Lucky Planet

Mediocrity

On St Valentine’s Day 1990 the voyager turned around for a last lingering look at the home she had left twelve years before. Thus, from beyond the orbit of Pluto and 6 billion kilometres from Earth, the space probe Voyager 1 took the most distant photograph of our world ever attempted. The result was a picture of an insignificant spot barely visible against a background of instrument-scattered sunlight, but this picture beautifully encapsulates our modern view of the Earth as a tiny, unimportant speck in space. Carl Sagan, one of the greatest-ever popularisers of science and the man who did most to encourage NASA to turn Voyager 1 around to capture this image, memorably described the Earth in this picture as just a ‘pale blue dot’. At that time we did not know whether stars, other than the Sun, had planets. It was still possible to believe that there was something special about our star’s entourage of six pale, variegated dots (Venus, Earth, Jupiter, Saturn, Uranus and Neptune had all been imaged) but this state of affairs didn’t last long. The first widely accepted exoplanet, a planet orbiting another star, was discovered just five years after Voyager 1’s farewell photograph and, by early in the 21st century, it has become clear that exoplanets are pretty common. These discoveries, along with Voyager’s photo, reinforce the perception of our world as small, insignificant and lost in the immensity of the Universe. In this book I plan to challenge that view and, to begin with, I want to look at the historical background to the idea that the Earth is a mediocre planet.

The idea that our world is just one planet among many has certainly not been mankind’s view through most of history. Until 400 years ago we generally placed the Earth at the centre of the Universe or, with a little less hubris, at the bottom of the ladder to the heavens. The first step towards an improved sense of perspective was taken by Nicolaus Copernicus, whose De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres) was posthumously published in 1543. This book revolutionised our view of the Universe by suggesting that the Earth and planets revolved around the Sun rather than all heavenly bodies revolving around a stationary Earth. Interestingly, De Revolutionibus’ title is the origin of ‘revolution’ as a word to indicate overthrowing of previously well-established ideas or organisations. It was another 150 years before this first revolution, the Copernican revolution, became widely accepted. Nevertheless, once the Earth had been knocked off its perch at the centre of the Universe, the next step in our planet’s demotion came along with remarkable rapidity: perhaps the Sun isn’t the centre of the Universe either!

Giordano Bruno, a 16th-century priest, was among the first to wonder whether the stars are just distant suns and whether these too have planets revolving around them. The story is that Bruno was burned at the stake for suggesting this and for supporting the views of Copernicus. Bruno is therefore held up as an early scientific martyr, someone who gave his life in the battle of truth against ignorance. However, this tale of scientific heroism is a 19th-century exaggeration promoted at a time when the supporters of Darwin’s new theory of evolution saw themselves in conflict with a church they regarded as superstitious and reactionary. The myth was magnified further by the resonance it had in a 19th-century Italy struggling to emerge as a nation and straining to free itself from the political dominance of the Vatican. As part of the propaganda campaign in this power struggle, a statue of Bruno was erected in 1889 near to the spot where he was executed, further fuelling the secular canonisation of this controversial and colourful character. Thus, 250 years after his death, Bruno was dragged into two new revolutions: one that began with the ‘Spring of Nations’ nationalist uprisings of 1848 and one that began in 1859 with the publication of Charles Darwin’s On the Origin of Species. However, the dreadful fate of Giordano Bruno was the consequence of a much earlier and even bloodier clash of ideas.

At a time of great religious strife in Europe resulting from the rise of Protestantism, the rather argumentative Bruno travelled through Italy, France, England, Bohemia and the Germanic countries and, as he travelled, he argued with almost everyone about almost everything. To his credit, he was trying to reunite a divided Western Europe behind his own ‘Hermetic’ version of Christianity. Hermeticism has existed in one form or another since the early years of the Roman Empire and still has its supporters in our own time. Over that 2,000-year history it has meant many things to many people but one of its more constant messages is that everything is divine, even the rocks of the Earth. The idea that all of creation is sacred contrasted starkly with the religious orthodoxy of 16th-century Europe, which held that everything below the orbit of the Moon, the sub-lunar world, was degenerate, while the heavens beyond were incorruptible, eternal and perfect. Despite this rather serious barrier to wide acceptance, Bruno saw Hermeticism as a way to bridge the theological divide between Catholics and Protestants and, at a time when many were tiring of bloodshed, his ideas might have been listened to but for his rather arrogant, tactless and belligerent manner. Instead, he merely succeeded in angering all sides and even managed to be simultaneously excommunicated by the Calvinist, Lutheran and Catholic churches; almost the full set. Then he suffered the possibly worse fate of being laughed at in Oxford, an experience he neither forgot nor forgave.

Despite his Oxford experiences Bruno remained in England for two years and, during this visit, made the fatal mistake of becoming embroiled in the intrigues of the French ambassador against the Spanish by acting as a spy (as well as, possibly, a double agent spying on the French for Queen Elizabeth). I’ll come back to the fatal consequences of this unwise move later, but it was also while in England that Bruno wrote La Cena de la Ceneri (The Ash Wednesday Supper) and De l’Infinito Universo et Mondi (On the Infinite Universe and Worlds), in which he explained his cosmological ideas. In these books Bruno took Copernicus’ suggestion that the Earth went around the Sun to its logical conclusion: if the Earth moved just like the other planets, then the Earth and the heavens were not fundamentally different. Clearly this fitted well with Bruno’s belief that the Earth and heavens were equally sacred and it explains why he supported Copernicanism so strongly. Bruno then took his ideas a breathtaking step further by reasoning that, if the heavens were made of the same stuff as the Earth, there was no reason why there could not be Earth-like places elsewhere in the heavens. Perhaps the other planets are just like the Earth and perhaps even the stars are just distant suns each with their own planetary companions. Here, Bruno was expressing for the first time what we now call the principle of mediocrity – the idea that there is nothing special about the Earth. The Earth is just a typical planet in orbit around a typical star.

Less than ten years after Bruno’s execution his compatriot, Galileo Galilei, became one of the first to turn a telescope onto the night sky and what he saw proved beyond all reasonable doubt that Copernicus was right – the Universe did not revolve around the Earth. Galileo was the first to see that Venus showed phases like the Moon, phases whose timing made sense only if Venus went around the Sun and not around the Earth. He also saw that Jupiter had its own moons and this again showed that the Earth did not lie at the centre of all things. Finally, he saw that Earth’s satellite is an entire world complete with its own mountains, valleys, cliffs and craters. It still took decades to convince those unwilling to accept the evidence of their own eyes but Galileo’s observations proved that Bruno had been right: the Earth is not the only world.

Over the next 400 years, the Earth was demoted further as we discovered that the Sun is just one star in 200 billion forming our galaxy. Our galaxy, in turn, is just one out of hundreds of billions of galaxies in the visible Universe. And it is probable that the visible Universe is only a small fraction of the entire Universe, with recent speculations even suggesting that the Big Bang was a local affair and that our Universe is just a tiny part of what some are calling a multiverse, a subject I’ll return to much later. Furthermore, the last few hundred years of research have shown that the principles of physics and chemistry are the same in the most distant parts of the visible Universe as they are on Earth. The conclusion then, following centuries of scientific work, is that the Earth is nothing special and its location is very ordinary.

Assuming the Earth to be mediocre has been a powerful tool enabling us to greatly expand our horizons and to see the true vastness and grandeur of the cosmos. The principle of mediocrity has served us well for nearly half a millennium but I believe that its very success has caused this invaluable working principle to slowly mutate into an unbreakable law. Ironically, it has become scientific heresy to question Bruno’s insight. An almost subconscious belief in the ordinariness of our world is making us blind to an important truth: there may be things about our planet that are far from typical. As I’ve already discussed, places suitable for the emergence of intelligent observers may be extremely rare. We might therefore need to return to a geocentric cosmology in the sense that the Earth may be the most interesting place in the observable Universe.

Before moving on, I’d like to complete Bruno’s tragic story. Several years after leaving England he returned to Italy and worked for the nobleman Zuan Mocenigo in Venice. The Catholic church already considered Bruno to be a dangerously unorthodox thinker but he should have been safe in a Venice that was proudly independent of papal influence. Unfortunately he angered Mocenigo by refusing to teach him black magic. His entirely reasonable excuses were that, despite rumours to the contrary, he didn’t know any magic and didn’t approve of such things anyway. However, he must have said this with all of his usual tact and diplomacy because Mocenigo took offence and denounced Bruno to the Venetian Inquisition. The Venetian Inquisition, in turn, passed him on to the Roman authorities and Bruno was snared.

As was the usual fate of heretics at this time, the Roman Inquisition locked Bruno up and threw away the key. He probably expected to spend the last few decades of his life in jail, but events in Spain and southern Italy led him to an even worse fate. A revolt broke out in Spanish-ruled Calabria and the leader of the revolt just happened to be another Hermetic philosopher. After suppressing this revolt, the Spanish authorities decided they wanted to make an example of Bruno, the most famous Hermetic philosopher in Europe as well as someone who had plotted against Spanish interests while in England. Spain therefore demanded that Bruno be executed. Rome, in turn, was looking for favours from the Spanish and so, on 17 February 1600, Bruno was led from his cell, had his tongue spiked to silence him for ever and was burned at the stake without the usual consolation of being strangled first.

It’s clear that Bruno was more a victim of political circumstance than a martyr to science. Indeed many of his ideas, and his reasons for supporting them, seem distinctly unscientific today. However, we all like to have heroes, and in the 400 years since these events, Giordano Bruno has been transformed into a free-thinker whose ideas were centuries ahead of his time. There is much truth in this, even though the details show him as frequently quarrelsome and only occasionally profound. But, when it comes to stars being suns each with their own systems of worlds, Giordano Bruno hit the nail on the head and started a revolution in thought that continues to this day.

As often happens to new ideas, the principle of mediocrity built up a bit of momentum before it became widely accepted and, as a result, its eventual triumph led to a dramatic switch from outright rejection to over-application. The three centuries following Bruno’s death were characterised by almost unquestioning certainty that the worlds of our solar system are fundamentally so similar to the Earth that they must all be populated by intelligent life. Ironically, this became the new religious orthodoxy since many thinkers could not understand the purpose of other worlds unless God had placed people on them.

However, a few dissenters did question this new doctrine. Of particular note is the mid-19th-century polymath William Whewell, the man who coined the term ‘scientist’ and a great thinker who made innovative advances in fields from geology to mathematics. Whewell is chiefly remembered today for writing an influential book on natural theology (the idea that nature’s ‘perfections’ demonstrate the existence of God), which was referred to by Darwin at the beginning of On the Origin of Species. At this point in my story, though, Origin lay in the future. In 1853, six years before publication of Darwin’s masterpiece, Whewell published Of the Plurality of Worlds in which he attempted to demonstrate that the Earth is special and that life is unlikely elsewhere in the cosmos. Whewell’s motivation was explicitly religious: he believed the existence of intelligent life on other worlds to be incompatible with mankind’s special relationship to God. But despite this, his core argument was pure science. Whewell used the new geological knowledge of his time to show that, for the vast majority of its history, the Earth had been a planet without sentient life. Hence, we have only to look at the ground beneath our feet to see that worlds uninhabited by people are logically possible.

In many ways Whewell’s arguments from 150 years ago are strikingly similar to some that will be put forward in these pages. For example, I can only applaud his statement that ‘the history of the world, and its place in the universe, are far more clearly learnt from geology than from astronomy’, although it should be admitted that, as I’ll discuss in the next chapter, that situation is now changing rapidly. Furthermore, Whewell expresses my own views perfectly when he writes that ‘the Earth, then, it would seem, is the abode of life … because the Earth is fitted to be so, by a curious and complex combination of properties’. I was also particularly struck on reading that ‘the Earth’s orbit is the Temperate Zone of the Solar System … [since] the Inner Planets bear no infrastructure of life; for all life would be scorched away along with water, its first element’. This may be the first-ever reference to what is now called the habitable zone; the zone around a star where temperatures allow the existence of liquid water. There is, however, one fundamental difference between Whewell’s book and my own. Whewell believed our good fortune in living on a well regulated world to be the result of divine providence, whereas I put it down to good fortune: a good fortune that is inevitable somewhere in a big enough universe.

Whewell aside, the 300 years from Bruno until the beginning of the 20th century were characterised by almost universal acceptance of the idea that intelligent life exists throughout the solar system and beyond. During the course of the 20th century, however, this came to look more than a little optimistic. The debate over whether there is life on Mars illustrates that century’s transition to pessimism particularly well and so it’s worth taking a look at that in some detail.

The story begins with yet another great Italian, one who lived 250 years after Bruno and Galileo. In 1877 Giovanni Schiaparelli made ground-breaking observations of Mars at a time when it was unusually close to us. Earth and Mars approach one another every two years; Mars circulates around the Sun once in the time it takes Earth to go around the Sun twice. However, the orbits of Earth and Mars are not perfectly circular and so their exact distance apart varies from one opposition (as the moment of closest approach is called) to another. In the recent 2003 opposition, for example, Mars was closer to us than it has been for 60,000 years. Schiaparelli used his drawings from the almost equally good 1877 opposition to create a new map of Mars that was significantly better than anything previously produced. Indeed, the map was so good that many of the names he chose for features still appear on modern maps based on space probe pictures. The high quality of Schiaparelli’s work shouldn’t surprise us. When he began his Martian work Schiaparelli had been chief astronomer at Milan’s observatory for fifteen years and his most important contributions to astronomy until then had been his discovery of the asteroid Hesperia in 1861 and his 1866 demonstration that comets generate meteor showers. He also made sophisticated breakthroughs in the mathematics of orbit calculation. It’s clear that Schiaparelli was an extraordinarily accomplished and talented man, but today he is chiefly remembered for a mistake he made on that, otherwise excellent, map of Mars.

Conditions for viewing at even the best-situated observatories are variable, but during those moments of exceptional atmospheric clarity that arise for a few seconds every now and again on the best nights, Schiaparelli glimpsed long, thin, dark lines crossing the surface of Mars. He dutifully marked these onto his map and called them canali. Many other respected astronomers confirmed his discovery and their existence was rapidly accepted by everyone. Then the trouble started. The word ‘canali’ was perfectly correctly translated into English as ‘canals’, but while the Italian word does not necessarily imply an artificial channel, the English translation certainly does. This was the origin of the myth, popular through much of the 20th century, that intelligent Martians are living on an old, dying world and have constructed a network of canals for transporting water from the poles towards an arid equator. This vision of ‘intellects vast and cool and unsympathetic, [who] regarded this earth with envious eyes’, was immortalised in The War of the Worlds in 1898 and H.G. Wells’s story of a Martian invasion remained plausible throughout the first half of the 20th century. However, by the time of a major Hollywood film version in 2005, the moviemakers were distinctly vague about where exactly the invaders came from. By the early years of the 21st century, Mars had become an unlikely site for complex life. Why?

Arguments over the origin of Martian canals began almost as soon as Schiaparelli announced his discovery. The chief proponent of the view that they had to be artificial was Percival Lowell who, in 1894, established one of the best observatories in the world in Flagstaff, Arizona, with the specific objective of investigating the non-natural features of Mars. Lowell published Mars and Its Canals in 1905 and his book included detailed drawings showing 400 canals that were geometrically straight, thousands of miles long and frequently double ‘like the rails of a railway track’ as Lowell described them. For Lowell the implications were incontrovertible; the canals were artificial.

A riposte came quickly and from one of the most venerable scientists of the day, Alfred Russel Wallace. Wallace has a slightly mixed reputation today. He is revered by biologists as the co-discoverer of the theory of evolution by natural selection and as the father of biogeography, the study of the influence of geography on the distribution of species. On the less positive side, he is also remembered for his belief in spiritualism and for his campaign against smallpox vaccination. The story of his co-discovery of natural selection is well known and widely discussed in other books; in brief, Wallace formulated the theory while working in Indonesia and sent an account of it to London, where it was forwarded to Charles Darwin. Darwin, who had arrived at the same theory years earlier but had not yet published it, was stung into action and the result was a joint presentation to the Linnaean Society in 1858 followed by publication of Darwin’s Origin of Species in 1859. Wallace’s own views concerning Darwin’s primacy in this discovery can be judged from the fact that Wallace published a book called Darwinism in 1889. This is testament both to the generosity of Wallace’s nature and to the strength and breadth of the stunningly detailed evidence that Darwin had amassed for their theory during the years before Wallace’s bombshell letter arrived from Indonesia.

In 1907, almost 50 years after this central role in one of the biggest scientific revolutions of all time, Wallace published Is Mars Habitable?, a short book that is almost a line-by-line demolition of Lowell’s Mars and Its Canals. At the age of 83, Wallace still had a keen mind and an ability to cut through to the key issues. He did not question the reality of the canals, since they had been seen by many highly skilled observers, but he did focus on the major error in Lowell’s arguments: his assertion that the Martian climate is warm enough to allow the presence of liquid water. Wallace and Lowell were both aware that the mean temperature of a planet depends on the amount of heat it receives from the Sun, the fraction of that heat absorbed rather than reflected and, finally, on the strength of the greenhouse effect. Lowell’s estimates of these factors yielded a mean Martian temperature of 9°C while Wallace, after consulting the eminent physicist John Henry Poynting, arrived at an estimate of –38°C. Our modern estimate is an even colder –55°C, a figure that is now, of course, supported by direct measurements from spacecraft on the surface of Mars (in a pleasing coincidence, just as I wrote that last sentence I learned that the latest of these, the Mars Science Laboratory rover Curiosity, landed successfully three hours ago in Gale Crater on Mars). Interestingly, Lowell’s temperature estimate was wildly incorrect for almost exactly the same reason that some modern climate-change sceptics overstate the resilience of Earth’s climate: he over-estimated the cooling effect of cloud reflectivity relative to the greenhouse warming effect of atmospheric water vapour. As a result, he believed that the large amount of water on the Earth made it relatively cool and that the much drier Mars would be almost as warm despite being further from the Sun. In reality, and as Wallace discussed in great detail, Mars was far too cold to allow Lowell’s concept of Martian snow melting at the poles each summer, or the flow of water in canals across its surface. Indeed, as Wallace strongly suspected and we now know, the apparent shrinking of the Martian polar caps each summer results from thawing of a thin carbon dioxide frost and not from melting of water at all.

Despite the work of Wallace and other sceptics, the canal myth was finally buried for good only in 1965 when the US space probe Mariner 4 passed by the planet and sent back the first close-up pictures. Canals were nowhere to be seen, neither in the Mariner pictures nor in any of the detailed pictures sent back by the numerous space probes that have visited Mars since that time. I have spent several surprisingly enjoyable hours staring at the maps made by Schiaparelli and Lowell, along with photographs from the Hubble telescope and images from spacecraft now orbiting Mars, to see if any of the proposed canals are real features. I have even deliberately blurred the modern pictures to try to recreate the difficulties of seeing Mars through Earth’s atmosphere. My impression is that a few of Lowell’s canals and the ‘oases’ that formed at their intersections do correspond to chance alignments of features such as large craters and volcanoes and, with the eye of faith, even with some of the very largest drainage features such as Eos Chasma. However, these possible correlations are extremely speculative and, when it comes to Schiaparelli’s map, I’m afraid I have been unable to explain any of his canal locations at all! Sadly, the Martian canals were a classic example of the human mind’s extraordinary ability to construct patterns from the flimsiest of evidence, a useful trait but one that easily fools us into seeing things that just aren’t there. The canals were ‘seen’ by many eminent astronomers but they are no more real than the constellations we see in the sky or ley-lines (supposed ‘lines of power’ constructed by imaginative individuals that connect ancient monuments on Earth). The human eye and brain just like to connect dots. Canali do live on, but on a different planet: Venus, where enigmatic but undoubtedly natural channels thousands of kilometres long have been found and that have been termed canali in, I hope, a respectful gesture to Schiaparelli. But, with the dismantling of the myth of Martian canals, the remaining slim evidence for intelligent life on Mars evaporated.

Since 1965 Mars has been visited by many spacecraft that have sent back incredibly detailed pictures of a not entirely alien world. A beautiful planet is revealed with volcanoes, canyons, plains and ice caps. It could almost be home except that there are no plant-covered continents and no blue seas. In fact, there is no stable liquid water at all because the surface is too cold and the atmosphere too thin. But something cut those canyons! Where has the water gone? The real geography of Mars demonstrates beautifully that there are no guarantees of climate stability. Mars once had liquid water and, possibly, warm seas on its surface. Sadly, the planet slowly lost much of its water to space, and the remainder became locked up in ice at the poles and below the surface. Lowell’s vision wasn’t entirely wrong. Mars really has turned from a water-rich world into a desert planet, but this happened billions of years ago and no sentient life-forms were there to rescue the planet by constructing a worldwide web of irrigation.

Despite the unpromising Martian environment now revealed to us by probes and landers, the debate over life on Mars continues to the present day – although we are now resigned to searching for past or present microbial life buried deep beneath the frigid, dry and caustic soil forming the surface. The idea of simple life on Mars is far from implausible and there is tentative evidence that life may have existed there 3.6 billion years ago. In December 1984 a meteorite was picked up in the Alan Hills area of Antarctica and taken back to the USA for study. The meteorite was labelled ALH84001 to indicate its provenance and discovery date but little else was done to it for several years. When it was looked at more carefully in the 1990s it proved to be the most significant meteorite to hit the Earth since the one that may have killed the dinosaurs. ALH84001 is a meteorite made from 3.6 billion-year-old Martian rocks and it appears to contain biochemicals along with fossil micro-organisms. This meteorite therefore suggests there is a plausible case for life on Mars, but how good a case is it?

The first part of this claim states that ALH84001 comes from Mars – but how could we possibly know that? Very large meteorites occasionally hit all planets and some material from the struck world is blasted into space when this happens. Eventually, these fragments may meet another planet and so, over millions of years, significant amounts of rock are exchanged between worlds. One interesting consequence is that Earth and Mars are not biologically isolated from each other. Rock fragments blasted from the Earth will contain bacteria that could survive the journey from one world to another, provided the fragments are large enough and do not spend too long in space. So, even if life really does exist on Mars, it may well be transplanted Earth life. It’s even possible that Earth life originally came from Mars!

As a result of this exchange of rocks, there must be many pieces of Venus, the Moon and Mars sitting on the Earth; the problem is in recognising them. However, since we have been to the Moon and have sent measuring instruments to Mars, we know something of the chemical composition of these worlds, which means that we can recognise chunks of the Moon or Mars when they are found here. Chips off the Martian block are pretty rare, though. Of the tens of thousands of meteorites that have ever been collected, only about 100 come from Mars. It is clear that these meteorites share a common origin because many aspects of their chemical compositions are similar. We also know that one of them, called EETA79001, came from Mars because it contains bubbles of gas with a composition identical to that of the Martian atmosphere as measured by NASA’s Viking landers in the mid-1970s. So, the claim that ALH84001 is a bit of Mars is pretty sound and not seriously disputed by any of the experts.

The second claim, that ALH84001 contains life-like chemical traces, is more contentious. The fractures in this meteorite contain iron-based minerals and complex carbon compounds that, on Earth, would definitely indicate the presence of bacteria. However, we cannot be sure that these minerals and compounds are only ever produced by living organisms. It is possible that they could be produced on Mars by exotic, non-biological, chemical processes unfamiliar to us on an Earth whose organic chemistry is utterly dominated by life. It is also possible that these biomarkers, as chemical fingerprints of life are sometimes called, resulted from contamination by Earth bacteria after the meteorite landed on the Antarctic ice sheet. So the biomarker evidence is interesting but not conclusive.

The final and most dramatic claim is that there are microscopic fossils within ALH84001. Electron-microscope images show worm-like features that certainly look bacterial but they are tiny compared to Earth-born microbes. The NASA team that made these discoveries worked hard to eliminate the possibility that these microfossils resulted from contamination or that so-called artefacts were created accidentally when the specimens were coated with gold and palladium (a standard procedure in electron-microscopy). Despite these precautions, most planetary scientists continue to believe that these creatures either crawled in after the meteorite fell to Earth or that they are tiny, artificially created, pellets of gold or palladium that just happen to look a bit bug-like.

So, the evidence for life on Mars in the distant past is worth taking seriously, but most experts remain unconvinced. The reason for continued scepticism is the widely accepted principle that extraordinary statements require extraordinary evidence. The evidence for life in ALH84001 would be uncontentious, not to say uninteresting, if this was a rock from somewhere on Earth – but it’s not from Earth, it’s from Mars. If the evidence is taken at face value and we all accept that there is, or at least has been, life on Mars, we might well discover later that understandable enthusiasm has clouded our judgement.

If it does turn out that ALH84001 has been over-optimistically interpreted, it wouldn’t be the first time. Throughout the modern scientific era there has been a tendency to over-optimism on the question of whether life in general, and intelligent life in particular, is common in the Universe. Of course, it doesn’t necessarily follow that present optimism concerning life around other stars is misplaced but it should give us pause for thought. We’ve seen that many of the arguments in favour of a Universe with widespread life have been based on wishful thinking rather than hard science. Indeed, at times, it has almost been seen as reprehensible to believe that we are alone. This view, that there is something morally wrong in regarding the Earth as special, persists to this day but now takes the form of accusations of arrogance against those of us who think Earth-like biological richness may be rare. Dictionaries define arrogance as a ‘display of superiority or self-importance’ and so, strictly speaking, it really is arrogant to assume that the Earth is special – but that doesn’t make the view incorrect. It’s possible that I am both arrogant and correct!

More generally, the anthropic principle itself is deeply disliked by many scientists because they fear it encourages lazy thinking. Why bother trying to find out the real reason why the Earth has particular attributes if you can just dismiss them as mere accidents that happen to be essential for our existence? In one scientific paper I read recently, such thinking was dismissed as ‘scientifically unsatisfying’. Again, this may be a fair criticism but it has no bearing on whether Earth really does have an unusual combination of properties that make it peculiarly well suited for life. In a similar vein, I have had correspondence with one deep thinker on the topic of the anthropic principle who, among the many other serious issues he raises, worries that it encourages a tendency towards atheism. My flippant response to this is that we shouldn’t tell God how to run the Universe! And while I do think my correspondent has a point, I don’t think it is relevant to the question of whether or not the anthropic principle is true. The anthropic principle may well encourage arrogant, lazy, naive and atheistic thinking but these anxieties concern the moral status of the theory, not its truth status.

These types of criticism remind me of similar ones made in the 19th century against Darwinism. There is a widely repeated story that a Victorian lady responded to news of Darwin’s ideas with: ‘Descended from the apes! My dear, we will hope it is not true. But if it is, let us pray that it may not become generally known.’ This apocryphal tale illustrates nicely the moral difficulties that 19th-century people had with accepting the theory of evolution by natural selection. Indeed, it could be argued that their concerns were vindicated; there really is a plausible link between Origin of Species and Kristallnacht but this no more falsifies Darwin’s ideas than the Enola Gay falsifies those of Einstein. Similarly, there are many excellent reasons for disapproving of the anthropic principle but none of them has any bearing whatsoever on whether it is true. Instead, we need to look fearlessly at the evidence and go with what that tells us, whether we like it or not.

In my view, the principle of mediocrity has become too much of an article of faith and the time has come when it should be questioned. Maybe there are at least some things that are special and different about the Earth compared to the vast majority of planets. On the other hand, we shouldn’t just throw mediocrity away. It has proved far too valuable over the centuries for that. Clearly, there is a conflict between a logically unassailable anthropic principle and an observationally well supported principle of mediocrity, but this tension can be resolved by combining them into a single principle: the Earth is no more special than it has to be to allow the existence of intelligent life. To put it another way, the Earth is probably a fairly typical inhabited planet since, by definition, we are more likely to be living on a typical inhabited world than on an atypical one. This principle of anthropic mediocrity answers a criticism I frequently hear: that the special conditions I claim are needed on Earth apply only to Earth-like life, and that perhaps other forms of life are possible and are less picky. However, this leaves me wondering: Why should Earth have such odd forms of life if other types of life are more common?

The conventional view discussed in this chapter, that the Earth is a fairly typical world, is challenged in the next chapter, which looks at how the latest discoveries concerning other worlds are overturning this centuries-old idea.