Lucky Planet E-Book

David Waltham

• Herausgeber: Icon Books
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 357

Veröffentlichungsjahr: 2014

Beschreibung

Science tells us that life elsewhere in the Universe is increasingly likely to be discovered. But in fact the Earth may be a very unusual planet – perhaps the only one like it in the entire visible Universe. In Lucky Planet David Waltham asks why, and comes up with some surprising and unconventional answers. Recent geological, biological, and astronomical discoveries are bringing us closer to understanding whether we might be alone in the Universe, and this book uses these to question the conventional wisdom and suggest, instead, that the Earth may have had 'four billion years of good weather' purely by chance. If Earth-like worlds don't have natural stabilising mechanisms, then intelligent observers such as ourselves will only ever look out onto those rare planets where, like the Earth, all the bad things that could have happened to the climate have fortunately cancelled each other out. So before you prepare to meet the aliens, consider that we are probably alone …

Leseprobe

Praise for Lucky Planet

‘[Lucky Planet is] a lively and well argued antidote to a widespread view that advanced life could arise frequently and in many places in the known Universe. Waltham explains why the Earth is a much more peculiar planet than you might think, and he shows that its friendliness to life does not just apply to the here-and-now, but must equally have pertained through a history of more than 3.5 billion years: life’s survival and prospering to the point where intelligent life could emerge was a product of extraordinary and exceptional luck. A sceptical response to ideas of inevitable evolution of intelligent beings among the stars, Waltham suggests that we may, after all, be lonelier than we could have thought …’

Richard Fortey, author ofSurvivors and The Hidden Landscape

‘David Waltham takes us on a delightful tour of the various factors that influence planetary habitability and the evolution of advanced life. That he thinks the prospects for it are unlikely is all the more reason for us to go up to space and take a good look!’

James Kasting, Penn State University, author ofHow to Find a Habitable Planet

LUCKY PLANET

DAVID WALTHAM

LUCKY PLANET

WHY EARTH IS EXCEPTIONAL – AND WHAT THAT MEANS FOR LIFE IN THE UNIVERSE

Published in the UK in 2014 by

Icon Books Ltd, Omnibus Business Centre,

39–41 North Road, London N7 9DP

email: [email protected]

www.iconbooks.net

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ISBN: 978-184831-656-0

Text copyright © 2014 David Waltham

The author has asserted his moral rights.

No part of this book may be reproduced in any form, or by anymeans, without prior permission in writing from the publisher.

Typeset in ITC Galliard by Marie Doherty

Printed and bound in the UK by Clays Ltd, St Ives plc

Contents

Prologue: A Tale of Two Planets

1    Almost Too Good To Be True

2    Mediocrity

3    Rarely Earth

4    Constant Change

5    Air Conditioning

6    Snowballs and Greenhouses

7    Staggering Through Time

8    Music of the Spheres

9    Force of Nature

10  Pond Weeds and Daisies

11  Life’s Big Bang

12  Eclipse

13  The Dark Side of the Moon

14  Gaia or Goldilocks?

Epilogue: Siblings

Further Reading

Acknowledgements

Index

About the author

David Waltham obtained a first-class degree and a PhD in Physics before moving into the oil industry in the early 1980s. This industrial experience led to his appointment, in 1986, as a lecturer at Royal Holloway, University of London, where he became Head of Earth Sciences from 2008–2012.

Prologue: A Tale of Two Planets

Far beyond the range of any telescope humanity will ever possess lies the doomed planet Nemesis. Nemesis, a near-twin to our own world, is named after the ancient Greek goddess responsible for the rebalancing of undeserved good fortune. As befits this name, Nemesis has been on a lucky roll far longer than any world could reasonably expect, but her streak of good fortune has come to an end.

Nemesis is dying. The immense herds that once swarmed across her vast plains are gone for ever. The huge beasts that swam her clear blue ocean waters are now extinct, and the previously verdant rainforests of her equatorial regions have withered and died. A beautiful and complex biosphere has vanished, leaving bacteria and a few species of worm as meagre representatives of the multifarious life-forms that once made Nemesis the biological pinnacle of her galaxy.

When Nemesis first formed she was a duplicate of the early Earth in almost every way. Most of her subsequent history, too, was strikingly similar to that of our own planet. Both of these initially sterile, over-heated hell-holes transformed into vibrant, microbe-infested globes within a few hundred million years of their births. Single-celled plants arose on both worlds within 2 billion years, giving them oxygen-rich atmospheres just as they reached their 3-billionth birthday. Breathable atmospheres in turn allowed large plants and animals to appear when Nemesis and Earth reached about 4 billion years of age. By middle-age, at 4.5 billion years, ferocious monsters stalked both worlds. Had they known it, the Earth-dinosaurs and Nemesis-dragons could have been proud of their status as the most complex organisms in their respective galaxies. Then disasters struck both planets. Only one world recovered.

The first indications of trouble on Nemesis were subtle; storms became a little bigger, droughts slightly longer and winters marginally colder. Animals and plants adapted to the changes and continued to prosper. Bit by bit, conditions worsened. During the most extreme periods the entire planet switched from stiflingly hot desert to frigid polar wasteland and back every few hundred thousand years while, at other times, much of Nemesis spent months in frozen darkness followed by months of continuous, blazing sunlight. The climate occasionally stabilised to give life a respite but, after a few tens of thousands of years, it became erratic once more and intolerable for many organisms. Eventually, plants could no longer evolve fast enough to keep up and when the plants died, so did the animals. Extinction outpaced species creation, and within a few million years of the start of the troubles, only the most robust organisms clung to life.

Earth too endured a dangerously variable climate at this time. Sulphur dioxide clouds, the result of intense volcanic eruptions, reflected sunlight into space to produce severely cold decades. When the clouds cleared, equally unforgiving hot periods followed, produced by greenhouse gases blasted into the air by those same volcanoes. And, just when it seemed that things could get no worse, Earth was hit by an asteroid. The climatic consequences of this impact proved to be the final straw for millions of struggling species; many, including the dinosaurs, died out. Yet this is where Earth’s fate finally diverged from that of Nemesis. The climate chaos calmed down and Earth’s biosphere began its slow journey back to full health. Our ecosystem took 10 million years to recover but new life-forms slowly emerged to repopulate a changed yet still vibrant world; a world that went on to produce Homo sapiens, 65 million years later.

How did two worlds enjoy billions of years of exuberant, parallel development and then eventually experience such dramatically different outcomes? There are many ways of getting a world wrong but few ways of getting it right. Nemesis stands for all the myriad failed worlds in the Universe, just one example of how not to build a habitable planet. Yet her difference from our own world is tiny, as we’ll see in the pages of this book. In contrast, Earth has been blessed with incredible good fortune, giving it all the right properties to sustain a complex and beautiful biosphere. It may just be the luckiest planet in the visible Universe.

1

Almost Too Good To Be True

The Earth is a precious jewel in space possessing a rare combination of qualities that happen to make it almost perfect for life. Lucky Planet investigates the idea that good fortune, infrequently repeated elsewhere in the Universe, played a significant role in allowing the long-term life-friendliness of our home and shows why it is unlikely we will succeed in finding similarly complex life elsewhere in the Universe.

The proposition that the Earth may be an oddball, a planet quite unlike any other we will ever find, has been discussed for centuries. Until recently such debates were built upon mere speculation, but times are changing. We now sit at one of those scientific crossroads where a field of study moves from being a disreputable, if interesting, subject for discussion to a real science with defendable conclusions based on substantial evidence. Such transitions occur when technological advances make previously impossible observations routine and, as a result, new data becomes available.

In the case of oddball Earth, the new data comes from advances in how we look at the rocks beneath our feet and at the stars above our heads. The rocks tell a tale of our planet’s constantly changing environment along with the story of life and its struggles to survive. The stars speak of many possible worlds, all unique in their own way. These parallel stories suggest that incredible good fortune was needed to allow our existence, although that proposal remains controversial. Many of my colleagues will tell you that the data are still too sparse to decide whether we live on a fairly typical planet orbiting a normal star in an unremarkable part of a common-or-garden galaxy or, alternatively, on the weirdest world in the entire visible Universe.

Personally, I no longer have doubts. The evidence points towards the Earth being a very peculiar place; perhaps the only highly-habitable planet we will ever find. This view has led some astrobiologists to describe me as ‘gloomy’, but I don’t see things that way. For me, these ideas merely emphasise how wonderful our home is and how lucky we are to exist at all.

My central argument is based on geological evidence showing the Earth to have had a surprisingly stable climate. At first glance this may seem like a trivial claim. Why shouldn’t the Earth have a stable climate? Quite simply, because the factors that control our planet’s surface temperature have all changed dramatically during the 4.5 billion years of Earth’s existence. Our Sun now gives off much more heat than she did when young, while geological and biological activity have produced a modern atmosphere with a completely different composition to that in the distant past. The scale of these natural variations dwarfs those imposed by mankind in the last few centuries. We have made minor adjustments to the atmospheric composition, have caused significant alterations to the amount of cloud cover, and have even destroyed entire ecosystems. Among many other nasty side-effects, our tinkering will produce a warming of the climate comparable to that experienced at the end of the last ice age. Now imagine the climatic result of atmospheric, oceanic and terrestrial changes hundreds of times bigger than those we have been able to generate. This is the scale of transformation imposed by Nature during the long history of planet Earth. And despite Nature’s massive modifications, the climatic fluctuations wrought by astronomical, geological and biological processes have always more or less cancelled each other out. I find that remarkable.

There is no dispute that the Earth’s climate has been continuously suitable for life for billions of years; we have incontrovertible evidence for life throughout that time. However, the reasons for the unbroken eons of life-friendly climate are hotly debated. Most scientists agree that the evolution of our beautiful, complex biosphere could never have occurred if the Earth had not enjoyed billions of years of reasonably good weather, but it is not at all clear whether there are processes that automatically stabilise our climate, and that would therefore also work on other worlds, or whether the Earth has simply been very, very lucky. It is also possible that life, once started, is more robust than we believe and would have survived even had there been more dramatic climate change over the long history of our planet. I’ll consider these possibilities in the pages of Lucky Planet.

The obvious questions this idea raises are: ‘Why should the Earth have been so lucky?’ ‘What’s so special about us?’ The answer is that we’re looking at the most severe case of observational bias in the history of science. This rather sweeping statement lies at the core of my book so I’d better explain what an ‘observational bias’ is. Observational biases occur whenever what you see is not what you get. For example, on mountain-sides, seashores, cliffs and other rocky places, harder rocks tend to stick out while softer ones erode away, with the resulting spaces being filled with mud, vegetation or rubble. Under these conditions it is easy to erroneously believe that the area contains only the harder rocks. Our view of what is really there has been misled by the accident of what we’re able to see.

A similar observational bias occurs when we look at the night sky. The majority of stars visible to the naked eye are more massive than our Sun even though 95 per cent of all stars are actually lighter. The reason is simple: bigger stars are brighter stars and our unaided eyes aren’t sensitive enough to see the faint ones. In addition, heavy stars are usually hot enough to shine with a white or blue light but the much cooler majority of stars would be distinctly reddish if we could only see them. The few thousand stars we see on a dark night are therefore unrepresentative of the hundreds of thousands of stars that inhabit our small corner of the galaxy. To eyes that could see these red-dwarf stars, the heavens would be awash with faint red points of light interspersed only rarely by the brighter white stars, blue stars and red giant stars that dominate the night skies seen with human eyes. Our view of what is really there has been misled by the accident of what we’re able to see.

The potential for observational bias becomes enormous when the Earth itself is the subject of enquiry. In the same way that we can’t see rocks that are buried, or stars that are faint, intelligent observers can’t see a home-world that is uninhabitable. We must be living on a planet suitable for intelligent life, even if such worlds are extraordinarily rare and peculiar. As a geologist I think this ‘anthropic selection effect’, as it is known, is a vital but almost universally ignored insight and we simply cannot understand our planet properly without taking it into account. Our view of what is really there has been misled by the accident of what we’re able to see.

As a consequence of this bias, we must acknowledge and take account of our privileged viewpoint when considering whether qualities of the Earth are typical or exceptional. An instructive example concerns the surprisingly early appearance of life on our planet. The fact that microbes appeared on Earth while our world was still very young is often taken as evidence that life appears easily and will be widespread throughout the Universe. This is mistaken. Planets are habitable for only a few billion years and so intelligent life probably doesn’t have time to evolve on worlds that drag their feet over life’s origin. All intelligent observers, including us, must find themselves looking out onto worlds where life began soon after conditions became suitable. The possibility that this is a chance event not repeated on most habitable worlds means that there could be an observational bias and an early start for life on Earth cannot be used as evidence that life is an easy trick for a planet to pull off. Maybe it is and maybe it isn’t.

From my perspective, the most important anthropic selection effect concerns the resilience of life. I’ve frequently heard it said that life is exceptionally robust, once it arises, as shown by the fact that it has survived every catastrophe thrown at it during Earth’s long history. But how could it be otherwise? Planets where life fails to survive do not give rise to sentient beings. Intelligent observers throughout the Universe, no matter how rare or common they may be, must look out onto home planets where life has managed to survive. Perhaps life doesn’t survive for long on the majority of planets where it appears and we simply wouldn’t be around to notice had the Earth been less fortunate.

A planet may therefore have to be pretty weird to allow a creature as odd as Homo sapiens to appear. However, for practical reasons, Lucky Planet will discuss the planetary preconditions necessary for complex life-forms in general rather than sentient life-forms in particular. From observations of the Earth’s biosphere we can say a great deal about the environments that favour complex organisms, but it is much harder to say anything concrete about the circumstances under which intelligence emerges.

Given this generalisation, I should be clearer about what I mean by complex life. In the case of Earth life it is helpful to draw a distinction between single-celled organisms and multicelled ones. The vast majority of organisms on this planet are microscopic, single-celled creatures such as amoebas and bacteria. These are anything but simple. However, some rather rare organisms have relatively recently evolved the trick of growing enormous colonies of cells tens of metres tall (e.g. trees) while their close relatives have evolved similarly large colonies able to move about to track down food (e.g. grazing cows). These multi-celled organisms have an even higher level of organisation than their single-celled relatives. Single-celled organisms do sometimes form colonies but the key characteristic of more complex organisms is that they are constructed from many different types of cell. Of course, we shouldn’t be too Earth-centric in our thinking. Perhaps complex life-forms on other planets are not multi-celled creatures with differentiated tissues but have a completely alien and utterly unimaginable architecture instead. Nevertheless, I think we can be sure that alien intelligences, if they exist at all, will be more complex than single-celled Earth organisms. As I’ll show in later chapters, simpler organisms tend to be much tougher than more complex ones and so this distinction is quite important.

Lucky Planet is an exploration of the idea that the Earth is a very strange place – perhaps the luckiest planet in the visible Universe. We’ll begin with the opposite idea, the scientifically conventional one that there is nothing particularly special about our world at all. We will then tour astronomy, geology, climatology, biology and cosmology to show why this conventional view needs to be reconsidered. In many places you will almost certainly come up with counter-arguments. However, I hope you will still conclude that ‘Is the Earth special?’ is a sensible question to ask. After this tour, I’ll return to Nemesis, the ‘unlucky planet’ with which I began. Once you see how trivial the difference was between Earth and our near-twin, I hope you will agree that our planet really is almost too good to be true.

2

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