mittens and imagination. Star WarsStar TrekAlien, Arthur C. Clarke and Isaac Asimov were merged seamlessly with Patrick Moore, Carl Sagan and James Burke, and they remain so; fact and fiction are inseparable in dreams. The superficially orthogonal desires to do science and to imagine distant worlds are closely related: shadows cast by different lights.

So the question ‘Are we alone in the universe?’ might make good science fiction, but it is not well posed in a scientific sense because the universe is too big for us to explore in its entirety. If we restrict the domain of the question, however, we can address it scientifically. ‘Are we alone in the solar system?’ is a question we are actively seeking to answer with Mars rovers and future missions to the moons of Jupiter and Saturn, where the conditions necessary for life may be present on multiple worlds. But even here, the use of the word ‘alone’ in the question is problematic. Would we be alone if the universe were full of microbes? Would you feel alone stranded in a deep cave with no means of escape and a billion bacteria for company? If not being alone means having intelligent beings to communicate with – sophisticated creatures that build civilisations, have feelings, do science and respond emotionally to the universe, then we have our answer in the solar system. Yes – Earth is the only world that is home to a civilisation, and we are alone.

How far might we reasonably expect to extend the domain of our question beyond the solar system? I find it impossible to believe that we’ll ever explore the universe beyond our own galaxy. The distance between the Milky Way and our nearest neighbour, Andromeda, is over 2 million light years, and that seems to me to be an unbridgeable distance, at least given the known laws of physics. But that still leaves an island of several hundred billion stars, 100,000 light years across. We will therefore rephrase our question so that we have a chance of interrogating it in a scientific way, and ask ‘Are we the only intelligent civilisation in the Milky Way galaxy?’ If the answer is yes, then we are in the cosmic equivalent of an inescapable cave and that would have made my 11-year-old self, gazing up at a dark sky of infinite possibilities, extremely sad. There may be others out there amongst the distant galaxies, but we’ll never know. If the answer is ‘No’, on the other hand, this would have profound consequences. Aliens would exist in a truly science-fiction sense; beings with spacecraft, culture, religion, art, beliefs, hopes and dreams, out there amongst the stars, waiting for us to speak with them. What are the chances of that? We don’t know, but at least we have posed a question that can be explored scientifically. How many intelligent civilisations are there likely to be in the Milky Way, given the available evidence today?

THE FIRST ALIENS

On 24 June 1947, Ken Arnold, an amateur pilot from Scobey, Montana, was flying over Mount Rainier, one of the most dangerous volcanoes in the world. Arnold was an experienced pilot with thousands of flying hours, and this implied he was a trustworthy observer. On returning to the airfield, he claimed to have seen nine objects flying in the mountain skies, describing them as ‘flat like a pie pan’ and ‘like a big flat disc’. He estimated the discs were flying in formation at speeds of up to 1920 kilometres per hour. The press jumped on the story – coining the term ‘flying saucer’ – and within weeks hundreds of similar sightings were reported from all over the world. On 4 July a United Airlines crew reported seeing another formation of nine discs over the skies of Idaho, and four days later, the mother of all UFO stories exploded at Roswell, New Mexico, with the confirmation and then rapid retraction by the United States Air Force of a recovered ‘flying disc’ – an alien craft crash-landed on Earth.

I’ll put my cards on the table here: I believe in UFOs. That is to say, I believe that there have been sightings of flying things in the sky that the observers were unable to identify, some of which were objects. But I do not believe for a moment that these were spacecraft flown by aliens. Occam’s razor is an important tool in science. It shouldn’t be oversold; nature can be complex and bizarre. But as a rule of thumb, it is most sensible to adopt the simplest explanation for an observation until the evidence overwhelms it.

My favourite response to the criticism that dismissing the possibility of alien visitations to Earth is unscientific was provided by physicist and Nobel Laureate Richard Feynman in his Messenger Lectures at Cornell University in 1964: ‘Some years ago I had a conversation with a layman about flying saucers – because I am scientific I know all about flying saucers! I said “I don’t think there are flying saucers”. So my antagonist said, “Is it impossible that there are flying saucers? Can you prove that it’s impossible?” “No”, I said, “I can’t prove it’s impossible. It’s just very unlikely.” At that he said, “You are very unscientific. If you can’t prove it impossible then how can you say that it’s unlikely?” But that is the way that is scientific. It is scientific only to say what is more likely and what less likely, and not to be proving all the time the possible and impossible. To define what I mean, I might have said to him, “Listen, I mean that from my knowledge of the world that I see around me, I think that it is much more likely that the reports of flying saucers are the results of the known irrational characteristics of terrestrial intelligence than of the unknown rational efforts of extraterrestrial intelligence.” It is just more likely. That is all.’

Irrespective of the veracity of the stories of mutilated cows, crop circles and violated Midwesterners at the hands of these alien visitors, the cultural impact of these early sightings was very real. America quickly entered into a media-fuelled love affair with alien invaders in shiny discs brandishing anal probes (why didn’t they use MRI scanners, a non-Freudian would surely ask?). Of all the hundreds of thousands of references to flying saucers that began to appear in the media, a cartoon by Alan Dunn published in the New Yorker magazine on 20 May 1950 found its way into the lunchtime conversation of a group of scientists at the Los Alamos National Laboratory in New Mexico.

Enrico Fermi was one of the greatest twentieth-century physicists. Italian by birth, he conducted his most acclaimed work in the United States, having left his native country with his Jewish wife Laura in 1938 as Mussolini’s grip tightened. Fermi worked on the Manhattan Project throughout World War Two, first at Los Alamos, and then at the University of Chicago, where he was responsible for Chicago Pile 1, the world’s first nuclear reactor. In a squash court underneath a disused sports stadium in December 1942, Fermi oversaw the first man-made nuclear chain reaction, paving the way for the Hiroshima and Nagasaki bombs.

After the war Fermi settled as a professor in Chicago, but he often visited Los Alamos. During one of these visits, in the summer of 1950, Fermi settled down for lunch with a group of colleagues including Edward Teller, the architect of the hydrogen bomb, and fellow Manhattan Project alumni Herbert York and Emil Konopinski. At some point, talk turned to the recent reports of UFO sightings and the New Yorker cartoon, stimulating Fermi to ask a simple question that turned a trivial conversation into a serious discussion: ‘Where are they?’

Fermi’s question is a powerful and challenging one that deserves an answer. It has become known as the Fermi Paradox. There are hundreds of billions of star systems in the Milky Way galaxy. Our solar system is around 4.6 billion years old, but the galaxy is almost as old as the universe. If we assume life is relatively common, and on at least some of these planets intelligent civilisations arose, it follows that there should exist civilisations far in advance of our own somewhere in the galaxy. Why? Our civilisation has existed for around 10,000 years, and we’ve had access to modern technology for a few hundred. Our species, Homo sapiens, has existed for a quarter of a million years or so. This is a blink of an eye in comparison to the age of the Milky Way. So if we assume we are not the only civilisation in the galaxy, then at least a few others must have arisen billions of years ahead of us. But where are they? The distances are not so vast that we cannot imagine travelling between star systems in principle. It took us less than a single human lifetime to go from the Wright Brothers to the Moon. What might we imagine doing in the next hundred years? Or thousand years? Or ten thousand years? Or ten million years? Even with rocketry technology as currently imagined, we could colonise the entire galaxy on million-year timescales. The Fermi Paradox simply boils down to the question of why nobody has done this, given so many billions of worlds and so many billions of years. It is a very good question.


FERMI’S PARADOX

The Fermi Paradox is the apparent contradiction between the high probability of extraterrestrial civilisations’ existence and humanity’s lack of contact with, or evidence for, such civilisations.


LISTEN VERY CAREFULLY

For three days in 1924, William F. Friedman had a very important job. As chief cryptographer to the US Army, Friedman was used to dealing with National Security responsibilities, but from 21 to 23 August he was asked to search for an unusual message. On these dates Mars and Earth came within 56 million kilometres of each other, the closest the two planets had been since 1845, and they would not be so close again until August 2003. This offered the best opportunity since the invention of radio to listen in on the neighbours.

To make the most of the planetary alignment, scientists at the United States Naval Observatory decided to conduct an ambitious experiment. Coordinated across the United States, they conducted a ‘National Radio Silence Day’, with every radio in the country quietened for five minutes on the hour, every hour, across a 36-hour period. With this unprecedented radio silence and a specially designed radio receiver mounted on an airship, the idea was to make the most of the Martian ‘fly-by’ and listen in for messages, intentional or otherwise, from the red planet.

Conspiracy theories notwithstanding, William F. Friedman didn’t decipher the first message from an alien intelligence, and the American public soon tired of the disruption to their news bulletins, but the principle of the experiment was sound. The idea that we might listen in to aliens had first been proposed 30 years earlier by the physicist and engineer Nikola Tesla. Tesla suggested that a version of his wireless electrical transmission system could be used to contact beings from Mars, and subsequently presented evidence of first contact. He wasn’t right, but in 1896, one year before the publication of War of the Worlds, it was certainly a plausible claim. Tesla wasn’t alone; other luminaries of the time shared his optimism, including the pioneer of long-distance radio transmission, Guglielmo Marconi, who believed that listening to the neighbours would become a routine part of modern communications. By 1921 Marconi was publicly stating that he had intercepted wireless messages from Mars, and if only the codes could be deciphered, conversation would soon begin.

The failure of the National Radio Silence Day brought a temporary halt to the organised search for extraterrestrial signals, and the idea dropped out of scientific fashion until the post-war flying saucer boom. One of the first scientists to make the search for ET scientifically acceptable again was Philip Morrison, a contemporary and colleague of Fermi. It is not known whether they discussed the Fermi Paradox directly, but the idea of answering it certainly played on Morrison’s mind throughout the 1950s. At the end of the decade Morrison published a famous and influential paper with another of Fermi’s collaborators, Giuseppe Cocconi, laying out the principles of using radio telescopes to listen for signals. ‘Searching for Interstellar Communications’ was published in the prestigious journal Nature, and proposed a systematic search of the nearest star systems on a very specific radio frequency – the so-called 21cm hydrogen line.

Morrison and Cocconi chose the hydrogen line because it is a frequency that any technological civilisation interested in astronomy will be tuned in to. Hydrogen is the most abundant element in the universe, and hydrogen atoms emit radio waves at precisely this frequency. If we could see these wavelengths with our eyes, the sky would be aglow, and this is why astronomers tune their radio telescopes to the 21cm line to map the distribution of dust and gas in our galaxy and beyond. If a technological civilisation wants to be heard, then under the assumption that anyone with any sense does radio astronomy, the 21cm line would be the most obvious choice for a message.

Morrison and Cocconi’s paper inspired the birth of one of the most widely debated and controversial astronomical projects of modern times. Within a year of its publication, the 85-foot radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia, was pointing towards two nearby stars – Tau Ceti and Epsilon Eridani – listening in to the 21cm hydrogen line for any signs of unnatural order in the signals from the stars. The project, known as Ozma after a character from L. Frank Baum’s Land of Oz, was the brainchild of Frank Drake, a young astronomer from Cornell University. Drake chose Tau Ceti and Epsilon Eridani as the first target star systems because of the stars’ similarity to our own Sun and their proximity, just 10 and 12 light years away from Earth. In 1960 Drake had no idea if these stars harboured planetary systems, because no planets had been detected outside our solar system at that time. We now know that Drake’s guess was a good one. Tau Ceti is thought to have five planets orbiting the star, with one of them in the habitable zone. Epsilon Eridani is also thought to have at least one gas giant planet with an orbital period of around seven years. After 150 hours of observation, Drake heard nothing, but for him this was the beginning of a lifetime dedicated to the search for extraterrestrial intelligence, a search commonly known by its acronym, SETI.


21CM LINE

Hydrogen atoms consist of two particles – a single proton bound to a single electron. Protons and electrons have a property called spin, which for these particular particles (known as spin ½ Fermions, named after Enrico Fermi himself) can take only one of two values, often called spin ‘up’ and spin ‘down’. There are therefore only two possible configurations of the spins in a hydrogen atom: the spins can be parallel to each other – both ‘up’ or both ‘down’, or anti-parallel – one ‘up’ and one ‘down’. It turns out that the parallel case has slightly more energy than the anti-parallel case, and when the spin configuration flips from parallel to anti-parallel, this extra energy is carried away as a photon of light with a wavelength of 21cm.

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Today SETI is a global scientific effort, analysing data from telescopes used primarily for radio astronomy. The organisation also has a dedicated collection of telescopes designed specifically to detect signals from extraterrestrial civilisations at the Hat Creek Radio Observatory near San Francisco. The Allen Array, named after Microsoft founder Paul Allen who donated over $30 million to fund the construction of the project, consists of 42 radio antennae able to scan large areas of the sky at multiple radio frequencies, including the 21cm hydrogen line. If there are any civilisations making a serious attempt to contact us with technology at least as advanced as our own within a thousand light years, the Allen Array will hear them.

In the early 1960s, the scientific community was sceptical about such endeavours and Frank Drake was perceived as a maverick. It’s important to be sceptical in science, but as Fermi understood, a back-of-the-envelope calculation with some plausible assumptions suggests that the search for ET may not be futile. Indeed, the alternative view that our civilisation is unique or extremely rare in a galaxy of a hundred billion suns appears outrageously solipsistic, and the sceptical finger might as easily be pointed at the cynics. There was, however, a handful of scientists who understood the importance of asking big questions, and together with Peter Pearman, a senior scientist at America’s prestigious National Academy of Sciences, Drake organised the first SETI conference in November 1961. The Green Bank meeting was small, but the list of attendees, who named themselves The Order of the Dolphin, was impressive.


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FIRST SETI CONFERENCE ATTENDEES

PETER PEARMAN

conference organiser

FRANK DRAKE

PHILIP MORRISON

DANA ATCHLEY

businessman and radio amateur

MELVIN CALVIN

chemist

SU-SHU HUANG

astronomer

JOHN C. LILLY

neuroscientist

BARNEY OLIVER

inventor

CARL SAGAN

astronomer

OTTO STRUVE

radio astronomer

GIUSEPPE COCCONI

particle physicist


Philip Morrison was there, as was his co-author of the seminal 1959 Nature paper, Giuseppe Cocconi. I have a professional connection with Cocconi, who was a noted particle physicist and director of the Proton Synchrotron accelerator at CERN in Geneva. Cocconi was instrumental in discovering early experimental evidence for the pomeron, an object in particle physics known as a Regge trajectory that I have spent most of my career studying. The eminent, highly respected astronomer Otto Struve also attended. Struve publicly stated his belief in the existence of intelligent extraterrestrial life, perhaps because he had recently suggested a method for detecting alien planets outside our solar system (see here). Nobel Laureate Melvin Calvin, most famous for his work on photosynthesis, was present, along with future Hewlett Packard vice president for R&D Barney Oliver, astronomer Su-Shu Huang, communications specialist Dana Atchley and the colourful neuroscientist and dolphin researcher John Lilly. The most junior attendee was a 27-year-old postdoc. called Carl Sagan. I would love to have been there, although I’d have spent the whole time chatting with Cocconi about pomerons.

In preparation for the meeting, Drake drew up an agenda designed to stimulate a structured conversation amongst the group. If the search for intelligent extraterrestrial life was to be taken seriously, it was clear in Drake’s mind that the discussion should be rigorous and provide a framework for future research. The way to do that is to address the problem quantitatively rather than qualitatively; to break it down into a series of probabilities that can be estimated, at least in principle, using observational data.

Drake focused on a well-defined question – the one we discussed above: how many intelligent civilisations exist in the Milky Way galaxy that we could in principle communicate with? Drake’s brilliant insight was to express this in terms of a simple equation containing a series of probabilities. What is the fraction of stars in the galaxy that have planets? What is the average number of planets around a star that could support life? What is the fraction of those planets on which life begins? What is the probability that, given the emergence of simple life, intelligent life evolves? Given intelligence, how likely is it that the intelligent beings build radio telescopes and are therefore capable of communicating with us? Multiply all these probabilities together, and multiply by the number of stars in the Milky Way, and you get a number – the number of intelligent civilisations that have ever existed in the Milky Way.

This isn’t all Drake did, however, because he was interested in the number of civilisations that we might be able to speak to now, and that requires the addition of a rather thought-provoking term – the average lifetime of civilisations from the moment they develop the technology to communicate. If a civilisation arose a billion years ago and vanished shortly afterwards, then we would never be able to talk to them. The question of the lifetime of a civilisation may have been more vivid in the early 1960s than it is today. The Manhattan Project had been the training ground for many of the great physicists, and the Cuban missile crisis was less than a year away, propelling the world, in Soviet Premier Khrushchev’s words to President Kennedy, towards ‘… the abyss of a world nuclear-missile war’. To me, and to the participants at the Green Bank conference, the idea that a civilisation might destroy itself is both ludicrous and likely. We are pathetically inadequate at long-term planning, idiotically primitive in our destructive urges and pathologically incapable of simply getting along. More of this later! Putting the lifetime term into the equation was therefore scientifically valid and a political masterstroke; merely confronting the question should give us pause for thought at the very least.

To complete the equation with the lifetime term included – recall that it should give the number of currently contactable civilisations in the Milky Way – a little thought will convince you that the whole lot must be multiplied by the current rate of star formation in the galaxy. That might not be immediately obvious, but I have confidence you can demonstrate to yourself that it’s the correct thing to do. Homework is good.

The completed equation, which is known as The Drake Equation, is shown here.


THE DRAKE EQUATION

N = R* × fs × fp × ne × fl × fi × fc × L

where:

N

the number of civilisations in our galaxy with which radio communication might be possible

(i.e. which are on our current past light cone)

R*

the average rate of star formation in our galaxy

fp

the fraction of those stars that have planets

ne

the average number of planets that can potentially support life per star that has planets

fl

the fraction of planets that could support life that actually develop life at some point

fi

the fraction of planets with life that actually go on to develop intelligent life (civilisations)

fc

the fraction of civilisations that develop a technology that releases detectable signs of their existence into space

L

the length of time for which such civilisations release detectable signals into space


When Drake wrote down his equation, only R was known with precision. Star formation had been closely studied in parts of our galaxy and the data suggested a value of around one new star per year. The rest of the terms were unknown in the 1960s, and we will spend the majority of this chapter exploring them, given over 50 years of astronomical and biological research. Despite the lack of experimental data, however, the Green Bank participants spent the meeting debating each one of the terms in the Drake Equation. This is the power of Drake’s formulation. It’s not yet possible to make a measurement of the fraction of planets on which life emerges with any sort of precision, but it is possible to look at the experience we have on Earth, and increasingly in the wider solar system, and make an informed guess. The probability of the emergence of intelligence given simple life is also a difficult question, but we do know that it took over 3 billion years on Earth, and that may give us a clue. Drake’s equation is valuable therefore because it provides a framework for discussion and debate, focuses the mind and suggests a direction for future research, just as Drake intended.

The Green Bank meeting did produce a consensus number, based on the not inconsiderable expertise of the participants: there are of the order of 10,000 civilisations present now in the Milky Way with whom we could communicate if we had enough radio telescopes and the will to conduct a systematic search. Interestingly, Philip Morrison, veteran of the Manhattan Project, felt that the lifetime of technological civilisations may be so short that this number could well be zero, although he observed that ‘… if we never search, the chance of success is zero’.

I had the privilege of meeting Frank Drake during the filming of Human Universe. In my view he is one of the greatest living astronomers. Frank collects and cultivates orchids, and by complete coincidence I arrived at his house when his Stanhopea orchid was flowering. These delicate and complex flowers bloom for only two days every year, and the chance of seeing one on a random visit is therefore small. Frank turned to me and said ‘well, so it is with SETI – we’ve learned that we must search over and over and over through the years, until we are in the right place at the right time to make the discovery’. There is ‘hope’ in its name, and there is nothing wrong at all with admitting a dash of hope.

Throughout the 1960s and 1970s, SETI projects both big and small continued to develop across the planet. Soviet scientists joined their American contemporaries in pointing radio receivers to the sky in the hope of detecting a signal in the noise. NASA considered funding Project Cyclops, a $10 billion super-array of 1500 dishes that could listen for signals originating up to 1000 light years from Earth. It never progressed beyond the planning stage, but the scale of the project demonstrates that SETI was considered to be a serious scientific endeavour. By the mid-1970s, various projects had come and gone but none had detected the faintest hint of a significant signal. This failure, combined with a lack of progress in pinning down any of the terms in the Drake Equation – it was not even certain that planets existed in large numbers beyond our solar system – made the search look increasingly futile. Not only was there a deafening silence, no one had much idea where to look or how hard to listen. NASA didn’t lose faith, however, and in 1973 Ohio University’s ten-year-old Big Ear telescope was optimised for a SETI survey and began taking data.

Four years later, on 18 August 1977, Jerry R. Ehman, then a volunteer at the Big Ear, received a knock on the door of his house. It was a Thursday morning and, as usual, standing at the door was a technician carrying reams of paper printouts. This was an age when a state-of-the-art hard disk could hold only a couple of megabytes, and every few days someone had to visit the telescope, print out the data and wipe the disks clean. Ehman put the three days’ worth of printed data onto his kitchen table and began searching. He was confronted with dozens of pages covered in hundreds of letters and numbers.

The list of numbers and letters depicts the strength of the signal hitting the telescope at different times. A space denotes low intensity, and higher intensities are registered as numbers from 0 to 9. For stronger signals still, letters between A and Z are used. Most of the data the ‘Big Ear’ recorded contained no letters; a stream of 1s and 2s signified sweeps across the general radio hiss of the sky. That morning, however, Ehman stumbled across something different. At approximately 10.16pm Eastern Standard Time on 15 August, a radio pulse of extreme intensity entered the antennae, recorded with the alphanumeric code 6EQUJ5. The signal lasted for 72 seconds, precisely the length of time a transmission of distant origin would register as the rotation of the Earth swept the telescope past the source. This is extremely important. If the signal had been caused by some kind of Earth-based interference, it would be highly unlikely to rise and fall in this manner, precisely and coincidently simulating the rotation of the Earth and the telescope’s field of view on the sky. The peak was marked by the letter U, the strongest signal ever recorded by the Big Ear, denoting an intensity over 30 times that of the background emission of the galaxy. Equally strangely, the signal had a wavelength of 21cm – the hydrogen line favoured by Morrison and Cocconi in their 1959 Nature paper. A smoking gun for extraterrestrial communication?

With a now-famous flourish, Ehman circled the six characters and scribbled ‘Wow!’ on the printout. He then continued as a research scientist should, and looked to see if it happened again. He flicked through page after page, but the event of 10.16pm on 15 August was a solitary blip in the background noise. This presented a problem, because it should have happened again. The Big Ear telescope scans each part of the sky twice, separated by 3 minutes, so there should have been a similar Wow! signal in the data 3 minutes afterwards. None was present. This doesn’t rule out an intelligent extraterrestrial origin; perhaps ET just turned the transmitter off a minute or so after it was first detected. Who knows?

The origin of the Wow! signal was narrowed down to a point in the sky in the direction of the constellation Sagittarius. Tau Sagittarii, a stable orange star twice the mass of our Sun and around 122 light years away, is the closest bright star to the source. Since August 1977 multiple attempts have been made to recover the signal using the world’s most sensitive radio telescopes. Many hours have been spent listening, but nothing unusual has ever been detected again. Today, over 35 years later, there is no satisfactory explanation, but no serious scientist, no matter how embedded in SETI, would claim it as definitive evidence of intelligent extraterrestrial communication. Scientific results have to be repeatable, and the observation has never been repeated. For the moment, the Wow! signal remains an interesting anomaly in an otherwise silent sky. It is the stuff of dreams; the faintest of whispers in a great silence.


PROBES ON TOUR

The Voyager probes have visited most of the outer planets on their way out of the solar system. Each visit has also used the planets’ gravitational pull to slingshot the probes on their journey.

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THE GOLDEN VOYAGE

Two days after Jerry Ehman spotted the Wow! signal, the human race responded with a long-planned contribution to the interstellar conversation. In an explosive, serendipitous moment, the Voyager 2 spacecraft blasted into the sky above Space Launch Complex 41 at Kennedy Space Centre, followed two weeks later by its twin Voyager 1.

The Voyager missions were designed to take advantage of a rare planetary alignment to study the outer solar system gas giants Jupiter, Saturn, Uranus and Neptune. I remember the launch – I had collected a series of PG Tips tea cards called ‘The Race Into Space’, in which the Grand Tour mission was described as ‘the most ambitious unmanned space project known’. Using the newly proposed gravity assist, a spacecraft could accelerate around Jupiter, Saturn and Uranus to encounter Neptune only a decade from launch. The Voyagers delivered, I suspect, way beyond their designers’ wildest dreams, returning the first detailed pictures of the esoteric moons of Jupiter and Saturn, and in the case of Voyager 2, sweeping onwards to become the only spacecraft to date to visit Uranus and Neptune, where it photographed the distant ice moon Triton in the summer of 1989.

At the time of writing, on 8 July 2014, Voyager 1 is the most distant man-made object at over 127 astronomical units from Earth, so distant that radio waves take over 17 ½ hours to reach it. This puts Voyager 1 at the very edge of the solar system, on its way into interstellar space. The bus-sized spacecraft has enough electrical power to continue to communicate with its home world until around 2020, at which point it will fall silent. In 40,000 years it will drift within 1.6 light years of the red dwarf star Gliese 445 in the constellation of Camelopardalis. Voyager 2 will reach Sirius, the brightest star in the night sky, in 296,000 years.

The Voyagers are accompanied on their lonely flights out of our solar system by a dream – an unusually sentimental and hopeful afterthought to a scientific mission bolted to their sides almost 40 years ago.

The Voyager Golden Record is our message in a bottle. An old-fashioned phonograph record constructed of gold-plated copper floating through the universe, it contains what some would term a surreal mixture of sound recordings, images and information. It was designed to provide an alien civilisation with information about who we are, what we know and what our planet is like. There are 116 images on the disc; the first 30 or so are scientific, illustrating our solar system, our home world, the structure of DNA, the anatomy of our bodies, our reproduction and our birth. Anatomy takes up more room than any other subject, perhaps reflecting our own fascination with what aliens might look like. In the most magnificently colloquial and futile gesture towards the aliens’ moral sensibilities, no nudity was allowed! I find it hard enough to imagine the inner workings of alien brains, but I cannot begin to fathom what it must be like inside the mind of a person who raised such an objection to the depiction of the human body. ‘How do these beings reproduce? Perhaps they use those ten dangly things on the ends of their arms? Disgusting!’

This is a present from a 
small, distant world, a token
of our sounds, our science,
our images, our music, 
our thoughts and our feelings.

We are attempting to
survive our time so we may
live into yours.

US President Jimmy Carter

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The illustrations go on to detail our planet’s landscapes and the variety of life on Earth, before dedicating 50 images to our lives and the civilisation we’ve constructed – from the Great Wall of China to a supermarket. Finally, there are images of the scientific instruments we have used to explore the universe from microscopes to telescopes, including the Titan rocket that launched the Voyagers into space. Chosen by a committee chaired by Carl Sagan, the disc also contains music and sounds, including human greetings in 55 languages, recordings reflecting ‘the sounds of the Earth’, and the ultimate 1977 mix tape featuring 90 minutes of music from Beethoven to Chuck Berry. Sagan wanted the Beatles’ ‘Here comes the Sun’ on the disc, but EMI refused copyright permission for the universe. I like to imagine that Carl Sagan put the song on the record anyway in a great cosmic two-fingered salute to corporate Earth. That would have been pure Sagan – ‘You’re most welcome to go fetch it’.

The outside cover of the golden disc is more functional. As well as instructions on how to play back the images and sounds at precisely Image Missing revolutions per minute for the audio, and how to build a record player, it also contains a map so that any extraterrestrial civilisation will be able to trace the record back to our planet. The map uses the position of 14 pulsars whose precise locations are marked relative to the Sun. The pulsars are identified by their fingerprints – each has a unique and unvarying rate of rotation. The most important piece of content on the cover is the key to unlock the information – a diagram illustrating the spin configurations of a hydrogen atom. The 21cm hydrogen emission line is a fundamental and universal property of nature, a Rosetta Stone that will allow an alien scientist to unlock the secrets of Earth. The disc also contains one last invisible source of information: electroplated onto the surface of the cover is an ultra-pure sample of uranium 238, an isotope with a half-life of 4.468 billion years. This is Voyager’s clock, a way for any civilisation to determine the age of the record, assuming that they aren’t creationists who disagree with radiometric dating. Perhaps these are the sorts of aliens that would also be offended by nudity.


THE 1977 PLAYLIST

Brandenburg Concerto No. 2 in F First Movement, Bach

‘Kinds of Flowers’ Court gamelan, Java

Percussion Senegal

Pygmy girls’ initiation song Zaire

‘Morning Star’ & ‘Devil Bird’ Aborigine songs, Australia

‘El Cascabel’ Mexico

‘Johnny B. Goode’ Chuck Berry

Men’s House Song New Guinea

‘Tsuru No Sugomori’ (‘Crane’s Nest’), Shakuhachi, Japan

‘Gavotte en rondeaux’ from the Partita No. 3, Bach

Queen of the Night aria, no. 14. The Magic Flute, Mozart

‘Tchakrulo’ Chorus, Georgian S.S.R.

Panpipes & Drum Peru

‘Melancholy Blues’ Louis Armstrong

Bagpipes Azerbaijan S.S.R.

Rite of Spring Stravinsky

The Well-Tempered Clavier Book 2, Bach

Fifth Symphony Beethoven

‘Izlel je Delyo Hagdutin’ Bulgaria

Night Chant Navajo Indians

‘The Fairie Round’ Holborne, Paueans, Galliards, Almains

and Other Short Aeirs

Panpipes Solomon Islands

Wedding Song Peru

‘Flowing Streams’ Ch’in, China

‘Jaat Kahan Ho’ Raga, India

‘Dark Was the Night’ Blind Willie Johnson

String Quartet No. 13 in B flat Beethoven


For all the thought and care that went into these discs, neither Voyager spacecraft is heading towards any particular star; these tiny craft constructed by human hands will almost certainly never be found. The vastness of space swallows travellers, and of course Voyager’s scientists and engineers knew this. That, however, is not the point; the act of launching these gilded emissaries into space expresses something important. It’s my childhood science fiction dream of living in a Star Wars galaxy filled with life and possibilities. It is a desire to reach out to others, to attempt contact even when the chances are vanishingly small; a wish not to be alone. The golden discs are futile and yet filled with hope; the hope that we may one day know the boundaries of our loneliness and lay to rest the unsettling internal noise that accompanies the enduring silence.

Friends of space, how are you all? Have you eaten yet?

Come visit us if you have time.

Margaret Sook Ching, Voyager Golden Record

ALIEN WORLDS

Let us now return to Frank Drake’s equation and use it as intended, as a framework to address in a systematic manner the question of our solitude. Recall that the equation consists of a series of terms which, when multiplied together, give an estimate of the number of currently contactable civilisations in the Milky Way galaxy. At the 1961 Green Bank meeting only the first term – the rate of star formation in the Milky Way – was known with any precision. Over half a century later, we can do much better. The next term in the equation is the fraction of stars in the Milky Way that have planets orbiting around them – most definitely a prerequisite for an intelligent civilisation to emerge. It’s true that the civilisation may not have remained confined to its home world, and we will discuss this possibility later on. But it must be true that for life to emerge and evolve to the point where it can build spacecraft, a planet of some sort is required.

This space we declare to be infinite …

In it are an infinity of worlds of the same kind as our own.

Giordano Bruno, 1584

The existence of alien worlds has been speculated about for many centuries. Ever since Copernicus began the process of demoting our solar system from its preferred place in the cosmos, it has been natural to assume that at least some of the stars in the sky must have planetary systems. Yet despite this seemingly common-sense conclusion, reached by virtually every right-thinking astronomer from Giordano Bruno onwards, the existence of other planets remained nothing more than an educated guess well into my lifetime. The vast distances between the stars and the limitations of technology locked us inside our own solar system with no way of seeing beyond. Throughout the nineteenth century a number of astronomers claimed to have detected distant planets, but all these observations proved to be flawed.

Today the picture couldn’t be more different; the night sky is known to be awash with worlds. One of the more enticing of the known solar systems is located around a slightly smaller, cooler version of our Sun called Kepler-62. About 1200 light years from Earth in the constellation of Lyra, the system has been widely studied because it has at least five planets. Two of them, Kepler 62-e and Kepler 62-f, are particularly interesting because they are Earth-like in both size and distance from the star. Bathed in Kepler-62-shine, these worlds may, if they have the right atmospheric conditions, support oceans of liquid water on their surfaces. We will discuss the significance of this in the context of life later on.

An intrinsically improbable
event may become highly 
probable if the number of events 
is very great … [I]t is probable 
that a good many of the billions 
of planets in the Milky Way 
support intelligent forms of life.

To me this conclusion is 
of great philosophical interest.

I believe that science has 
reached the point where it is 
necessary to take into account 
the action of intelligent beings, 
in addition to the classical 
laws of physics.

Otto Struve

The discovery of extra-solar planets has been possible due to the rapid development of precision astronomical instruments, both space-based and terrestrial, that allow us to see beyond the bright glare of stars to the worlds that lie in the shadows. Imagine looking at our solar system from the nearest star system to Earth, Alpha Centauri. The system is 4.37 light years away, and consists of two sun-like stars – one slightly more massive than the other – orbiting each other with a period of approximately 80 years. The red dwarf Proxima Centauri is probably a distant gravitationally bound component of the system, making it a loosely bound triple star. Looking back towards Earth from 40 trillion kilometres with the naked eye, our sun would look like any other solitary star. Detecting exoplanets is no easy task because planets are vanishingly small and faint, masked by the brightness of their parent stars, and directly imaging them remains a major technical challenge.

To step out of the glare has required the development of indirect methods of detection based on surprisingly sensitive technologies. On 21 April 1992 the first conclusive detection of an exoplanet was made by radio astronomers Aleksander Wolszczan and Dale Frail, working at the Arecibo Observatory in Puerto Rico. They were hunting for planets around a pulsar known as PSR 1257+12, located 1000 light years from Earth, using a delicate method of indirect observation known as pulsar timing. Pulsars are spinning neutron stars, some of the most exotic objects in the universe. PSR 1257+12 is 50 per cent more massive than our Sun, but has a radius of just over 10 kilometres. It is, in effect, a giant atomic nucleus, spinning on its axis every 0.006219 seconds – that’s 9650 rpm. As you may gather from this rather precise statement, it is possible to measure the spin-rates of pulsars with great precision by timing the interval between pulses of radio waves emitted from the stars like a lighthouse. Wolszczan and Frail reasoned that if a large enough planet was orbiting a pulsar, the gravitational tug should shift the arrival times of the radio pulses by enough to be detectable. And sure enough, they found two planets orbiting PSR 1257+12, and measured their masses and orbits. Planet A has a mass of 0.020 times the mass of Earth and orbits the star once every 25.262 days. Planet B is 4.3 times the mass of Earth, and orbits once every 66.5419 days. Subsequently, a third planet has been discovered, with a mass of 3.9 times that of Earth and orbiting every 98.2114 days. Pulsar astronomy is indeed a precision science.


KEPLER-62

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THE HABITABLE ZONE

The most important requirement for the evolution of life as we know it is liquid water. This can only exist on the surface of a planet if that planet is far enough away from the star at the centre of its planetary system: too close and the surface is too hot, resulting in any water boiling off into space; too far away and the surface is too cold and the water will exist only as ice. The too hot/too cold scenario is what is known as the Goldilocks Zone. The distance and width of the Goldilocks Zone also depend on the size and temperature of the central star – it is further away from large, hot stars and closer in systems with small, cold stars. Using the Hertzsprung-Russell diagram and the known size of the star allows the calculation of each system’s Goldilocks Zone, thus allowing us to determine whether the observed planets are likely to have liquid water and are therefore candidates for the evolution of life.

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This was an historic observation, but of limited direct interest to SETI since there is absolutely no chance that life could survive the hostile environment around such a violent astronomical object. It was, however, an existence proof – the first discovery of planets beyond our solar system, and a surprising one at that.

To search for Earth-like planets around Sun-like stars required the development of different but equally beautiful methods of observation. The first of these to be deployed was the radial velocity method. A star doesn’t sit still at the centre of a solar system with planets orbiting around it. Rather, the star and planets orbit around their common centre of mass. The centre of mass of a solar system with a single star will always be inside the star itself, because it carries virtually all of the mass, but the star will still wobble around the centre of mass of the system as seen from Earth.

This planetary-induced wobble is small but measurable. In our solar system Jupiter causes our Sun to wobble backwards and forwards with a velocity change of approximately 12.4m/s across a period of twelve years. The Earth’s effect is minute in comparison, inducing a velocity change of just 0.1m/s over a period of a year.

In the 1950s, future Green Bank pioneer Otto Struve suggested that such a planetary-induced wobble could be detected using the Doppler Effect. When a star moves towards the Earth, its light is shifted towards the blue part of the spectrum, and when it moves away from the Earth its light is shifted towards the red part of the spectrum. By making measurements of the specific frequencies (i.e. colours) of light absorbed by chemical elements in the star’s atmosphere, and measuring how much these are shifted relative to the known frequencies as measured here on Earth, the motion of the star backwards and forwards can be determined over a period of time, and this can be used to calculate the orbital period of the planet and to estimate its mass. If there is more than one planet, the motion of the star will be more complicated, but since the orbital periods of the planets are regular, the contributions of the different planets to the star’s wobble can be figured out.

One of the most exciting areas of current astronomical research is the hunt for planets around other stars – known simply as exoplanets – which are potential homes for extraterrestrial life. Until recently, such a search would have been impossible, as planets are too faint to see over interstellar distances. However, thanks to new instrumentation, we are now able to detect the telltale signals of exoplanets using two main techniques: the radial velocity method and the transit method.


RADIAL VELOCITY METHOD

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The radial velocity method measures the variation in the wavelength of the radiation transmitted by a star. The variation is due to the star ‘wobbling’ as the exoplanet rotates around it, causing the distance from us to the host star to vary minutely. The dedicated planet hunter – the Kepler Space Telescope – uses the transit method (see here).


Struve was one of the first respected scientists to publicly state his belief in extraterrestrial life. In the 1950s, however, the spectrographs used to measure red and blue shift were only able to detect velocity changes of a few thousand m/s, and at the Green Bank meeting he could only speculate that his technique would one day confirm his prejudice that planetary systems are common. Struve didn’t live long enough to see his method applied, dying just two years after Green Bank, long before technology caught up with his ambition. It took until 1995 for two Swiss astronomers, Michel Mayor and Didier Queloz, to detect a planetary-induced Doppler shift using the Observatoire de Haute-Provence in France. The team discovered a planet orbiting the Sun-like star 51 Pegasi, located 50.9 light years from Earth.

This planet is named 51 Pegasi b, but its nickname is Bellerophon, after the mythological Greek hero who rode Pegasus, the winged stallion. Since its historic discovery, Bellerophon has been observed and examined in quite some detail, and it is no second Earth. It is a deeply hostile world, orbiting its parent star every four Earth days on a trajectory that takes it far closer than Mercury approaches our own Sun. Unlike Mercury, Bellerophon is a gas giant planet with a mass 150 times that of the Earth and a surface temperature approaching 1000 degrees Celsius. Although only half the mass of Jupiter, it may have a greater radius because the high surface temperature causes it to swell. Such exoplanets are known as Hot Jupiters – big enough and close enough to cause a significant wobble in their parent stars, which is why these types of worlds were discovered first by the early planet hunters.

The first evidence of a potential Earth-like planet arrived in 2007, when Stephan Audrey and his team at the European Southern Observatory in Chile announced the discovery of a planet around the red dwarf star Gliese 581, just over 20 light years from Earth. This was the second planet to be discovered in this system, but Gliese 581-c made headline news around the world because of its apparent Earth-like qualities. This planet is a rocky world, about five times as massive as Earth, and possibly the right distance away from its parent star to support liquid water on the surface: the stuff out of which science-fiction dreams are made. Further research has cast doubt on the idea that Gliese 581-b might have the necessary conditions to support life, but in March 2009 the second-Earth hunters got their own dedicated scientific instrument, and with it a cascade of new data became available.

The Kepler Space Telescope has transformed our knowledge of the distribution of planets in the Milky Way. Kepler is not a general-purpose instrument with multiple detectors and myriad ambitions; the telescope was designed for one purpose: to look for Earth-like planets. Free of the distorting effects of the Earth’s atmosphere, Kepler carries a high-precision photometer, an instrument that has measured the light intensity from over 100,000 stars considered stable enough to support life on planets around them. Kepler searches for planets using a technique known as the transit method. If a planet passes across the face of a star as seen from Earth, the observed brightness of the star will drop by the tiniest of margins. Kepler’s photometer is so sensitive it can measure changes in brightness (to use precise astronomical language we should say changes in the apparent magnitude) of less than 0.01 per cent. Observing repeated dips in brightness allows the orbital period of the planet to be measured, and the details of the changes in the brightness, combined with knowledge of the orbit, allow the size and mass of the planetary candidate to be estimated. The transit method has been extremely successful in the hunt for exoplanets, but the technique is not entirely reliable, often throwing up false positives. Once a promising candidate is found, the location is passed to ground-based telescopes for further analysis, and, if confirmed, the planets are classified as discoveries. Kepler has used the transit method of planet hunting on a quite extraordinary scale since it became fully operational in May 2009. As I write in July 2014, NASA’s Exoplanet Archive lists 1,737 confirmed planets, over 50 per cent of which have been discovered using the Kepler data. This number is all the more staggering because Kepler is only capable of detecting a very small number of the planetary systems in our galaxy. Kepler views around 0.3 per cent of the sky in the constellations of Cygnus, Lyra and Draco, and even in this small patch, the telescope can only detect planets that pass directly in between their parent star and Earth. If the plane of the planetary orbits is orientated at the wrong angle, which is more likely than not, Kepler will not see any planets. Furthermore, Kepler only observed for four years, and because it has to see more than one transit to measure an orbit, it is blind to planets that orbit with periods greater than four years – which is the case for all the outer planets in our solar system. And finally, Kepler only sees stars out to a distance of approximately 3000 light years, whilst our galaxy has a diameter of 100,000 light years. Kepler’s data set, then, contains only a tiny fraction of the planetary systems out there. All of these losses can be corrected for in a statistical sense, and when the numbers are crunched we have a reliable observation-based number to put into the Drake Equation. The fraction of stars that have planetary systems is close to 100 per cent! On average, there is at least 1 planet per star in the Milky Way galaxy, and we can insert the second term with confidence: fp = 1.

The extraordinary Kepler mission was expected to last until 2016, but technical malfunctions may mean the telescope has now finished its planet-hunting activity. Even so, the huge volume of data is still being worked through and indications suggest it may have captured evidence for up to 3000 more planets circling distant stars.

This is encouraging for SETI enthusiasts, but in the hunt for civilisations, it’s not the number of planets out there that really matters; rather, it is how many of these planets are capable of supporting life. This is the next term in the Drake Equation – the average number of planets per star that has planets that can support life – ne. This is sometimes referred to as the Goldilocks question: how many of those billions of planets are not too hot and not too cold, but just right to allow life to exist on their surface?


TRANSIT METHOD

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The transit method of exoplanet identification depends on the measurement of the brightness of the light emitted by a star. This is very slightly dimmed as a planet passes between the star and the telescope. The Kepler Space Telescope can measure a variation of less than 0.01 per cent and has discovered 1,737 planets since its launch in May 2009.


THE RECIPE FOR LIFE

Why Earth? What is it about our planet that makes it a home for life? In 2008 NASA brought together a team of scientists to define in the most basic terms the properties a planet needs to have a chance of supporting life, given our current scientific knowledge. Top of the list was liquid water – an ingredient virtually every biologist would agree is necessary for life. Water is a uniquely complex liquid, with its simple H2O molecules forming great complexes held loosely together by hydrogen bonds. It forms the scaffolding around which biology happens, holding molecules and orientating them in just the right way for chemical reactions to take place. It is a superb solvent, and remains a liquid over an unusually large range of temperatures and pressures. It has been said that we will never truly understand biology until we understand water, such is its role in the chemistry of life on Earth. Fortunately, water is abundant in the universe. Hydrogen is the most common element, making up 74 per cent of the matter in the universe by mass. Oxygen is the third most abundant, at around 1 per cent, and these two reactive atoms combine to form water whenever they can. Water has been present in the universe for over 12 billion years, which we know because we’ve seen it. In July 2011, a giant reservoir of water was detected around an active galaxy known as APM 08279+5255. The cloud contains over 140 trillion times the amount of water in Earth’s oceans, and is over 12 billion light years away, having formed less than 2 billion years after the Big Bang. So water is necessary for biology and, fortunately, extremely common throughout the universe.

Earth is unique in the solar system, however, because it is currently the only place where the surface conditions are right for water to exist in all three of its states: solid, liquid and gas. There are ice sheets at the poles and on the summits of the highest mountain peaks. In the atmosphere, clouds of water vapour form and fall as rain and snow, flowing back through rivers into the oceans that cover over 70 per cent of the surface. Mars has water, but on the cold red planet it can only be found as ice trapped in the poles and deep below ground and, just possibly, as sub-surface liquid lakes. Venus may once have been wet, but its proximity to the Sun and runaway greenhouse effect boiled any primordial oceans off into space long ago. This appears to suggest that it is Earth’s distance from the Sun that defines its suitability for life. Drag the Earth closer to the Sun and the temperatures would rise, the oceans would evaporate into the atmosphere, and if things got too hot the water molecules would escape into space, leaving Earth a dry, Venusian world. Drag the Earth further out towards Mars, and temperatures would drop until eventually the surface water would freeze.

Extended regions of liquid 
water, conditions favourable
for the assembly of complex 
organic molecules, and energy 
sources to sustain metabolism.

NASA, 2008

It might appear tempting, therefore, to look for planets at roughly the same distance from their stars as Earth in the search for living worlds. This would be oversimplistic, because things are a lot more complicated. The conditions on the surface of a planet depend on many factors, the distance to the star being only one. The mass of the planet determines the gravitational pull it exerts on the molecules in its atmosphere, and this determines which atmospheric molecules it can hang on to at a given temperature. This is important because the atmosphere plays a critical role in setting the surface temperature of a planet. Venus has the hottest surface in the solar system other than the Sun because of its greenhouse gas-laden atmosphere, despite being much further away from the Sun than Mercury. The Moon, on the other hand, has very little atmosphere due to its small mass, and even though it is the same distance from the Sun as the Earth, its surface temperatures range from over 120°C in direct sunlight to below -150°C at night. NASA’s Lunar Reconnaissance Orbiter measured the coldest temperature ever recorded in the solar system, -247°C, in the limb of a crater at the Moon’s North Pole, which never receives sunlight because the Moon’s spin axis is almost perpendicular to its orbital plane. The composition of the atmosphere is determined in part by the geology of the planet; on Earth, plate tectonics play an important role in regulating the amount of carbon dioxide in the atmosphere. CO2 is a greenhouse gas, and higher concentrations of such gases raise the temperatures. The presence of sulphur dioxide in the atmosphere from volcanic eruptions can cool the surface of a planet, however, because sulphate aerosols reflect sunlight back out into space. The Mount Pinatubo eruption in June 1991 cooled the Earth’s surface by up to 1.3 degrees for the three years following the eruption. And we shouldn’t forget that life itself alters the composition of planetary atmospheres quite radically. Earth’s atmosphere today is a product of the action of living things; before photosynthesis evolved, there was very little free oxygen in the atmosphere, and plants play an important role in removing CO2 and locking it up in biomass. The planet’s mass, spin axis, orbit, geology and atmospheric composition all conspire in a complex way to set the average surface temperature and atmospheric pressure, which ultimately determine whether liquid water can exist on the surface. And if life gets going, its effects have to be folded in as well.

Beyond the planet, a vitally important ingredient for producing a potentially living world is, of course, the parent star itself, and all stars are most definitely not alike. There are over two hundred billion stars in the Milky Way galaxy. The largest known supergiant stars are over 1500 times the diameter of our Sun. If such a star were located at the centre of our solar system, it would engulf Jupiter. At the other end of the spectrum are tiny red dwarfs, with diameters from around half that of our sun to as small as a tenth of it. The smallest known star at the time of writing goes by the name of 2MASS JO5233822-1403022, which shines eight thousand times less brightly than our sun and is smaller (but denser) than Jupiter.

As with virtually everything in physics, a good way to make sense of this stellar menagerie is to draw a graph. The most famous graph in all of astronomy is known as the Hertzsprung-Russell diagram, after astronomers Ejnar Hertzsprung and Henry Norris Russell, who drew it independently in 1911. They plotted the surface temperature of the stars (which is directly related to their colour – hot stars are blue or white hot, cool stars are red) against their brightness. It is immediately obvious that the stars are not distributed randomly on the diagram. Most lie on a sweeping line ascending from the bottom right to the top left. This line is known as the Main Sequence. Our yellow sun lies around the middle of the main sequence, and all the stars on this line are generating their energy in the same way – by fusing hydrogen into helium in their cores. These are the ‘standard stars’, if you like, although their masses, lifetimes and suitability for the support of living solar systems are very different.

The basic physics underlying the Main Sequence line is simple. Stars are clouds of hydrogen and helium, which is pretty much all there is in the universe to a good approximation, collapsing under their own gravity. As the cloud collapses, it heats up. This is not surprising – all gases get hot when they are compressed – try pumping up a bicycle tyre. Eventually, the collapsing ball of gas gets so hot that the positively charged hydrogen atoms overcome their mutual electromagnetic repulsion and fuse together in a nuclear reaction to make helium. This releases a tremendous amount of energy, which further heats up the gas, increasing the rate of nuclear reactions and continuing to heat the gas. Hot gases want to expand, and so ultimately a balance will be reached between the crushing force of gravity and the outward pressure exerted by the nuclear-heated gas. This is the current state of our Sun, happily converting 600 million tonnes of hydrogen every second into helium to counteract the inward pull of gravity. For less massive stars, the equilibrium will be reached at a lower temperature because the inward pull of gravity is weaker. Having a lower surface temperature, these stars will be redder than our sun, and also less luminous. These are the dim, red stars at the bottom right of the diagram, known as red dwarfs. We’ve already met an example of a red dwarf – our nearest stellar neighbour, Proxima Centauri. Red dwarfs also have the longest lifetimes of the stars on the Main Sequence, simply because they have to burn their fuel at a lower rate in order to reach a stable equilibrium with gravity.

At the other end of the Main Sequence are the massive blue stars. Ten times the mass of our Sun or more, the inward pull of gravity is strong, and they have to burn their hydrogen fuel at a profligate rate to resist collapse. This makes them hot, and therefore blue, but also short-lived. The largest Main Sequence stars will use up their nuclear fuel in ten million years or less, at which point they will move off the Main Sequence to become red giant stars. The red giants, like the famous Betelgeuse in the constellation of Orion, are stars nearing the end of their lives. Starved of hydrogen in their cores, they begin to fuse helium into heavier elements like carbon and oxygen. These stars are the origin of most of the heavy elements in your body. Their cores become superheated in their ultimately futile battle against gravity, causing their outer layers to expand and cool. This is why the red giants sit at the top right of the Hertzsprung-Russell diagram. They are vast, and therefore bright, but their cool surfaces cause them to glow a deep red. Red giants will last for only a few million years before they run out of nuclear fuel, at which point they shed their outer layers, forming one of the most beautiful sights in nature – a planetary nebula. It is these clouds, rich in carbon and oxygen, which ultimately distribute the building blocks of life into the galaxy. Your building blocks are likely to have been part of a planetary nebula at some point over five billion years ago. Cooling at the heart of the nebula is the fading core of the star, exposed as a white dwarf. These stars populate the bottom left of the Hertzsprung-Russell diagram.

There are a handful of other exotic stars out in the Milky Way. The vast blue supergiant stars like Deneb are extremely hot and extremely luminous. Deneb, the brightest star in Kepler’s field of view in the constellation of Cygnus, is almost 200,000 times more luminous than our Sun, and 20 times more massive. It burns its nuclear fuel at a ferocious rate, and will probably explode in a supernova explosion within a few million years, leaving a black hole behind.

The Hertzsprung-Russell diagram, then, is the key to understanding stellar evolution, and also contains vital information for planet hunters. Stars that do not lie on the Main Sequence are highly unlikely to support planetary systems with the right conditions for life. They are either short-lived and ferociously bright, or have had a life history fraught with violence and change. The Main Sequence, containing the stable, hydrogen-burning stars, is where we should look for stability. But even there, the more massive, brighter stars are likely to be too short-lived for complex life to emerge. On Earth, life existed for over three billion years before complex organisms emerged in the Cambrian explosion just 550,000 years ago. We will discuss the history of life on Earth in more detail a little later, but for now we might venture an educated guess that stars with lifetimes significantly shorter than a billion years or so are unlikely to preside over planets with intelligent civilisations. This rules out the blue stars at the top left of the Main Sequence. Even familiar stars like Sirius, the brightest star in the night sky and only twice the mass of the Sun, can probably be ruled out as its lifetime on the Main Sequence is expected to be a billion years at most. We are therefore left with stars on the Main Sequence with masses within a factor of two or less of our Sun as candidates for solar systems that could support complex life.

There may also be a lower limit on the masses of life-supporting stars, although this is very much an active area of research. Around 80 per cent of the stars in the Milky Way are red dwarfs, and many are known to have solar systems. Red dwarfs have potential lifetimes measured in the trillions of years, so there is no issue with their longevity. Despite their frugal use of fuel, however, red dwarfs tend to be volatile and variable in their light output. Sunspots can reduce their brightness by a factor of two for long periods of time, and violent flares can increase their brightness by a similar factor over time periods of days or even minutes. Planets in orbit around red dwarfs are therefore subject to significant and rapid changes in the amount of light and radiation they receive. Furthermore, because of their low light output, planets must be extremely close to the star if they are to be warm enough for liquid water to exist on the surface, irrespective of the details of their atmospheres. When planets orbit close to stars, they become tidally locked, with one hemisphere permanently facing the star and the other always facing into the darkness of space. We only see one face of our Moon for the same reason – tidal locking is inevitable for moons orbiting close to planets or planets orbiting close to stars. This results in a strange kind of climate for potentially habitable planets around red dwarf stars; there will be regions of permanent day, and regions of permanent night.

Despite all these problems, however, recent computer modelling suggests that red dwarf planets may be able to maintain stable surface conditions if they have thick, insulating atmospheres and deep oceans, and life has plenty of time to evolve in these unfamiliar (to us) conditions. The jury is still out as to whether the red dwarfs that populate the low-mass region of the Hertzsprung-Russell diagram could be candidates for living solar systems.

Where does all this leave us? If we take the conservative path, and focus our attentions on the Sun-like orange and yellow stars on the main sequence, we can look at the Kepler data to estimate how many of these so-called F, G and K-type stars in the Milky Way have rocky planets in the right orbits to allow liquid water to be present on the surface, at least in principle. These planets orbit within what is known as the habitable zone, and this is the number we want to measure and insert into the Drake Equation. This has been done, and the results are surprising. In a recent study, ten planets were identified as Earth-like in the Kepler data set, in the sense that they have the right mass and composition, and are in the right orbits around their parent Main Sequence F, G or K stars, to support liquid water on their surfaces for long periods of time. Applying all the statistical corrections to account for the alignment of the solar systems relative to Earth, the lack of ability to see planets with longer orbital periods, and so on, we can estimate with a reasonable degree of certainty that there are around 10,000 Earth-like planets capable of supporting life in Kepler’s field of view. This in turn suggests that around a quarter of F, G and K stars in the Milky Way have potentially life-supporting planets in orbit around them, corresponding to ten billion habitable planets. If we allow the possibility that planets around red dwarfs may also be habitable, then we can more than double that number.

There is one final point worth making about habitable zones around stars. In our solar system, Venus, Mars and Earth are within the habitable zone as commonly defined, but there are other places where life may exist. Several of the moons of Jupiter and Saturn are planet-sized worlds, and it is known that the Jovian satellites Europa and Ganymede, and quite possibly Saturn’s giant moon Titan and the small but active Enceladus, have sub-surface oceans or lakes of liquid water. Europa in particular is considered to be one of the most likely places beyond Earth that may support life, even though it is outside the more commonly defined habitable zone around the Sun. If we admit the possibility that planet-sized moons may extend the habitable zone around stars, then the number of potentially life-sustaining worlds in the Milky Way increases significantly.

Over 50 years after the Green Bank meeting, the first three astronomical terms in the Drake Equation are now known from experimental data, and they are encouraging for SETI. There are, of course, large uncertainties, and one can find differing interpretations of the data in the academic literature. What is absolutely clear, however, is that the number of potential homes for life in the Milky Way is measured in hundreds of millions at the very least – most likely billions. From an astronomical perspective, the Milky Way could be teeming with life. The next three terms in the Drake Equation are biological; they concern the probability that life will emerge spontaneously on a planet that could support it, and the probability that the necessarily simple life that first appears evolves into complex, intelligent beings capable of constructing a technological civilisation. It is to these difficult questions that we now turn.

ORIGINS

Earth formed 4.54 +/-0.07 billion years ago out of the flattened disc of dust orbiting our young Sun. The planet was far from hospitable for the first few hundred million years of its life; it was an intensely hot and volcanic world, bombarded by asteroids and comets and, at least once, it collided with another planet, which resulted in the 23.5-degree tilt of our spin axis and the formation of the Moon.

Slowly, the solar system became a more ordered place, and Earth cooled to the point where liquid water could exist on its surface. There is evidence that liquid water existed as far back as 4.4 billion years, but it is certain that our planet was blue by the end of the late heavy bombardment 3.8 billion years ago, and around this time we find the first evidence of life. Structures known as microbially induced sedimentary structures were discovered in 2013 at a remote site in the Pilbara region of Western Australia. They were found in a sedimentary rock layer laid down in the early Archean period, 3.48 billion years ago. Similar structures are found today along ocean shorelines and in rivers and lakes, formed by the interaction of microbial mats with sediments carried through them by water currents. They indicate the presence of a complex microbial ecosystem, most likely a purple layer of slime that thrived in the warm, wet, oxygen-free environment of the early Earth, filling the atmosphere with the sulphurous stench of anaerobic breath. Early Earth would not appear welcoming to our eyes or noses.

Beyond 3.5 billion years, there is indirect evidence for the existence of life as far back as 3.7 billion years. Geologists studying some of the oldest sedimentary rocks on Earth in the Isua Supracrustal Belt in Western Greenland analysed the ratio of carbon isotopes in sedimentary rocks. The ratio of the heavier carbon 13 isotope to the more common carbon 12 can be used as a biomarker, because organisms preferentially use the lighter carbon 12 isotope in metabolic processes. Around 98.9 per cent of naturally occurring carbon is carbon 12, and if the concentration is significantly higher in a particular rock deposit then this is taken as evidence that the carbon was laid down by biological processes.

What can this evidence tell us about the probability of life emerging spontaneously on other worlds? The problem is that Earth is a sample size of one, so it would be erroneous to draw firm conclusions. It is interesting to observe that life emerged very early in the Earth’s history – probably as soon as the conditions were right. The first half a billion years after Earth’s formation is known as the Hadean Eon, named after the Greek god of the underworld. It is likely that the carbon dioxide atmosphere, volcanism and frequent bombardment from space made life impossible on the surface during the Hadean. From the start of the Archean Eon 4 billion years ago, and certainly after the violent period of the solar system’s history known as the Late Heavy Bombardment – which is known from analysis of lunar rocks to have ended 3.8 billion years ago – Earth became a more stable planet, and this date coincides with the earliest evidence for life. It is tempting, therefore, to suggest that life began on Earth pretty much as soon as it could have done after the violence of its formation. If this is taken as a working hypothesis, then we might venture that the probability of life arising on a planet that could support it – the term fl in the Drake Equation – is close to 100 per cent. This is, of course, speculative to say the least, and we would know this number with much greater certainty if we found that life arose independently on Mars, Europa, or one of the many bodies in the solar system that had or still have large bodies of liquid water on or below the surface. This is one of the most important motivations for the exploration of Mars and the moons of the outer solar system.

A BRIEF HISTORY OF LIFE ON EARTH

At this stage in the analysis of the Drake Equation, it’s looking promising for the alien hunters. There are billions of potentially habitable worlds in the Milky Way galaxy, and it is possible to interpret the early emergence of life on Earth as a hint (evidence would be too strong a word) that simple life may be inevitable, given the right conditions. The next term in the equation turns out to be more problematic for the optimist, however. We need to estimate fi, the fraction of planets with life that go on to develop intelligent life, and fc, the fraction of those worlds on which civilisations develop the technology to be contactable. As for the origin of life, the only evidence we have can be found in the history of life on Earth, so let us briefly summarise what we know.

The first population of living things whose ancestors survived to the present day is commonly known as LUCA – the Last Universal Common Ancestor. These four words mean something very specific; because all living things on the planet today share the same basic biochemistry, including DNA, we may assert that all living things are related and share a common origin. Specifically, if you trace your personal lineage back – to your parents, grandparents, great-grandparents and so on – you will find an unbroken line stretching all the way back to LUCA. It is possible that life emerged more than once on Earth, with different biochemistry, but we have no evidence of it. LUCA may have been unrecognisable when compared to today’s life – they may not even have been cellular in nature, but rather a collection of biochemical reactions involving proteins and self-replicating molecules, possibly contained inside rocky chambers around deep-sea hydrothermal vents. They would certainly have been simpler than the earliest known microbial mats, but somewhere in your genome there will be sequences of DNA that have been faithfully passed down across the great sweep of geological time, and if you have children, you’ll pass these four-billion-year-old messages on to them.

Our task is to try to estimate how likely it is that, given enough time, LUCA will evolve into organisms capable of building a civilisation. This is, of course, not precise; no accurate scientific statements can be made with a sample size of one! All we know for sure is that it happened here. The best we can do is trace our lineage back through time and try to identify potential bottlenecks along the way.

Our species, Homo sapiens, emerged around 250,000 years ago in the Great Rift Valley of East Africa. Given that Homo sapiens is the only species to have built a civilisation, the probability of our evolution from earlier hominin species is what we need to know to estimate fc. To summarise, the emergence of Homo sapiens was undoubtedly fortuitous, dependent on many factors including, it appears, the geology of the Rift Valley itself and the details of cyclical changes in the Earth’s orbit. But given enough time and the existence of large numbers of relatively intelligent animals on Earth, it is at least possible to imagine that some other creature may have made the long journey towards civilisation at some point in the future had we not emerged when we did. This is, of course, simply my opinion, and you should make up your own mind after reading further. Incredibly fortunate as we are to exist, therefore, I don’t think the ascent from primates to humans is the most important evolutionary bottleneck in the road to technological civilisation, given the pre-existing biological diversity on Earth and a few tens or hundreds of millions of years of stability into the future. Rather, I think we should direct our attention back over the much longer time periods between the origin of life on Earth and the emergence of the first intelligent animals. We are mammals, which first appeared 225 million years ago in the Triassic era. Dinosaurs also appeared around this time, a subgroup of archosaurs to which birds and crocodiles are related. The first evidence of large numbers of complex animals can be found around 530 million years ago, during a period of rapid biological diversification known as the Cambrian explosion. The earliest fossils of multicellular organisms, known as Ediacaran biota, have been identified as far back as 655 million years. Many of these organisms appear sponge-like or quilted, and nothing like them survives today. There is evidence of animal-like body plans in some Ediacaran fossils, with a clearly differentiated head, but because of their soft bodies fossils are rare and relatively little is known about them. Beyond 655 million years ago, there is no evidence of multicellular life on Earth.

The half a billion years or so from the Cambrian explosion to the present day is, in geological terms, relatively short, and life seems to have marched towards greater complexity ever since. This is a gross oversimplification, and we certainly do not suggest that evolution can be viewed as an inevitable march towards intelligence. One might be tempted to assert, however, that given something akin to a Cambrian explosion, the probability of developing intelligent life may be non-negligible, although there are scientists who will strongly disagree.

There is a significantly longer stretch of time between LUCA and the Cambrian explosion – over 3 billion years – and if we are looking for potential barriers to the emergence of intelligence we should investigate the vast expanse of time before complex, multi-cellular life appeared. Why did single-celled organisms remain ‘simple’ on Earth for so long? Most biologists would point to at least two crucial evolutionary innovations that were necessary, though not sufficient, to trigger the Cambrian explosion. The first was oxygenic photosynthesis. An oxygen atmosphere is probably a necessary precursor for the development of complex living things. All multicellular animals today breathe oxygen. This is not a coincidence or a biological fluke; it is chemistry. We release the stored energy from our food by oxidising it – a chemical reaction that is around 40 per cent efficient in the presence of oxygen. Food can be oxidised by other elements such as sulphur, but these reactions typically have an efficiency of 10 per cent or less. If a food chain is to be supported, with predators eating prey that eat plants and so on, then oxygen is probably essential. Without it, the energy available for predators would diminish by 90 per cent at each step in the food chain. This wouldn’t simply mean that an oxygen-starved planet could be full of grazing animals like sheep and cows but no predators such as cats or sharks or humans. The arms race between predators and prey was a vital evolutionary driver towards living complexity on Earth; eyes, ears and brains offer a survival advantage whether you are the hunter or the hunted, and if predation had been impossible for energetic reasons it is far less likely or perhaps impossible that complex animals would have evolved.

Photosynthesis has been around for a long time. The 3.5-billion-year-old Western Australian microbial mat structures are bacterial and they were probably early photosynthesisers, using light from the Sun to grab electrons off hydrogen sulphide and force them onto carbon dioxide to form sugars. They would not have used a pigment as complex as the green chlorophyll that colours the landscapes of Earth today; more likely they would have used simpler molecules from the same family known as porphyrins, which occur naturally and whose precursors have been found in Moon rocks and in interstellar space. Living things are like electrical circuits – they need a flow of electrons to power their metabolism, and given the ready availability of sunlight and naturally occurring molecules that can be assembled into machines to capture it and deliver electrons, it is not too difficult to see how primitive photosynthesis might have appeared very early in the history of life on Earth.

Given the obvious advantage of using the light from the Sun to power the processes of life, it’s not surprising that some early bacteria used photosynthesis for a different purpose – to synthesise a molecule known as adenosine triphosphate, or ATP, the energy storage system for life. ATP is one of the molecules that all living things share, and must therefore be very ancient, perhaps dating back to LUCA and the origin of life.

The type of photosynthesis found in modern plants, trees and algae is a hybrid of these two processes, with an important twist. Crucially, the electrons are no longer taken from hydrogen sulphide, but from water. The fusion of these two slightly different types of photosynthesis, and the use of sunlight to grab input electrons off water, was the great evolutionary leap that led to the oxygenation of the Earth’s atmosphere. Known as oxygenic photosynthesis, it evolved at some point earlier than 2.5 billion years ago. We know this because at this time Earth started to rust, forming great orange iron oxide layers known as banded iron formations, and this requires the presence of large amounts of free oxygen in the atmosphere. Molecular oxygen is an unstable and highly reactive gas, and must be constantly replenished. Astronomers in search of life on exoplanets would consider the detection of an oxygen atmosphere as a smoking gun for the presence of photosynthesis. Oxygenic photosynthesis is a terrifically complicated process, though; the molecular machinery is known as the Z-scheme, and its operation has only been understood in detail in the last few years. The sugar-manufacturing part alone, known as photosystem 2, consists of 46,630 atoms. The structure of the part that holds water molecules in place ready for their electrons to be harvested, known as the oxygen-evolving complex, was discovered in 2006. It is perhaps not surprising, therefore, that the more primitive forms of photosynthesis were not combined together into the oxygen-releasing Z-scheme for well over a billion years.

Beyond the long timescales involved in the evolution of oxygenic photosynthesis, however, there is another piece of circumstantial evidence that may suggest an evolutionary bottleneck. All the green plants and algae that fill our atmosphere with oxygen today perform their photosynthesis inside structures called chloroplasts. Chloroplasts look for all the world like free-living bacteria, and that is because they were, long ago. The story is that a bacterium, most likely one of the great family of early photosynthesisers known as cyanobacteria, was swallowed up by another cell and became co-opted to perform the complex task of grabbing electrons off water and using them to manufacture ATP and sugars, releasing the waste product oxygen in the process. This engulfing of one cell by another, and the merging of their properties, is known as endosymbiosis, an ability possessed by some cells that allows for step changes in living things through the wholesale merger of capabilities that evolved separately and over vast periods of time in different organisms. But here is the key point: everything on the planet today that performs oxygenic photosynthesis does it using the Z-scheme, and this strongly implies that it only evolved once, most probably in a population of cyanobacteria over 2.5 billion years ago. This tremendously advantageous innovation was so useful that it became co-opted into every plant, every tree, every blade of grass and every algal bloom on the planet, flooding the atmosphere with the oxygen necessary for the Cambrian explosion to populate Earth with endless forms most beautiful. If there were ever a smoking gun for a bottleneck, this is it.

But how on earth does a cell ‘learn’ how to engulf another one and survive? How did endosymbiosis arise? A clue, and perhaps an even more significant bottleneck, may be found in another prerequisite for the Cambrian explosion – the eukaryotic cell. All multicellular organisms are made up of cells known as eukaryotes – cells with a nucleus and a host of specialised structures each charged with performing specific tasks. The eukaryotic cells in every living thing look so similar that an alien biologist, knowing nothing about planet Earth, would immediately recognise that human eukaryotes are closely related to those from a blade of grass. The earliest known eukaryotic cells date from around two billion years ago. Beyond this, simpler cells known as prokaryotes were the only living things on the planet. Bacteria and archaea, the two single-celled kingdoms of life that still flourish today, are prokaryotes. They are simple in the sense that they lack the vast, specialised machinery of the eukaryotes, although as we’ve seen they do possess some vital and extremely complex abilities – photosynthesis being a very good example.

The most striking difference between eukaryotes and prokaryotes is the eukaryotes’ cell nucleus, which contains most of its DNA. In the story of evolution of life on Earth, however, it is the small amount of DNA stored outside the nucleus that is most revealing. Almost all eukaryotic cells contain structures called mitochondria. The word ‘almost’ is used a lot in biology. Unlike physics, there always seem to be one or two exceptions that ruin sentences in books like this. Most biologists believe that even the eukaryotes that don’t possess mitochondria did so at some point in the past, however, so we can take it that these structures are ubiquitous. Mitochondria are the power stations of the cell, and their job is to produce ATP. Around 80 per cent of your energy comes from the ATP produced in mitochondria, and without them you certainly wouldn’t exist. A clue as to their evolutionary origin is contained in their DNA, which is stored in loops and kept separate from the genetic material in the cell nucleus. Bacteria also store their DNA in loops, and this is not a coincidence. The mitochondria were once free-living bacteria.

The obvious question is, how did the bacterial mitochondria get inside the cells of every complex organism on the planet? The answer is through endosymbiosis, just as for the chloroplasts, but there is not universal agreement on the detail, and the detail matters a great deal. What is not in question is that the mitochondria are bacterial in origin. The debate surrounds the nature of the original host cell. One camp of biologists believes that the host cell was already a eukaryote, which over many millions of years had evolved an ability called phagocytosis – the ability to ingest other cells. This is a traditional Darwinian explanation – one in which complex traits evolve gradually over time via mutations and natural selection. If this is true, then it is possible to view the eukaryotic cell as just another evolutionary innovation, albeit a very important one, that might crop up anywhere given enough time. The other possibility, which is favoured by many biologists, has different implications. The idea is that the swallowing of the proto-mitochondrial cell was the origin of the eukaryotic cell itself. There was no such thing as phagocytosis or the eukaryotic cell before this singular event, and this ‘fateful encounter’ changed everything. Recent DNA evidence suggests that the host cell was probably an archaeon, one of the two great prokaryotic domains. Somewhere, in some primordial ocean, this simple prokaryote managed to swallow a bacterium – a trick that neither cell possessed before – and against terrific odds the pair survived and multiplied. The archaeon gained a huge advantage – a previously unimaginable energy supply from the bacterium’s sophisticated ATP factory. The bacterium also gained an advantage – it was protected and, over aeons, could specialise and concentrate entirely on producing energy for its host. If this theory is correct, the origin of complex life on Earth was a complete accident. Without access to the energy supply from the mitochondria, all the complexities of the eukaryotic cell, which are absolutely necessary for complex multicellular life, would never have evolved. Earth would be a living planet today, but a planet of prokaryotes, and certainly not home to a civilisation.

I cannot tell you which of these two theories is true. If it were obvious, then all academic biologists would agree. But my impression is that the fateful encounter is currently the more widely accepted theory, and if it is correct then this has very important consequences for estimating the probability of the evolution of intelligent life. Eukaryotes are absolutely essential for intelligence. There is no biologist who would suggest that the prokaryotes, for all their ingenuity in developing photosynthesis and mitochondrial machinery, would have managed to construct radio telescopes given enough time and a following wind. Without eukaryotes, there would be only slime.

I think these are very important points to consider in the Drake Equation. If it is correct that at least two of the necessary foundations for the emergence of complex multicellular life on Earth arose from barely credible accidents, then they might be seen as potential bottlenecks in the evolution of intelligence elsewhere in the Milky Way.

So where are we in our attempt to estimate the chances that, given the origin of life on a planet, intelligence will arise? This is where we move from science to speculation and opinion, and with these caveats, let me give you my personal view.

Given the eukaryotic cell and an oxygen atmosphere, life on Earth became diverse and complex relatively quickly. It is almost certainly no coincidence that the Cambrian explosion followed soon after a rapid rise in the oxygen content of the atmosphere. Whether it is possible to claim that intelligence on the scale necessary to build a civilisation is likely given the right biological building blocks and enough time – half a billion years, let’s say – is another question. We simply don’t know, and the very specific conditions in the African Rift Valley that led to the emergence of early modern humans only 250,000 years ago might suggest that civilisation-level intelligence is a rare development, even given animals as sophisticated as primates, never mind a eukaryote and an oxygen atmosphere.

An optimist would assert that there are billions of potential homes for life in the Milky Way, and that since life emerged on Earth pretty much as soon as it could at the end of the violence of the Hadean, then the Milky Way must be teeming with life and therefore civilisations. I would agree that the Milky Way must be teeming with life – I think there is a sense of chemical inevitability about it. Even accepting this line of argument, however, a pessimist would surely point to the evolution of the eukaryotic cell and oxygenic photosynthesis as being potential bottlenecks. On Earth, it took life over three billion years to get to the eve of the Cambrian. That’s three billion years of planetary stability – a quarter of the age of the universe. If just one of the necessary steps – the fateful encounter, let’s say – was at the fortunate end of a probability distribution, then one can easily imagine that the 20 billion Earth-like worlds in the Milky Way could all be covered in prokaryotic slime. A living galaxy, yes, but a galaxy filled with intelligence? Given what we know about the ascent from prokaryote to civilisation on Earth, I’m not so sure.

A BRIEFEST MOMENT IN TIME

Let’s take one final journey back to Green Bank in 1961. Drake and his colleagues, with far less evidence than we have today, concluded that our galaxy seems remarkably conducive to life, full of Earth-like worlds warmed by the glow of benign stars. They too believed that a good fraction of these billions of worlds must be home to life, and given that Darwin’s law of evolution by natural selection must apply across the universe, they concluded that intelligence must have emerged on at least some of these planets. As I’ve argued above, I’m not so sure about intelligence, but we must at least consider the possibility that potential evolutionary bottlenecks like the eukaryotic cell and oxygenic photosynthesis aren’t as bad as they might appear. In this case, the final term in the Drake Equation becomes all-important. Perhaps it is L, the lifetime of civilisations, that is the fundamental reason for the great silence. This is a sobering thought. The reason we have made no contact with anyone is not because of a lack of stars, or planets, or living things; it’s because of the in-built and unavoidable stupidity of intelligent beings.

This might seem a bit strong, but it is a view shared at Green Bank by Manhattan Project veteran Philip Morrison. Morrison was intimately involved in the design and development of the first atomic bomb, and he helped load Little Boy onto Enola Gay destined for Hiroshima. The fact that human beings had deployed a potentially civilisation-destroying weapon twice, against civilian targets, and that Morrison had personally loaded one of the bombs, must have never left him, and on the eve of the Cuban missile crisis, it must have seemed likely that we would do it again on a much grander scale.

Drake realised this as well, which is certainly one of the reasons why he introduced the time that a technological civilisation can endure into his equation: we can after all only communicate with nearby civilisations if they exist at the same time as us. This is a possible resolution to the Fermi Paradox. Civilisations inevitably blow themselves up soon after acquiring radio technology, and therefore the Milky Way will remain forever silent apart from the briefest, non-overlapping flickers of intelligence. This might seem like a solipsistic conceit; how can we possibly assume that human stupidity is universal? We can’t, of course. But just as for the biological arguments we made against the inevitable emergence of complex life on an otherwise living world, we only have the Earth as a guide, and extrapolating from our own experience is the best we can do. On Earth, Rutherford discovered the atomic nucleus in 1911 and we destroyed two cities and killed over 200,000 of our fellow human beings with nuclear technology 34 years later. About 17 years after that, having seen the devastation nuclear weapons can cause, Khrushchev and Kennedy came close to ending it all, and to this day we don’t know how close we came to eliminating the fruits of almost four billion years of evolution. Here on Earth it appears that sanity, perspective and an appreciation of the rarity and value of civilisation emerges after, and not before, the capability to build big bombs. We have the bombs, but I don’t think enough of us have the rest. Why should other young civilisations be any different? If this is the reason for the Great Silence, then I suppose we might take comfort in the fact that we are not the only idiots to have existed in the Milky Way, but that’s the coldest comfort I can imagine.

The above might be seen as a naïve rant, of course. One could argue that mutually assured destruction, the guiding principle of the Cold War, did act to stabilise our civilisation and is still doing so today. Perhaps no intelligent beings will knowingly destroy their civilisation, which is what global nuclear war on Earth would surely do; after all, Kennedy and Khrushchev ultimately took this view. Similarly, one assumes that the submersion of Miami and Norwich by rising sea levels would silence the so-called climate change sceptics (I’d call them something different) and trigger a change of policy that will avert catastrophic, civilisation-threatening climate change in good time. It seems to me, however, that a small planet such as Earth cannot continue to support an expanding and flourishing civilisation without a major change in the way we view ourselves. The division into hundreds of countries whose borders and interests are defined by imagined local differences and arbitrary religious dogma, both of which are utterly irrelevant and meaningless on a galactic scale, must surely be addressed if we are to confront global problems such as mutually assured destruction, asteroid threats, climate change, pandemic disease and who knows what else, and flourish beyond the twenty-first century. The very fact that the preceding sentence sounds hopelessly utopian might provide a plausible answer to the Great Silence.

SO, ARE WE ALONE?

What, then, is the range of estimates for the number of civilisations in the Milky Way, given the limited evidence we have at our disposal? During the filming of Human Universe, Frank Drake told me that the Green Bank meeting came up with a number of around 10,000, and he sees no reason to change that estimate. This would be wonderful, and makes the search for signals from these civilisations one of the great scientific quests of the twenty-first century. I strongly support SETI, because contact with just one alien civilisation would be the greatest discovery of all time, and it’s worth the investment on that basis alone.

There is, however, one piece of evidence that might suggest a more lonely position for us on our little home world. In 1966 the mathematician and polymath John von Neumann published a series of lectures entitled ‘Theory of Self-Reproducing Automata’ in which he analysed in great detail the possibility of constructing machines capable of building copies of themselves. Such machines exist in nature, of course – all living things do this routinely. In principle, therefore, one might imagine a sufficiently advanced civilisation building a self-replicating Von Neumann space probe and launching it out to explore the galaxy. On reaching a solar system, the probe would mine the planets, moons and asteroids, extracting the materials necessary to build one or more copies of itself. The newly minted probes would launch themselves out to neighbouring solar systems and repeat the process, spreading across the Milky Way. Even given the vast distances between the stars, computer models assuming currently envisioned rocketry technology suggest that such a strategy could result in the exploration of the entire Milky Way galaxy within a million years.

Science fiction? It certainly sounds like it, but if there is no objection in principle to the construction of a Von Neumann probe, then one has to develop an argument as to why we don’t see any. The reason that this is difficult to do is due to timescales. The Milky Way has been capable of supporting life for over ten thousand million years. It is possible to envisage many millions of civilisations rising and falling over such vast expanses of time, and if only one had developed a successful Von Neumann probe, then the galaxy should be filled with its progeny; there should be at least one Von Neumann probe operating in our solar system today. Carl Sagan and the astronomer William Newman noticed a flaw in this line of argument. If the probes multiply exponentially and unchecked, then one can show that they consume the resources of the entire galaxy relatively quickly, and we’d certainly have noticed that! Or more accurately, we wouldn’t be here to notice that. Sagan reasoned that this obvious risk would be sufficient to prevent any civilisation intelligent enough to build Von Neumann probes from actually doing so. They would be doomsday machines. Other astronomers have countered that it wouldn’t be beyond the wit of such an advanced intellect to build in some fail-safe mechanism that guaranteed, for example, only one probe per solar system, or a finite lifetime for each probe. Others have argued that there may indeed be a Von Neumann probe operating in our solar system today, with appropriate fail-safe mechanisms installed to stop it eating everything. If such a probe were relatively small, perhaps sitting amongst the asteroids or even in the Kuiper Belt of icy comets beyond the orbit of Neptune, then we’d almost certainly be unaware of its presence.

Von Neumann probes wouldn’t be the only signatures of ultra-advanced civilisations. Imagine a civilisation many millions of years ahead of us, carrying out engineering projects on a galactic scale. Imagine interstellar starships or great space colonies constructed in otherwise uninhabitable solar systems. Why not? As I said at the start of this chapter, we went from the Wright Brothers to the Moon in a single human lifetime, so, I ask again, how far will we travel, if the laws of physics allow, given another thousand years? Or ten thousand? Or a million? What signature will we leave on the sky if we survive and prosper that long? None of these questions is trivial, because the sheer immensity of the timescales available for life to evolve in the Milky Way galaxy forces us to consider them. Why should we be the most advanced civilisation in the galaxy when we’ve only been building spacecraft for half a century in a 13-billion-year-old universe? I don’t have an answer to this. It bothers me. Perhaps the distances between the stars are indeed too great, or perhaps there are insurmountable difficulties in building self-replicating machines or starships, but I can’t think what they might be.

I am tempted, therefore, to make the following argument for the purposes of debate. I think that advanced, space-faring civilisations are extremely rare, not because of astronomy, but because of biology. I think the fact that it took almost four billion years for a civilisation to appear on Earth is important. This is a third of the age of the universe, which is a very long time. Coupled with the remarkable contingency of the evolution of the eukaryotic cell and oxygenic photosynthesis – not to mention the half a billion years from the Cambrian explosion to the very recent emergence of Homo sapiens and civilisation – I think this implies that technological civilisations are stupendously rare, colossally fortuitous accidents that happen on average in much fewer than one in every two hundred billion solar systems. This is my resolution to the Fermi Paradox. We are the first civilisation to emerge in the Milky Way, and we are alone. That is my opinion, and given our cavalier disregard for our own safety, it terrifies me. What do you think?