Lucky Planet

Why Earth is Exceptional-and What That Means for Life in the Universe


By David Waltham

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Why Earth’s life-friendly climate makes it exceptional—and what that means for the likelihood of finding intelligent extraterrestrial life

We have long fantasized about finding life on planets other than our own. Yet even as we become aware of the vast expanses beyond our solar system, it remains clear that Earth is exceptional. The question is: why? In Lucky Planet, astrobiologist David Waltham argues that Earth’s climate stability is what makes it uniquely able to support life, and it is nothing short of luck that made such conditions possible. The four billion year-stretch of good weather that our planet has experienced is statistically so unlikely that chances are slim that we will ever encounter intelligent extraterrestrial others. Citing the factors that typically control a planet’s average temperature—including the size of its moon, as well as the rate of the Universe’s expansion—Waltham challenges the prevailing scientific consensus that Earth-like planets have natural stabilizing mechanisms that allow life to flourish.

A lively exploration of the stars above and the ground beneath our feet, Lucky Planet seamlessly weaves the story of Earth and the worlds orbiting other stars to give us a new perspective of the surprising role chance plays in our place in the universe.




Earth is a precious jewel possessing a rare combination of qualities that happen to make it almost perfect for sustaining 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 that it is unlikely we will succeed in finding similarly complex life elsewhere in the Universe.

The proposition that 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 on 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 history of life-forms and their 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 human existence. However, this 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, garden-variety galaxy or, alternatively, on the weirdest world in the entire visible Universe.

Personally, I no longer have doubts. The evidence points toward 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 emphasize how wonderful our home is and how lucky we are to exist at all.

My central argument is based on geological evidence showing Earth to have had a surprisingly stable climate. At first glance this may seem like a trivial claim. Why shouldn’t 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 has produced a modern atmosphere with a completely different composition than in the distant past. The scale of these natural variations dwarf those imposed by mankind in the last few centuries. Yes, we humans 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. But imagine the climatic result of atmospheric, oceanic, and terrestrial changes that are hundreds of times bigger than those we have generated. This is the scale of transformation imposed by Nature during the long history of our planet. 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 this remarkable.

There is no dispute that Earth’s climate has been continuously suitable for life for billions of years; we have incontrovertible evidence of the existence of 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 Earth had not enjoyed billions of years of reasonably good weather, but it is not at all clear whether there are processes that automatically stabilize our climate and that would also work on other worlds, or whether 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 Earth have been so lucky? What’s so special about us? The answer to both of these 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 I mean by “observational bias.” Observational biases occur whenever what you see is not what you get. For example, on mountainsides, 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 percent 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 that 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 Earth itself is the subject of inquiry. 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 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 only habitable for 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 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. We simply wouldn’t be around to notice had 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 Earth’s biosphere we can say a great deal about the environments that favor complex organisms; it is much harder to say anything concrete about the circumstances under which intelligence emerges.

Given this generalization, I should be clearer about what I mean by complex life. In the case of Earth, 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. They are anything but simple. However, some rather rare organisms have relatively recently evolved the trick of growing enormous colonies of cells tens of meters 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 multicelled organisms have an even higher level of organization than their single-celled relatives. Single-celled organisms occasionally also form colonies, but the key characteristic of more complex organisms is that they are constructed from many different types of cells. Of course, we shouldn’t be too Earth-centric in our thinking. Perhaps complex life-forms on other planets are not multicelled 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 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 counterarguments. However, I hope you will still conclude that “Is 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.



On Saint 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 six billion kilometers 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 Earth as a tiny, unimportant speck in space. Carl Sagan, one of the greatest-ever popularizers of science and the man who did the most to encourage NASA to turn Voyager 1 around in order to capture this image, memorably described 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 twenty-first 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 will look at the historical background for the idea that 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 four hundred years ago we generally placed Earth at the center of the Universe or, with a little less hubris, at the bottom of the ladder to the heavens. The first step toward 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 revolutionized our view of the Universe by suggesting that Earth and the other planets in our solar system revolved around the Sun rather than all heavenly bodies revolving around a stationary Earth. Interestingly, De Revolutionibus’s title is the origin of “revolution” as a word to indicate overthrowing of previously well-established ideas or organizations. It was another one hundred and fifty years before this first revolution, the Copernican revolution, became widely accepted. Nevertheless, once Earth had been knocked off its perch at the center of the Universe, the next step in our planet’s demotion came along with remarkable rapidity: perhaps the Sun isn’t the center of the Universe either!

Giordano Bruno, a sixteenth-century priest, was among the first to wonder whether the stars are distant suns and whether they, 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 nineteenth-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 nineteenth-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 the spot where he was executed, further fueling the secular canonization of this controversial and colorful character. Thus, two hundred and fifty 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 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 traveled through Italy, France, England, Bohemia, and the Germanic countries and, as he traveled, 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 two-thousand-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 beneath our feet. The idea that all of creation is sacred contrasted starkly with the religious orthodoxy of sixteenth-century Europe, which held that everything below the orbit of the Moon, the sublunar world, was degenerate, and 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, England, 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’s suggestion that Earth went around the Sun to its logical conclusion; if Earth moved just like the other planets, then Earth and the heavens were not fundamentally different. Clearly this fit well with Bruno’s belief that Earth and the heavens were equally sacred and 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 Earth, there was no reason why there could not be Earth-like places elsewhere in the heavens. He suggested that perhaps the other planets are just like Earth and perhaps even the stars are just distant suns each with its 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 Earth, that it is just a typical planet in orbit around a typical star.

Assuming 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. 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. What Galileo saw proved beyond all reasonable doubt that Copernicus was right; the Universe did not revolve around Earth. Galileo was the first to see that Venus showed phases like the Moon, phases whose timing only made sense if Venus went around the Sun and not around Earth. He also saw that Jupiter had its own moons and this again showed that Earth did not lie at the center 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: Earth is not the only world.

Over the next four hundred years, Earth was demoted further as we discovered that the Sun is just one star in the two hundred 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 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 in this book. 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 Earth is nothing special and that its location is very ordinary.

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 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. Bruno’s entirely reasonable excuses were that, despite rumors to the contrary, he didn’t know any magic and he didn’t approve of such things anyway. However, he must have said this with all of his usual tact and diplomacy because Mocenigo took offense and denounced Bruno to the Venetian Inquisition. The Venetian Inquisition, in turn, passed him on to the Roman authorities.

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 favors from Spain and so, on February 17, 1600, Bruno was led from his cell, had his tongue spiked to silence him forever, 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 four hundred years since these events, Giordano Bruno has been transformed into a freethinker 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 its own system 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 overapplication. The three centuries following Bruno’s death were characterized by almost unquestioning certainty that the worlds of our solar system are fundamentally so similar to Earth that they must all be populated by intelligent life. Ironically, this became the new religious orthodoxy; 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-nineteenth-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 Earth is special and that life is unlikely elsewhere in the cosmos. Whewell’s motivation was 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, Earth had been a planet without sentient life. Hence, we only have 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 one hundred and fifty years ago are strikingly similar to some that will be put forward here 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, as I’ll discuss in the next chapter, that the situation is now changing rapidly. Furthermore, Whewell expressed my own views perfectly when he wrote 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 three hundred years from Bruno until the beginning of the twentieth century were characterized by almost universal acceptance of the idea that intelligent life exists throughout the solar system and beyond. During the course of the twentieth 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 it in some detail.


On Sale
Apr 8, 2014
Page Count
208 pages
Basic Books

David Waltham

About the Author

David Waltham is an astrobiologist, geophysicist and head of the department of Earth Sciences at Royal Holloway College, the University of London.

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