Lucky Planet

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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...